Preamble

Vol. 43, No. 2

June 2025

PROGRESS IN ASTRONOMY Vol. 43, No. 2 June, 2025 doi: 10.3969/j.issn.1000-8349.2025.02.01

Research Progress on Physical Properties of Active Asteroid (3200) Phaethon ZHANG Xinyi¹;², JI Jianghui¹;², JIANG Haoxuan¹;³ (1. CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China; 2. University of Science and Technology of China, Hefei 230026, China; 3. Chuzhou College, Chuzhou 23909, China)

Abstract

This paper provides a comprehensive review of perihelion observational data and significant observational events of the active asteroid (3200) Phaethon, analyzing the mechanisms of activity near perihelion, particularly the formation mechanisms driven by thermal fracturing, water ice sublimation, and Na sublimation. Based on spectral data, albedo, and polarization studies of Phaethon, we summarize research findings on its surface physical properties and composition, providing abundant evidence for a comprehensive understanding of this celestial body. The Phaethon-Geminid Complex, classification of active asteroids, and their traceability are thoroughly discussed. In studies of asteroid orbital evolution and thermophysical models, the MERCURY6 integrator was employed to conduct millennial-scale inversion of Phaethon's orbital elements, preliminarily obtaining its motion patterns including perihelion distance. Additionally, based on the Advanced Thermophysical Model (ATPM), we performed integrated fitting of infrared multi-band observational data to derive Phaethon's thermal inertia, albedo, and diameter. Finally, we prospect space exploration missions for active asteroids, discussing JAXA's DESTINY+ mission and China's Tianwen-2 mission.

Keywords: asteroid; activity mechanism; near-Earth asteroid

1 Introduction

The Geminid meteor shower in mid-December each year is one of the most intense and regular annual meteor showers. In 1983, the Infrared Astronomical Satellite (IRAS) discovered its parent body to be asteroid (3200) Phaethon (hereinafter referred to as Phaethon) during a sky survey near the orbit of Geminid meteoroids [1], temporarily designated as 1983 TB at the time [2]. Phaethon is currently considered an active near-Earth asteroid, likely originating from the main belt, representing a key celestial body between comets and asteroids, also known as an active asteroid [3–5]. Multi-color photometric measurements show that Phaethon's surface exhibits a blue spectral slope [6], in stark contrast to the red spectra of cometary nuclei [7].

According to observations from the Arecibo Observatory, Phaethon is a top-shaped asteroid with an effective diameter of 5.7 km [8] and a rotation period of approximately 3.6 h [9]. In 2021, more precise size measurements were obtained through occultation observations, indicating that Phaethon's equatorial diameter is 6.13 ± 0.05 km and its polar diameter is 4.40 ± 0.06 km, revealing a more flattened ellipsoidal shape with flatter polar regions than previously thought [10]. [FIGURE:1] shows Phaethon's shape model from four different viewing perspectives.

Note: a) and b) are views rotated 90° in the equatorial direction, c) is the polar view, and d) is a 20° oblique view from above.

Phaethon possesses a highly elliptical orbit with semi-major axis a = 1.271 AU, eccentricity e = 0.890, inclination i = 22.3°, and orbital period of 523.5 days; its perihelion distance q = 0.14 AU and aphelion distance Q = 2.40 AU, located within Jupiter's 5.2 AU orbital radius. Two other asteroids, (155140) 2005 UD and (225416) 1999 YC, have orbital elements and spectra similar to Phaethon, and together with the Geminid meteor stream they are collectively referred to as the Phaethon-Geminid Complex (PGC) [12–15].

Initially, Phaethon was considered inactive [16]. In 2009, the Heliospheric Imager (HI-1) on the Solar Terrestrial Relations Observatory-A (STEREO-A) first reported brightening of Phaethon near perihelion [17]; its brightness peaked a few hours after perihelion and faded within two days [18]. In 2012 and 2016, anomalous brightening still occurred near Phaethon's perihelion, accompanied by an anti-solar dust tail, indicating that this activity could recur repeatedly [4, 19].

In terms of asteroid classification, Phaethon was once classified as an F-type asteroid within the C-type branch, later merged into B-type asteroids [20–24]. The Japan Aerospace Exploration Agency (JAXA), in collaboration with the Planetary Exploration Research Center at Chiba Institute of Technology, plans to explore Phaethon through a project named "DESTINY+" (Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science). Originally planned for launch in 2024, it has been postponed to 2028, with a subsequent flyby of Phaethon [25]. China is also preparing to deploy missions for active asteroids, with the "Tianwen-2" probe scheduled for launch in 2025, targeting active asteroid "311P" as one of its objectives, aiming to pave the way for subsequent Jupiter and asteroid exploration missions.

Chapter 2 primarily introduces important observational events of Phaethon; Chapter 3 focuses on discussing activity mechanisms near perihelion; Chapter 4 presents analysis of observational results; Chapter 5 introduces related model studies such as orbital evolution models and thermophysical models; Chapter 6 discusses Phaethon's relationship with other asteroids and group classification; Chapter 7 introduces future launch missions; finally, the paper concludes with a summary and prospects for directions worthy of in-depth exploration.

2.1 Perihelion Activity Observations

Phaethon's perihelion is close to the Sun, where the solar elongation angle is less than 8°, making direct observation extremely difficult. Jewitt and Li [26] first observed Phaethon's activity using the HI-1 camera on STEREO-A [27], showing that it brightened by approximately 2 mag after perihelion (June 20.2 ± 0.2, 2009 UTC), reaching peak brightness a few hours later and fading within two days [18]. After excluding geometric effects and plasma impact excitation, the brightening was interpreted as being caused by dust ejection, with the total ejected dust mass estimated at 2.5 × 10⁸ kg for millimeter-sized particles [26].

Subsequently, Li and Jewitt [19] discovered comet-like tail structures in optical imaging near Phaethon's perihelion. In 2009 and 2012, astronomers observed the tail growing to its full length (approximately 2.5 × 10⁵ km) within one day, indicating that dust ejection acceleration was at least 0.07 m·s⁻², consistent with radiation pressure acting on 1 μm-sized spherical dust grains. Based on this, the total ejected dust mass was estimated at approximately 3 × 10⁵ kg [19]. Analysis of observations from 2009, 2010, and 2012 showed that this brightening recurred [19], with tail-like structures appearing only during the two-day brightening period [4].

On August 19.82, 2016, Phaethon brightened by 2 mag shortly after perihelion passage [28], forming a tail structure 0.1° long the following day. This elongated structure quickly disappeared, essentially identical to previous tail structures [4, 19]. Based on the Schleicher-Marcus phase function, micron-sized dust would be enhanced by forward scattering effects at phase angles up to 166° [29, 30]. If the brightening is attributed to this mechanism—that Phaethon experienced forward scattering phase angles and brightened, then dimmed as the phase angle decreased—the dust size can be estimated as 0.5 μm, with total mass loss of 10⁴–10⁵ kg. However, calculations of effective scattering cross-section from large-phase-angle detection data using STEREO's COR2 coronagraph field of view place a 3σ upper limit of 300 kg on dust mass ejected during perihelion [31], far below the total mass of 3 × 10⁵ kg derived from HI-1 observations [4]. Therefore, more evidence is needed to confirm the correlation with micron-sized dust particle ejection.

Considering wavelength differences compared to COR2 camera observations [31], HI-1 has transmission near 400 nm and 1000 nm [32], and HI-1 camera filter bandpasses were found to have non-negligible transmission at the Na I D line [33]. Consequently, Phaethon's brightening behavior was speculated to be caused by Fe I emission lines near 400 nm or Na I D emission lines at 589.0/589.6 nm [31]. In May 2022, the Large Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) [34, 35] and the HI-1 imager on STEREO-A captured Na I D lines released by Phaethon [36], with Phaethon's activity brightness through orange filters being much brighter than through blue filters that cannot transmit Na I D lines [36]. Using SOHO LASCO coronagraph, observers detected resonance fluorescence at the Na I D line (589.0/589.6 nm) from Phaethon's tail, demonstrating that its brightening and tail development resulted from Na release [36]. This observation strongly supports Phaethon's Na-driven brightening theory, with detailed analysis presented in Section 3 of this paper.

On December 16, 2017, Phaethon passed Earth at a distance of 0.069 AU, the closest approach since 1974 until 2093. Numerous detection campaigns were conducted using ground-based telescopes to search for small fragments [5, 37] and dust particles [38] to investigate whether Phaethon's activity persists beyond perihelion. However, no activity or fragment traces were detected, further supporting the conclusion that the Geminid meteor shower is not produced by steady-state activity.

To date, Phaethon has only exhibited weak activity near its perihelion. Ye et al. [39] searched for gas and dust emissions and fragments from Phaethon during December 14–18, 2017, finding no fragments of 15–100 m near it; comparison with comet C/2017 O1 (ASASSN) spectra confirmed the absence of cometary emission lines in Phaethon. These observations established a 3σ upper limit on dust production rate of 0.007 ± 0.2 kg·s⁻¹ [39]. Additionally, no dust emission or fragments were found in Very Large Telescope observations of Phaethon at 10.7 μm [38].

Ground-based observations from the Canada-France-Hawaii Telescope (CFHT) and Xingming Observatory in Xinjiang, China, also found no cometary activity or meter-sized fragments from Phaethon, estimating upper limits on mass loss of 0.06 ± 0.02 kg·s⁻¹ at 1.449 AU from the Sun, and 0.2 ± 0.1 kg·s⁻¹ at 1.067 AU [22]. This activity intensity is insufficient to supply the Geminid meteor stream.

The geometric relationship between Phaethon, the Sun, and Earth's orbit makes it difficult to directly obtain absolute magnitude at phase angles of 0°, thus complicating diameter calculations from absolute magnitude and albedo derived from infrared observations. This close approach to Earth created conditions for radar to accurately determine Phaethon's size. [FIGURE:2] shows rotation images from Arecibo Observatory observations, based on which Phaethon's equivalent diameter was estimated at 5.7 km [8], larger than previous thermophysical model estimates (5.1 ± 0.2 km [3]). The observations revealed that Phaethon likely has an oblate shape: large kilometer-scale depressions in equatorial and low-latitude regions, and a distinct radar-dark feature near one pole, speculated to be a crater [8].

Note: A set of range-Doppler images of Phaethon from Arecibo observations on December 16, 17, and 18, 2017, showing its complete rotation.

Ground-based observations obtained clearer light curves, colors, and polarization data for Phaethon. Tabeshian et al. [22] presented its light curve for phase angles ranging from 20°–100° (shown in [FIGURE:3]), deriving Phaethon's mean colors as B–V = 0.702 ± 0.004 mag, V–R = 0.309 ± 0.003 mag, and R–I = 0.266 ± 0.004 mag. The B–V color varies with observation latitude, and its photometric variations are consistent with the depressions reported by Arecibo [22], indicating that Phaethon's surface may have large craters, which could be related to the Geminid meteor stream.

Note: Letters a and b indicate candidate boulders, c and d indicate candidate concavities, e indicates a linear facet, and f indicates a dark spot.

The occultation method can also determine asteroid size with high precision by directly measuring the shadow cast on Earth when the asteroid occults a background star, with precision determined by timing accuracy. To obtain more accurate size measurements, DESTINY+ science team members used occultation observations to estimate Phaethon's diameter, with 18 stations detecting stellar occultation phenomena [40]. On October 3, 2021, astronomers in western Japan observed Phaethon occulting a 12-magnitude star in Auriga. The occultation cross-section could be approximated by an ellipse with major and minor diameters of 6.12 ± 0.07 km and 4.14 ± 0.07 km, respectively, at a position angle of 117.4° ± 1.5° [40]. Observations from October 13, 2021, showed Phaethon's major and minor diameters as 6.13 ± 0.05 km and 4.40 ± 0.06 km [10]. The DESTINY+ science team will modify Phaethon's three-dimensional shape model based on these occultation results to more accurately develop the spacecraft.

From October 31 to November 11, 2018, the Wide-field Imager for Solar Probe (WISPR) on the Parker Solar Probe (PSP) mission first encountered dust in Phaethon's orbit within 0.0277 AU, with observed dust trails shown in [FIGURE:4] [41, 42]. The two most prominent dust trails observed were concentrated near perihelion, with distributions highly coincident with Phaethon's orbit but offset beyond perihelion [41]. In 2022, Battams et al. [42] summarized and analyzed dust observations from nine independent flyby missions between October 2018 and August 2021, finding that dust trails did not completely follow Phaethon's orbit, with discrepancies increasing with true anomaly. If Phaethon's orbital elements are modified by reducing its argument of perihelion by exactly 1.0° [42], its orbit nearly matches the observed dust trails.

Note: Phaethon's orbit is indicated by dashed lines, and black X marks represent the points where Phaethon's orbit was closest to PSP during observations.

On May 2, 2021 (E8), WISPR-I and WISPR-O on PSP recorded observations. The estimated dust mass ranges from 10¹⁰–10¹² kg, far exceeding the amount of dust ejected at Phaethon's current activity level and comparable to the total mass of the Geminid meteor stream. The observed dust trail brightness is roughly uniform, with no clear relationship between brightness and heliocentric distance, suggesting that dust has uniformly filled the orbit [42]. Battams et al. proposed that this dust trail may be produced by partial Geminid meteoroids near Phaethon and therefore may not share the same origin as meteoroids encountered by Earth [41]. It remains unclear whether this dust trail is caused by massive dust ejection from Phaethon.

3 Asteroid Activity Mechanisms

Active asteroids have various activity mechanisms, including radiation pressure [43], electrostatic repulsion [44], sublimation (e.g., 133P) [45], thermal fracturing [45], rotational instability (e.g., 311P) [46], and impacts [47]. Researchers currently believe that Phaethon's primary activity mechanisms may be water ice sublimation, thermal fracturing, and Na-driven activity [28], with additional consideration of rotational instability, radiation pressure debris motion [48], and cohesive forces [49]. These activities can interact, such as rotation promoting thermal fracturing [26].

Ryabova [50] suggested that dust in perihelion activity is difficult to distinguish from earlier Geminid meteoroids.

3.1 Water Ice Sublimation

Most meteor stream parent comets experience mass loss through ice sublimation [45], where dust grains are exposed on the surface and dragged into space as ice sublimates. Unlike comets, Phaethon's activity is only observed during perihelion passage, with no dust or gas ever detected near or beyond Earth's orbit [38, 39]. If sublimation is the cause of its activity, it must be demonstrated that volatiles exist somewhere on Phaethon that can sublimate at high temperatures. Jewitt and Li [26] argued that temperatures near perihelion reach approximately 1000 K, excluding the presence of water ice on Phaethon's surface; while its spectrum shows no clear features in the 3 μm region, meaning no surface water ice or hydrates, but not ruling out internal water [51]. If Phaethon originated in the outer solar system, internal water ice may be preserved, but whether it can drive current activity requires further study.

Assuming Phaethon's surface heat conducts to deep subsurface layers, according to Jewitt and Hsieh [15], the core temperature (Tc = 300 K) calculations show that water ice cannot persist [5]. Maclennan and Granvik [52] conducted thermodynamic analysis using the orbTPM model based on the possibility of deeply buried water ice, estimating that subsurface ice sublimation occurs on a timescale of 50 Ma, while water ice within 200 m of the surface would completely sublimate within 5.5 Ma, thus internal water ice remains possible.

Yu et al. [53] proposed a dust-ice two-layer system, discussing the possibility that internal water ice could be preserved to the present day after long-term thermal evolution. The sublimation/condensation cycle in this system could produce transient gas outbursts at perihelion, explaining the observed dust tail. Calculation results show that Phaethon would lose all buried ice on a timescale of 6 Ma, with dust outer layer thickness reaching hundreds of meters during evolution, providing the possibility for buried water ice in Phaethon today [53]. Moreover, Phaethon would have been more active in early stages when the dust outer layer was thinner.

3.2 Thermal Fracturing

Phaethon's dust tail only appears near perihelion, requiring consideration that brightening may result from pulsed release of dust grains increasing the effective cross-section and thus scattering more sunlight [4]. Its high acceleration of 0.07 m·s⁻² is consistent with radiation pressure acting on 1 μm-sized dust grains, indicating dust small enough to be strongly accelerated by radiation pressure [4].

Observations of the 2016 brightening event showed that a tail composed of dust particles with radii of about 0.5 μm formed within 1 day after perihelion passage [28]. Li and Jewitt [19] argued that high temperatures near perihelion are more likely to cause thermal fracturing than ice sublimation. Thermal fracture and decomposition of mineral materials such as hydrous silicates due to desiccation-induced shrinkage is a process that can both produce dust and eject it from the surface, with dust being a product of thermal fracture [19, 26, 54].

Four processes in the thermal fracturing mechanism can eject dust [26]: (1) tension during thermal fracture can eject dust; (2) electrostatic mechanisms can remove dust, with surface ions promoting dust adhesion and release [55]; (3) solar wind radiation pressure also removes dust smaller than 1 mm at perihelion [54]; (4) rotation causes surface dust removal during spin, with Phaethon's rotation period of approximately 3.6 h [3], and Phaethon's wider equatorial shape increases the size limit for dust removal by radiation pressure [5]. Due to rock cracking from heating, Phaethon's brightening and dust tail phenomena recur at perihelion [19].

Dust production at high temperatures is accompanied by rock structural changes, loss of chemically bound water, uneven thermal expansion, and rapid heating, causing thermal fracturing. Phaethon's expected perihelion temperature ranges from 743–1050 K, reaching the minimum temperature at which many rocks can decompose [54]. Temperature increases cause gradual loss of chemically bound water, changes in crystal structure, and overall rock contraction leading to internal stress insufficient to prevent cracking and producing dust. However, gas drag from this process is insufficient to overcome gravity for ejection. Cyclic heating and cooling of rocks cause non-uniform internal thermal expansion [56, 57], and when local stress exceeds material strength, cracks initiate and propagate [56, 58]. Meteorite sample experiments show that crack growth and decomposition are consistent with assumed regolith formation timescales [59], suggesting this process may be more effective than impacts at producing regolith [56, 57, 60]. Additionally, rapid heating causes insufficient solid thermal conduction, leading to thermal expansion, with large temperature gradients acting locally on timescales shorter than thermal conduction timescales sufficient to trigger thermal fracture. Using the rotation period to calculate diurnal temperature differences (ΔT ≈ 500 K) [61], thermal gradient simulations show that thermal fracturing can eject particles up to 2 cm from Phaethon's equatorial region [62]. If temperature changes are slow, shear stress between rocks with different expansion properties can also cause thermal fracturing when exceeding structural strength [26].

The Geminid meteor stream mass is 10¹³–10¹⁴ kg [63], with loss rates of 320–3200 kg·s⁻¹ on millennial timescales [63]. Simplified model calculations estimate brightening-ejected dust mass at 2.5 × 10⁸ kg [26]. Jewitt et al. [4] re-estimated brightening mass loss at 3 × 10⁵ kg based on 1 μm-sized dust conditions, with a mass loss rate of approximately 0.1–3 kg·s⁻¹. Hui and Li [28] calculated dust ejected during Phaethon's 2016 activity event, obtaining a mass of 10⁴–10⁵ kg, with an average mass loss rate of 0.1–1 kg·s⁻¹, consistent with previous results. This shows Phaethon does not provide stable replenishment for the Geminid meteor stream, leaving the origin of meteoroids unresolved [45]. However, thermal fracturing cannot be excluded as a possible source for the Geminid meteor stream. Additionally, the phase lag between perihelion in 2009 and 2012 and anomalous brightness increases was approximately 0.5 days [19], with the relationship between this phenomenon and thermal fracturing dust ejection remaining unclear.

3.3 Na-Driven Theory

Research on Phaethon indicates that temperatures on the sunward side exceed 1000 K, sufficient to vaporize rocks [3]. Sublimation of rock components can also serve as a trigger [64] to remove surface material and expose subsurface layers, enabling sustained activity and leading to catastrophic disruption. Na in rock composition is also one of the long-term volatile elements, and Na-driven activity may occur in water-free bodies like Phaethon. Geminid meteorite meteoroids show Na depletion with low abundance [65], possibly due to Na depletion after dust ejection at high temperatures [66]. Masiero et al. [67] proposed that Na sublimates from asteroid surfaces at high temperatures, causing dust ejection rather than depletion after ejection. To verify this, the study used the thermophysical model NIMBUS [68] for simulation, showing that Na concentrations at least twice the original abundance could form locally, which combined with impacts could cause outbursts [67]. Additionally, Na ionization and migration can promote dust particle adhesion and release on Phaethon's surface [44].

Before discovering bandpass leakage beyond 630–730 nm in HI-1 cameras, Phaethon's brightening tail was not initially linked to Na sublimation [4], and brightening could be explained by forward scattering enhancement from dust [28]. Researchers believed micron-sized dust required 1 day to be accelerated by radiation pressure into a tail [4, 28], making the tail appear elongated after perihelion. However, micron-sized dust scattering inferred from HI-1 did not match dust mass from COR2 data, suggesting Phaethon's activity mechanism warrants further investigation. Considering previously excluded leakage components [33], this brightening could include Fe I emission lines near 400 nm or Na I D emission lines at 589.0/589.6 nm [31]. According to observations in May 2022 from SOHO LASCO coronagraph and HI-1 heliospheric imager on STEREO-A, the brightening cause is likely Na I D lines [36]. By modeling emission line flux, researchers found that the peak atomic production rate from gas sublimation was delayed by 1 day after perihelion, with this asymmetric model better reproducing light curves similar to HI-1 observations. Simulations of brightening tail morphology support the gas sublimation-driven brightening explanation [31].

Zhang et al. [36] simulated the effects of radiation pressure, Doppler shift, and solar Fraunhofer lines on Na I tails. Changes in fluorescence efficiency and acceleration can reproduce tail luminosity and morphology observed by LASCO and HI-1 in 2022 and 17 earlier observations, with asymmetry before and after perihelion similar to observations. According to Na I tail models, physical elongation of the tail after perihelion is expected due to the Greenstein effect. This effect manifests as when Phaethon's radial velocity ṙ < 0 near perihelion, surface Na atom velocities are similar, where radiation pressure acts as a decelerant, suppressing D-line emission when velocities approach ṙ ≈ 0, coinciding with solar spectrum Na I D absorption lines, thereby reducing fluorescence excitation rates and inhibiting Na atom acceleration and volatilization. At perihelion, Na atoms accelerate under radiation pressure, rapidly producing a bright tail accelerating anti-sunward [36]. At this time, tail brightening does not represent actual activity enhancement but reflects overall fluorescence efficiency improvement in Na I tails. Through MCMC processes establishing Na tail models, researchers derived Na I production of approximately (1.09 ± 0.15) × 10²⁹ atoms, finding temperature dependence consistent with thermal desorption mechanisms, which are therefore considered the primary cause of Na I production [36]. This finding suggests Phaethon is related to sun-grazing comets that produce Na emission observed by SOHO.

3.4 Rotational Instability

Asteroid rotation rates can accelerate through external torques, chance impacts, gas release, and electromagnetic radiation to the point where surface centrifugal acceleration reaches its upper limit, causing mass loss during rotation. Shorter rotation periods intensify thermal cycling, thereby enhancing thermal fracture and driving fragmental rock toward equatorial regions with lowest gravitational potential [69]. For ice-free asteroids, radiation torque is the primary mechanism bringing rotational stability to critical values [45].

The Yarkovsky effect refers to the net recoil force per second from thermal photon radiation on asteroid surfaces [45, 70]. Its difference from radiation pressure on the anti-sunward side changes the orbital semi-major axis, significantly affecting meter-to-kilometer-sized bodies, with Phaethon's semi-major axis continuously decreasing under this influence. If net force produces torque relative to the center of mass, this torque is called the YORP effect [45], which can change rotation magnitude and axis orientation, exciting precession or rapid rotation that alters morphology. The Geminid meteor shower is currently believed to have formed on thousand-year timescales [71], while YORP effect-driven evolution requires 1 Ma [72], suggesting YORP effect-induced rotational mass ejection may be insufficient to form the meteor stream.

If Phaethon's bulk density is 500–1500 kg·m⁻³ (typical for B-type asteroids), its 3.6 h spin state may be near or above its critical rotation period [73], and combined with other activity mechanisms could cause large-scale deformation and trigger mass ejection. Indeed, its top shape [3] and detected equatorial ridge morphology [10] are consistent with expectations of surface material reshaping toward the equator driven by rotation [74]. Rotational factors also enhance Phaethon's structural disruption, while cohesive forces composed of van der Waals forces and structural friction can resist centrifugal forces from rotation and maintain shape [45].

Nakano and Hirabayashi [73] established a semi-analytical model to study rotation's role in mass shedding mechanisms. They proposed that throughout its orbital lifetime (26 Ma), the YORP effect causes Phaethon to begin rotating randomly, with an initial spin period shorter than the current period and greater structural deformation amplitude, which combined with other activity factors may have caused large mass shedding events that produced the Geminid meteor stream [75], after which spin speed slowed, evolving into the current top shape [73]. More research is needed to address rotational reshaping processes and deformation mechanisms.

3.5 Analysis of Other Brightening Causes

Besides activity, Phaethon's brightening could have other causes such as thermal radiation, solar wind effects, and phase angle scattering. Considering thermal radiation, with perihelion distance of 0.14 AU, calculations of isothermal spherical blackbody in equilibrium with sunlight yield a lower temperature limit of approximately 743 K [19], with an upper limit of about 1050 K for a blackbody with its rotation axis pointing at the Sun. However, Phaethon's surface temperature would need to reach 1650 K to explain current anomalous brightening through thermal radiation mechanisms. Therefore, Phaethon's perihelion brightening is not caused by surface thermal radiation [38]. Considering solar wind effects, Phaethon's brightening occurs about 0.5 days after coronal brightness increases, not perfectly coincident in time. After subtracting coronal background from photometric profiles, light curves still show significant changes before and after Phaethon's brightening [26], and based on Phaethon's 2 mag brightness increase, coronal particle density was calculated as 5 × 10¹⁴ m⁻³, far exceeding the coronal density of 10¹⁰ m⁻³ at that location [76], which cannot explain the brightening intensity. Given Phaethon's large and variable phase angles, phase angle scattering effects were considered. Compared with the Moon's phase function brightness, Phaethon brightened by more than 1 mag near phase angles of 80°–100° [19], lasting for 2 days, contradicting theoretical gradual brightness decay, thus brightening is not considered to be caused by phase angle scattering. Additionally, specular reflection effects from rocky asteroid surfaces have also been excluded [19].

4.1 Spectral and Composition Analysis

Phaethon is a B-subtype of C-type asteroids [6], exhibiting an unusual "blue" spectrum, i.e., a negative spectral slope [15]. As shown in [FIGURE:5], based on observed spectral properties, Phaethon shows a negative slope in the visible band 0.37–0.7 μm without absorption features, with slope decreasing further (becoming bluer) near 0.7–0.75 μm; near-infrared data show minimal variation, with a noticeable negative slope at 1 μm and spectral depression after 1.2–1.3 μm, with negative slope gradually flattening with increasing wavelength, becoming flat at 1.8 μm. Overall, the spectral slope shows slight variation across the entire visible wavelength range, with larger variations over smaller wavelength ranges.

Figure 5: Phaethon's rotation-averaged visible-near-infrared data (0.4–2.55 μm)

Initially classified as an F-type body due to lack of absorption features in the ultraviolet (UV) band [77]. Today, F-type classification has been merged into B-type classification [78], showing featureless reflectance spectra in the 0.5–1 μm wavelength range, with flat or slightly "blue" shape (negative slope in this band), similar to B-type Pallas family asteroids [79]. Phaethon's special characteristic is its more prominent negative spectral slope, leading de León et al. [75] to suggest it could be a separate classification. Spectral classification and comparison will be discussed in Chapter 6 of this paper.

Phaethon's spectral absorption varies with location, such as slight differences in spectral slope at different rotational phases [21]. Hanuš et al. [3] suggested that Phaethon's north pole may have distinctly different UV spectral absorption compared to other regions; Borisov et al. [20] also inferred that the north pole may have different polarization characteristics. Further research is needed to determine the physical mechanisms in the polar region and whether such spectral characteristics exist in other bands. Since grain size also affects spectral slope, meteorite type comparison studies are subject to interference from surface particle size and other factors. Because asteroid surfaces may not be uniform, latitude variations in surface properties cause changes in observation angles and asteroid attitudes [80]. However, Lee et al. [81] found insufficient evidence for surface inhomogeneity through spectral studies.

Phaethon's reflectance spectrum shows its surface composition consists of severely thermally altered carbonaceous chondrite material [78, 79, 82–84], with albedo increasing by about 30% from 1.0 μm to 0.5 μm, containing about 36% Mg-rich olivine [52]. This may result from heating experienced during perihelion passage that altered surface composition, related to decomposition of phyllosilicates from extreme heating [52]. Carbonaceous chondrites contain water that reacts with olivine, pyroxene, serpentine, etc., to form hydrated silicate minerals, while B-type asteroids experience some degree of dehydration, with composition between hydrated and anhydrous [83]. The feature at 3 μm indicates no hydrated minerals on Phaethon's surface [64], a characteristic resulting from high-temperature processing [84].

Many chondrites are considered possible surface components of Phaethon. Licandro et al. [82], based on near-infrared observations (0.4–2.45 μm), suggested that highly thermally processed CI and CM meteorites are the best compositional matches. Madiedo et al. [85] used atmospheric scintillation spectra to infer that Geminid meteorite composition is consistent with CM chondrites, also finding Phaethon's near-infrared spectrum matches water-free CK chondrites well. Clark et al. [83] found the best spectral matches were thermally processed CK4 meteorites ALH85002 and EET92002. CK4 meteorites are highly oxidized, containing CAI, olivine, refractory metal sulfides, FeOx, and highly refractory graphite matrix components, resulting from extensive hot water circulation reprocessing of original Fe-Mg silicates, sulfides, and complex organics [64]. In related studies of B-type classification, asteroid (101955) Bennu's spectrum best matches samples of CI chondrite Ivuna heated above 1000 K [86]. Kareta et al. [87] demonstrated through experiments that Phaethon's surface spectrum from near-ultraviolet to at least 2.5 μm is consistent with heated CI chondrites. Recently, Maclennan and Granvik [52] calculated mid-infrared emissivity spectra through spectral mixture modeling, modeling the presence of various mineral species, concluding that Phaethon most closely resembles heated carbonaceous chondrite CY, with consistent olivine abundance; its phyllosilicates undergo sufficient dehydration and dehydroxylation at 300–500 K, transforming into olivine at 870–970 K. This process can simulate lithological changes in Phaethon from high-temperature exposure.

4.2 Origin of the Blue Surface

Phaethon's special characteristic is its relatively "blue" surface, which may result from large grain size in its regolith layer, surface roughness, thermal alteration, or any combination of these three factors [62]. Slope variations in near-infrared data can be interpreted as effects related to particle size [21], a factor relevant to polarization measurements and thermophysical modeling calculations. Spectra of meteorite and asteroid materials tend to have more negative slopes (bluer) with increasing effective particle size [88], such as carbonaceous chondrites becoming bluer as grain size increases [89]. Experiments show that carbonaceous chondrites undergo surface sintering metamorphism at high temperatures, leading to surface grain coarsening [88]. If Phaethon's surface consists of carbonaceous chondrites and similar materials, this phenomenon would also occur.

After long-term thermal effects, Fe and organic solids in Phaethon's surface layer near the solar direction, along with more easily decomposable rocks, would sublimate [3], leaving no Fe in the spectrum and causing rapid increase in negative slope [90]. Pallas, with similar spectra but lacking similar thermal environments, may have reduced bluing due to relatively increased nano-Fe [83]. Lisse and Steckloff [90] proposed that any asteroid body on Phaethon-like orbits with perihelion less than 0.15 AU may have a blue surface, testable through observations of numerous sun-grazing asteroids near perihelion to search for blue surfaces and Fe- and CHON-rich gas comets, where sublimation processes change surfaces faster than space weathering [91, 92]. Solar wind continuously exposes new surfaces on Phaethon during each perihelion passage, maintaining surface blueness through sublimation, with calculated Fe loss rates at perihelion of Qgas = 10²² mol·s⁻¹ [62].

In addition to the above components, Na in chondrites volatilizes at high temperatures and carries dust out [67]. After sublimation of components like Fe and Na, some rocks such as pyroxene decompose into SiO and O₂ vapor, leaving solid refractory olivine residues [90]. Such sublimation processes weaken the solid matrix of pyroxene surfaces, with remaining solids detaching as entrained dust in sublimating gas outflows. This theory also applies to similar-environment asteroids (e.g., 2005 UD), with further verification possible through DESTINY+ mission in-situ measurements of Phaethon's surface near perihelion [93].

4.3 Albedo

Many asteroids' geometric albedos are calculated by combining thermal infrared observations with visible photometry [88]. Additionally, since albedo and polarization follow the inverse Umow law, albedo can also be derived from polarization observations [79]. Phaethon's currently measured albedo range is primarily 0.08–0.13. Based on IRAS thermal infrared data, Tedesco and Desert [7] calculated Phaethon's albedo as 0.11, higher than typical values for F-type spectral asteroids (0.03–0.07). Hanuš et al. [3] used the Thermophysical Model (TPM) based on IRAS and Spitzer data to derive Phaethon's geometric albedo as 0.122 ± 0.008, higher than Tedesco and Desert's calculation. Masiero et al. [94] calculated Phaethon's geometric albedo as 0.16 ± 0.02, confirming high albedo.

Some researchers have obtained lower albedo results. Zheltobryukhov et al. [95] estimated Phaethon's geometric albedo in R-band as 0.075 ± 0.007, consistent with initial F-type classification [77]. Kareta et al. [21] observed a thermal tail at 2.0 μm that did not appear at greater heliocentric distances, fitted Phaethon's thermal radiation tail, and derived an albedo of 0.08 ± 0.01. Currently, radar measurements give Phaethon's diameter as 5.7 km [8], larger than previous results (5.1 km [3]), which could also cause albedo underestimation.

Phaethon's surface shows strong polarization, meaning geometric albedo calculated theoretically would be lower than estimates from radiation observations [20, 79, 88]. If its albedo is high, Phaethon deviates from Umow's law, which states an inverse relationship between polarization and geometric albedo for solar system small bodies. Regions with higher albedo have weaker polarization due to more multiple scattering of light on high-albedo surfaces [88]. Devogèle et al. [79] derived Phaethon's albedo as approximately 0.05 from polarization observations, significantly different from Phaethon's determined radiation albedo [3]. Combining Phaethon's strong polarization with larger radar-observed diameter supports the low albedo case found by Kareta et al. [21]. However, Geem et al. [96] derived an albedo of 0.11 from low-phase-angle polarization observations, consistent with high albedo. The albedo controversy may be caused by heterogeneous and non-uniform Phaethon surfaces, with grain size and other factors also affecting albedo.

4.4 Polarization Analysis

Polarization measurements of small bodies at different phase angles can better reveal surface material properties, help determine spectral classification, and infer whether C-type asteroids contain water through low-phase-angle polarization observations [97]. After obtaining Phaethon's high albedo [3], polarization observations are needed to determine whether corresponding polarization conditions meet expectations. Phaethon shows high polarization levels, with Devogèle et al. [79] obtaining multi-color phase polarization curves from 36°–116°, showing extremely strong linear polarization at high phase angles, predicting polarization would reach maximum at approximately 130° phase angle (Pmax ≈ 45%). According to Umow's law, with surface particle size d fixed, empirical formulas can relate Pmax to albedo A [98]:

d = 0.03 exp[2.9(lg A + 0.845 lg Pmax)]

Devogèle et al. [79] calculated using Umow's law, finding Phaethon's polarization albedo much lower than the radiation albedo of 0.122 determined by Hanuš et al. [3] through radiation observation modeling. Geem et al. [96] measured albedo as 0.11 through polarization, more consistent with high albedo. Geem et al. [96] measured Phaethon's polarization curve at lower phase angles (8.8°–32.4° range), as shown in [FIGURE:6]. Phaethon's minimum polarization Pmin = (–1.3 ± 0.1)%, with polarization inversion angle of 19.9° ± 0.3°, where minimum polarization values can prove Phaethon's surface is closer to water-free meteorite conditions [96].

Note: Solid and dashed lines in the figure represent curves fitted to data using trigonometric and linear exponential functions, respectively.

Maximum linear polarization Pmax and phase angle are related to surface particle size [88]. When the surface is dominated by large particles, fewer particles cover the surface, reducing multiple light scattering and resulting in stronger polarization. As mentioned above, Phaethon's high-temperature sintering process makes high polarization consistent with blue surface characteristics. With albedo held constant, larger surface particles produce relatively stronger polarization. Higher surface porosity also leads to stronger polarization [88]. Ito et al. [88] observed Phaethon polarization as high as (50.0 ± 1.1)% at higher phase angles (α = 106.5°), estimating surface coverage by 150 μm particles; Geem et al. [96] estimated surface particle diameter at approximately 300 μm, differing from millimeter-sized grains derived from thermal modeling [99], likely indicating surface heterogeneity [100]. Maclennan et al. [80] analyzed heterogeneous surface grain sizes and found Phaethon's polarization consistent with coarse-grained surfaces in the northern hemisphere.

5.1 Orbital Evolution Model

Taking a reasonable variation range for Phaethon's current orbital elements and performing the same orbital inversion for different orbital elements within this range can reveal multiple possibilities of Phaethon's past orbital configurations that have evolved to the present. For example, taking 1σ deviation relative to Phaethon's orbital elements, 100 different elements were generated for inversion [3]. If multiple stable evolutions share common features, they can indicate the general evolutionary path of the asteroid. [FIGURE:7] shows the results of Phaethon's dynamical inversion [3], selecting 50 clones with minor differences (2 × 10⁻⁹ in semi-major axis, 2 × 10⁻⁸ in eccentricity, 3 × 10⁻⁷ in inclination) for inversion. Semi-major axis evolution shows clear signs of planetary encounters, dominated by random walk effects from brief gravitational perturbations caused by planetary encounters, with clone semi-major axes beginning to diverge about 4 × 10³ years ago. Over approximately 10⁵ years shown in [FIGURE:7]a, orbital semi-major axis evolution is primarily affected by major planet gravitational perturbations, while eccentricity e and inclination i evolutions show no obvious disturbances but are affected by long-period frequency variations from accumulated semi-major axis perturbations. Overall, Phaethon's orbital eccentricity shows a uniform increasing trend over the past 3 × 10⁵ years.

Note: a) shows the entire integrated timespan of 1 Ma, b) shows only the first 5 × 10⁴ years. Nominal orbit (black line) and orbits of 50 clones (gray lines) are shown. Past real orbital evolution could be any of these. Red lines represent clone evolution where eccentricity steadily decreases to.

This study used the MERCURY6 integrator [101] to calculate Phaethon's orbital dynamical evolution. As shown in [FIGURE:8], approximately 2 × 10³ years ago, Phaethon's orbital eccentricity reached its maximum, causing its perihelion distance q to be about 0.126 AU, much smaller than the current value (approximately 0.14 AU). As seen in [FIGURE:8], eccentricity increased regularly and slowly before reaching maximum near 2 × 10³ years.

Note: a) shows perihelion distance evolution in orbital inversion, b) shows eccentricity e evolution on 7 × 10⁴ year timescale.

Phaethon's origin can also be investigated through orbital inversion. The Pallas family is one of the most discussed sources for Phaethon [62, 102]. In dynamical mechanism studies, most primitive asteroid families in the inner main belt (C-type and B-type) have nearby secular or mean motion resonances [62]. Among Pallas family asteroids, some can resonate with Jupiter at 8:3 MMR and 5:2 MMR positions. Calculations show the lower size limit for asteroids at these positions is 4.95 km, and eccentricity can be further excited at Jupiter resonances, enabling asteroids to cross to near Mars or Earth orbits, providing a possible evolutionary path for near-Earth asteroids like Phaethon [75, 103]. Therefore, de León et al. [75] attempted clone numerical integration at Jupiter's 8:3 MMR and 5:2 MMR positions, taking 1σ deviation corresponding to Phaethon's orbital element averages for 1 × 10⁵ year evolution. Results showed about 93% of particles would fall into the Sun, about 5% would encounter Jupiter and fall, and only 2% (21 clones) could evolve to Mars-Earth crossing regions; when asteroids enter near-Earth space, their eccentricity begins oscillating between 0–0.9, with resonances pumping the asteroid's eccentricity to higher values until crossing near-Earth space. Through inversion, this asteroid can achieve Phaethon-like orbits with semi-major axes of about 1.2 AU and high inclination, thus Pallas family fragments could indeed become near-Earth asteroids and evolve to orbits similar to Phaethon [75]. [FIGURE:9] shows the evolution process of an effective clone evolving into a Phaethon-like orbit.

Todorović [103] conducted more rigorous dynamical research, finding 43.6% and 46.9% of particles from Pallas family's Jupiter resonance portions at 5:2 MMR and 8:3 MMR positions, respectively, could satisfy Phaethon-like orbital evolution pathways [103], more effective than previous results. This indicates that near-Earth object populations may contain evolved Pallas family members with Phaethon-like orbits.

Note: Evolution of semi-major axis, eccentricity, and inclination for particles in Jupiter's 8:3 MMR resonance position.

5.2 Thermophysical Model

Thermal radiation measurement is an important method for studying asteroid physical properties. Using thermophysical models, we can calculate infrared radiation from asteroids at specific orbital positions and fit observations to obtain parameters including thermal inertia (Γ), size D, albedo A, and surface roughness fr [104]. Asteroid thermophysical models are generally divided into two categories. One category includes simple models assuming spherical asteroids, such as the Standard Thermal Model (STM), Near-Earth Asteroid Thermal Model (NEATM), and Fast Rotating Model (FRM), where surfaces instantly reach equilibrium under solar radiation. Using relevant empirical formulas to calculate different temperature distributions on asteroid surfaces, fitting with observational data yields asteroid albedo and size information [105]. The other category includes thermophysical models treating asteroids as polyhedrons composed of triangular facets, including the Classical Thermophysical Model (TPM), Advanced Thermophysical Model (ATPM) [106, 107], and Regolith Simulating Thermophysical Model (RSTPM) [108]. These models can approximate real asteroid shapes [109] and solve heat conduction equations on each facet, considering multiple physical processes including multiple scattering of solar radiation, inward heat conduction, and outward thermal radiation in boundary conditions [104].

This study used the Advanced Thermophysical Model (ATPM) [106] to investigate Phaethon's thermophysical parameters. Combined with Phaethon shape model data [11], the temperature of any surface facet is determined by:

ρc ∂T/∂t = κ ∂²T/∂z²

where ρ, c, and κ are density, specific heat capacity, and thermal conductivity, respectively. According to energy conservation, external boundary conditions can be determined, where the sum of received solar radiation and radiation from other facets equals inward heat conduction plus outward thermal radiation:

(1 - A_B)F_s cosθ + F_sc + (1 - A_th)F_r = εσT⁴ - κ(∂T/∂z)

where A_B is Bond albedo, F_s is solar constant, r_h is heliocentric distance, s is the visibility coefficient of the facet relative to the Sun (s = 1 when illuminated, s = 0 when shadowed), cosθ is the cosine of the angle between incident sunlight and facet normal, F_s = 1367.5 W·m⁻² is the solar constant, F_sc is scattering of solar radiation from other facets, and F_r is self-heating between facets. For simplified calculations, following the standard transformation proposed by Lagerros [110], temperature T, time t, and depth z are transformed as: x = z/l_s; τ = ωt; u = T/T_e, where l_s = √(2κP_rot/(ρc)) is skin depth, P_rot is asteroid rotation period, and T_e is effective temperature. The heat conduction equation then becomes:

∂u/∂τ = ∂²u/∂x²

Boundary conditions become:

u⁴ - u₀⁴ = s·cosθ + (1 - A_B)F_sc/(εσT_e⁴) + (1 - A_th)F_r/(εσT_e⁴)

where u₀ is the thermal parameter: u₀ = Γ/(εσT_e³), with Γ = √(ρcκ) being thermal inertia. By solving these equations, asteroid surface temperature distribution T can be obtained, and theoretical radiation flux F_λ can be calculated using Planck function B(λ, T_i). Letting f_i be the view factor for facet i, we obtain theoretical radiation flux:

F_λ = Σ f_i B(λ, T_i)

This study used Phaethon infrared data from WISE, Spitzer, IRAS, AKARI, and UKIRT for fitting (totaling 67 observation epochs, see [TABLE:1]). Among these, Spitzer telescope data are abundant [3], but its observation duration is only 10 minutes. To avoid excessive weighting of this data in fitting, it was binned by calculating averaged flux values at integer wavelengths (binned data), see [TABLE:2]. WISE full-band observation data from 2010 were used, considering only W3 and W4 band observations (12 μm and 22 μm), along with IRAS and UKIRT multi-band infrared observation data [3, 11, 111, 112].

Asteroid surface roughness significantly affects thermal radiation characteristics; for example, rough surfaces cause shading of solar radiation and local temperature increases while making thermal radiation directional. This study uses a hemispherical crater model to simulate asteroid rough surfaces, with fr representing roughness (proportion of crater area to total facet area). Effective Bond albedo then represents reflection of solar radiation from rough and smooth surfaces:

A_eff = fr·(2/3)A_B + (1 - fr)A_B

Additionally, Bond albedo relates to geometric albedo as A_eff = p_v q_ph, where q_ph = 0.29 + 0.684G represents phase integral, G is slope parameter, and geometric albedo relates to diameter through:

D_eff = 1329 × 10^(-H_v/5) / √p_v

Based on these assumptions, theoretical radiation flux contains three free parameters: thermal inertia, roughness, and geometric albedo, i.e., F_λ = F_λ(Γ, fr, p_v). During fitting, we search in (Γ, fr, p_v) parameter space, setting thermal inertia Γ range as 0–500 J·m⁻²·s⁻⁰·⁵·K⁻¹, roughness range as 0–1, using least squares fitting. Parameters corresponding to minimum χ² value are optimal:

χ² = Σ [F_λ_i(Γ, fr, p_v) - F_obs_i]² / σ_i²

As shown in [FIGURE:10], we calculated this asteroid's thermal inertia as 550⁺³²⁰₋₁₆₀ J·m⁻²·s⁻⁰·⁵·K⁻¹, corresponding diameter of 5.160⁺⁰·⁰⁴⁰₋₀·₀₀₆₉ km, albedo of 0.1253⁺⁰·⁰⁰³⁴₋₀·₀₀₂₀, with minimum χ² value of 3.590. Due to large amount of data, fitting degrees of freedom are high, but results are same order of magnitude as previous work and relatively low [3, 80, 94], consistent with existing thermophysical parameter fitting. This asteroid has strong activity near perihelion, and its small-grained regolith components may detach from the surface, leaving larger-grained rock structures, resulting in higher surface thermal inertia than other asteroids. We compared our results with Phaethon thermal inertia calculated in other literature, see [TABLE:3].

Figure 10: Least squares fitting results for Phaethon thermal inertia

Table 3: Phaethon thermal inertia

Γ (J·m⁻²·s⁻⁰·⁵·K⁻¹) p_v D_eff (km) min χ² Reference 550⁺³²⁰₋₁₆₀ 0.1253⁺⁰·⁰⁰³⁴₋₀·₀₀₂₀ 5.160⁺⁰·⁰⁴⁰₋₀·₀₀₆₉ 3.590 This study 600⁺²⁰⁰₋₂₀₀ 0.122⁺⁰·⁰⁰⁸₋₀·₀₀₈ 5.1⁺⁰·²₋₀·² - [3] 630⁺⁸⁰₋₇₀ 0.16⁺⁰·²²₋₀·₂₂ 5.4⁺⁰·¹₋₀·¹ - [94] 880⁺⁵⁸⁰₋₃₃₀ - 4.6⁺⁰·²₋₀·³ - [80]

Note: Γ represents thermal inertia, p_v represents albedo, D_eff represents effective diameter.

6.1 Discussion on Comet and Asteroid Classification

Comets and asteroids are two different types of small bodies, while active asteroids are strictly defined as a cross between asteroids and comets, most of which are often called "main-belt comets" [15]. The distinction between comets and asteroids has three classification aspects. From an observational perspective, small bodies with unbound atmospheres caused by volatile substances are called comets, while those without such atmospheres are asteroids [71]; compositionally, comets are ice-rich small bodies formed outside the protoplanetary disk snow line, while asteroids are ice-free small bodies formed inside the protoplanetary disk [45]; dynamically, researchers mainly distinguish comets from asteroids through dynamical parameters, most commonly the Tisserand parameter measured relative to Jupiter [113]:

T_J = a_J / a + 2√[(a/a_J)(1 - e²)] cos i

where a, e, and i are orbital semi-major axis, eccentricity, and inclination (relative to Jupiter's orbit), and a_J = 5.2 AU is Jupiter's orbital semi-major axis.

Jewitt et al. [45] provided the definition of active asteroids: (1) semi-major axis a < a_J; (2) T_J > 3.08; (3) small bodies showing mass loss signs, such as comet-like tail phenomena. As mentioned above, activity causes may include sublimation, impact, rotational disruption, thermal fracturing, etc. [FIGURE:11] shows statistical results of known active asteroid activity mechanisms. Although Phaethon's activity has been confirmed through multiple observations, further discussion is needed on whether it is more comet-like or asteroid-like. Orbital dynamical analysis shows Phaethon's Tisserand parameter relative to Jupiter is T_J = 4.509, far above the dividing line, thus orbital characteristics identify it as an asteroid. Dynamical simulations indicate Phaethon likely originated from the main belt like most near-Earth asteroids, while comets typically come from the Kuiper Belt or Oort Cloud, representing significant differences [75].

Compared with typical cometary nuclei spectra, Phaethon has higher albedo, bluer optical and near-infrared spectral features [111], and higher bulk density [11]. Analysis of Geminid meteor observations shows meteorite densities between 0.7–1.3 g·cm⁻³, lower than typical cometary meteorite densities, indicating Phaethon is indeed not a typical comet [114].

Note: Different symbols represent different activity mechanisms for active asteroids.

In spectral classification, asteroids initially classified as F-type show comet-like characteristics [79]. Although Phaethon was once considered F-type, more research suggests this classification is not entirely certain. Past F-type asteroids showed abnormally low polarization spectral inversion angles of about 17°, while currently observed Phaethon polarization inversion angle is 19.9° ± 0.3° [96], slightly deviating from F-type classification. Regarding albedo, Zheltobryukhov et al. [95] estimated Phaethon's geometric albedo in R-band as 0.075 ± 0.007, seemingly consistent with dark F-type asteroids; but Hanuš et al. [3, 94] derived high albedo of about 0.122 ± 0.008 using thermophysical models, higher than typical cometary geometric albedo and F-type spectral asteroid values (0.03–0.07), but consistent with B-type asteroids forming the Pallas collision family [115]. Therefore, whether albedo supports Phaethon's classification requires deeper research.

Devogèle et al. [79] fitted polarization spectra, finding Phaethon more correlated with B-type body Pallas. This study fitted phase polarization using Phaethon and other F-type asteroid spectra, with results deviating from Phaethon's polarization spectrum [79]; while fitting with modern B-type showed good agreement with Pallas polarization data, with significantly improved best-fit curve rms [79]. In summary, Phaethon's properties as an active asteroid are more asteroid-like, and more similar to Pallas asteroids among B-types, even though Phaethon's special negative spectral slope makes it bluer than other B-type asteroids [75].

6.2 Discussion on Pallas Family Evolution

Lemaitre and Morbidelli [102] first noted that the Pallas family could be Phaethon's source. Pallas is the third-largest body in the asteroid belt, with mean diameter of 550 km, semi-major axis of 2.77 AU, classified as B-type [62]. Identifying it as Phaethon's source requires satisfying dynamical mechanism and physical property requirements, meaning Pallas family members of Phaethon's size can form planet-crossing orbits to become highly inclined near-Earth asteroids, with physically similar compositions reflected in spectral similarity [62]. Additionally, Kareta et al. [21] proposed that Phaethon's origin question is fundamentally about its current and past activity nature.

In dynamical mechanism research, Pallas family asteroids exist at positions resonant with Jupiter at 8:3 MMR and 5:2 MMR, with lower size limits of 4.95 km, comparable to Phaethon's size [75, 103]. Asteroids here can be excited to higher eccentricities to cross to Mars or Earth orbits, representing a reasonable evolutionary path. As mentioned, de León et al. [75] clone numerical integration showed 2% of clones could evolve to Mars-Earth crossing orbits, eventually achieving Phaethon-like orbits; Todorović [103] also indicated the existence of evolved Pallas family members with Phaethon-like orbits in near-Earth object populations.

Nevertheless, Phaethon cannot be definitively identified as an evolved Pallas member. Maclennan et al. [62] argued that no convincing dynamical evidence shows Pallas is most likely Phaethon's parent; they suggested inner main-belt Svea family asteroids with ~16° inclination may match Phaethon better, approaching Phaethon's high-inclination characteristics during evolution. Using the SWIFT RMVS integrator [116] and OpenOrb software to calculate clone orbital elements, they found multiple close encounters with inner planets make inclination parameters random, thus rejecting inclination-based similarity as support for Pallas being Phaethon's parent [62]. Even if Phaethon's mean inclination 5 Ma ago approached Pallas's 33°, large variation ranges from inner planet influences make averages statistically insufficient to demonstrate correlation [62]. Svea itself is classified as C-type, with some B-type family members [117], near the 3:1 MMR Jupiter resonance position, also possessing evolutionary potential to near-Earth orbits. Notably, if Phaethon's orbit changed significantly during Geminid meteor stream formation, orbital inversion work would produce substantial changes [118].

In spectral comparison, as shown in [FIGURE:12], Phaethon and Pallas and its family members have some similarity in visible and near-infrared spectra [75]. Data near 3 μm show Pallas composition matches heated CM chondrites and is similar to CR chondrite composition [119], while Phaethon's near-infrared spectrum currently matches CK and CI meteorites better [3, 83]. Atmospheric scintillation spectra inferred Geminid meteorite composition consistent with CM chondrites [85], showing some compositional similarity between Geminid meteorites and Pallas. Differences within dashed boxes in [FIGURE:12]a stem from size and surface grain size differences [75]. As mentioned above, carbonaceous chondrite spectra become bluer as grain size increases [89], with larger bodies retaining finer surface grains while smaller bodies lose fine grains and become covered by coarser grains, making Phaethon's spectrum bluer than Pallas's. Additionally, Phaethon's sintering and removal of Fe and other components [83, 90] can further explain its bluer characteristics compared to Pallas.

Note: a) Comparison of visible and near-infrared reflectance spectra between Phaethon (blue line) and Pallas (red line), with similar behavior beyond 1 μm indicating similar composition, dashed boxes highlight differences (0.4–0.9 μm); b) Comparison of visible spectra between Pallas and Pallas family members; c) Comparison of average visible spectra between Pallas family members and Phaethon (blue line).

Although Phaethon's geometric albedo remains controversial, Hanuš et al. [3, 94] derived high albedo of about 0.122 ± 0.008, close to Pallas's high albedo of 0.145 [120]. Devogèle et al. [79] derived Phaethon's phase polarization curve similar to asteroid Pallas. Kareta et al. [21] argued that spectral differences cannot be ignored, thus opposing the Pallas family as Phaethon's source. In summary, Phaethon has orbital and surface property similarities with Pallas, but controversies remain; Phaethon may also originate from other main-belt asteroid families [21], such as the Svea family.

6.3 PGC Relationship Research

Asteroids (155140) 2005 UD and (225416) 1999 YC are considered possible split fragments or former components of a single body with Phaethon due to similar orbital elements and colors [12–15], especially the color similarity and size ratio between Phaethon and 2005 UD further support their connection. Together with the Geminid meteor stream they are collectively called the Phaethon-Geminid Complex (PGC) [13]. PGC formation may be ancient [13], while Geminid meteor stream dynamical lifetime does not exceed several thousand years [121]. Beech [122] analyzed properties of 3 Geminid fireballs, finding meteor stream ages between (1–4) × 10³ years. Phaethon had unique perihelion and eccentricity changes at its position 2000 years ago, with minimum perihelion distance [3, 62], possibly related to Geminid meteor shower origin. Jewitt et al. [5] found Geminid meteor showers may have resulted from catastrophic events at some point within thousands of years.

Geminid meteor streams are denser than typical cometary meteor streams [114], with asymmetric spatial distribution near peaks [118], composed of particles ranging from 10 μm to 4.5 cm [123]. Meteor stream structural characteristics include [71, 118]: dust particles ejected from points around the parent orbit rather than a single point; particles ejected from the sunward-facing hemisphere; ejection velocity related to particle size, with larger particles having lower ejection velocities. These properties give Geminid meteor showers unique double-peaked activity profiles in observations [71]. After particle ejection, reaction forces affect the parent asteroid's orbit and attitude. Ryabova [124] quantitatively characterized Geminid meteor stream radiation distribution using semi-analytical numerical models applicable to meteor shower observation simulations. Besides ground-based observations, PSP observed Geminid meteor stream dust trails, deriving dust masses in the 10¹⁰–10¹² kg range, likely representing partial accumulation of meteoroids near Phaethon [41].

Regarding mass loss at perihelion, the most discussed causes are thermal fracturing assisted by rotation and radiation pressure removal, or Na sublimation driving. Kasuga and Masiero [125] conducted space-based thermal infrared observations of PGC-related asteroids, calculating Phaethon's upper mass loss rate limit as 2 kg·s⁻¹, and 2005 UD and 1999 YC's upper mass loss rate limits as 0.1 kg·s⁻¹, suggesting mass loss differences difficult to maintain through steady state, supporting the theory that Geminid meteor streams formed through episodic events.

Cukier and Szalay [126] simulated expected meteor stream positions for Phaethon under sudden disruption events and cometary dust activity conditions, using 1 km·s⁻¹ velocity-separated dust to simulate sudden disruption events, and rate inversely proportional to distance from Phaethon and Sun to simulate comet-like activity, with zero-velocity dust as control. Results showed meteor stream positions best match disruption formation mechanisms rather than comet formation mechanisms, indicating Geminid meteor streams may have formed through catastrophic destruction near perihelion [126]. However, considering differences between laboratory and actual space environments for thermal fracturing and other mass loss mechanisms, quantitative studies of these processes should be conducted through more space experiments [12].

Besides the relationship between Geminid meteor streams and asteroids, whether Phaethon and the other two asteroids originate from the same parent body also warrants further study. 2005 UD has rotation period of about 5.2 h, perihelion distance of about 0.16 AU [99], with color indices similar to Phaethon [127]. 1999 YC has rotation period of about 4.5 h, perihelion distance of about 0.24 AU [128]. Spectrally, Phaethon and 2005 UD show greater similarity, both currently belonging to B-type bodies with extremely similar visible spectra and blue characteristics [99]; while 1999 YC's spectrum indicates C-type, with overall redder color [128]. Additionally, the period (3.6 h) and size ratio of the Phaethon-2005 UD asteroid pair match the asteroid spin-fission formation model proposed by Pravec et al. [129], suggesting 2005 UD may be a fragment split from Phaethon.

As shown in [FIGURE:13], Devogèle et al. [99] compared polarization phase curves and spectra between the two asteroids, showing Phaethon and 2005 UD polarization curves are almost identical but significantly different from Bennu. Through NEOWISE thermophysical modeling, they indicated similar surfaces, giving 2005 UD albedo p_v = 0.10 ± 0.02, size of 1.3 ± 0.2 km, and thermal inertia of 300⁺¹²⁰₋₁₁₀ J·m⁻²·s⁻¹/²·K⁻¹. Studies show Phaethon's surface grain size range is 3–30 mm, while 2005 UD's surface grain size range is 0.9–10 mm [99], possibly because Phaethon's thermal environment ejects finer grains or due to differences in surface component thermal inertia. Near-infrared reflectance spectra show differences, with 2005 UD showing more positive "red" slope compared to Phaethon, unlike Phaethon's blue negative slope [99]. Compositionally, Phaethon is more similar to CI chondrites, while 2005 UD basically doesn't match [87], and even if explained by different thermal environments, no meteorites have been found that can present spectra of both asteroids at different temperatures.

Note: a) Phase-polarization curve comparison of 2005 UD (red), Phaethon (blue), and B-type (101955) Bennu (black); b) Comparison of 2005 UD, Phaethon, and B-type spectra.

Studies show Phaethon's orbital evolution has strong correlation with 2005 UD's over thousands of years [130], with similar orbital element changes about 4600 years ago [12]; 1999 YC's orbital inversion results differ from the other two asteroids, but Ohtsuka et al. [131] argued that if differences can be explained by orbital perturbations, the three asteroids can still be considered related. Hanuš et al. [3] repeated 2005 UD evolution calculations for nominal orbit and 50 clone orbits, confirming orbital similarity. As shown in [FIGURE:14], Maclennan et al. [62] used the SWIFT integrator to perform clone orbital inversion integration for 1000 random orbits of Phaethon and 2005 UD, calculating best-fit orbital elements in the least-squares sense; all clone orbits underwent similar periodic eccentricity changes, providing possibility for fission theory. When entering near-Earth object regions after multiple encounters with inner planets, inclination undergoes significant changes, making current orbital similarity particularly special [12].

However, even if originating from the same body, these two bodies may have separated from a common parent about 10⁵ years ago [3], meaning their formation may be unrelated to meteor streams [12]. Additionally, Ryabova et al. [132] questioned this common origin claim dynamically, arguing that if the three were related, they would exist in each other's debris streams. The study found 2005 UD and Phaethon fragment intersection points far apart, while 1999 YC and Phaethon fragment separation intersection points are closer, but this is insufficient to demonstrate strict correlation. Subsequently, using the DSH criterion [133] that quantifies orbital similarity: Phaethon and 1999 YC have DSH ≈ 1.0, greater than the maximum value of 0.058 for determining correlation [134], thus denying their correlation.

Note: a) Comparison of semi-major axis time inversion; b) Comparison of eccentricity time inversion.

7 Prospective Missions and Outlook

Current research on Phaethon is relatively comprehensive among active asteroids, but many controversial aspects remain in thermophysical models, surface composition, and anomalous brightening mechanisms, awaiting further in-depth study. Therefore, targeted exploration missions can provide more precise data support. The Japan Aerospace Exploration Agency (JAXA) DESTINY+ mission has selected Phaethon as a探测目标, scheduled for launch in 2028 [25, 135], expected to fly by Phaethon and possibly extend to 2005 UD [136].

The DESTINY+ mission aims to achieve high-resolution imaging, close flyby, high-precision navigation, and wide-range observation of Phaethon. DESTINY+ has three scientific payloads: Tracking Camera (TCAP), VIS-NIR Multi-band Camera (MCAP), and Dust Analyzer (DDA), for in-situ detection of Phaethon's surface and composition near perihelion [135]. TCAP and MCAP will perform high-speed (36 km·s⁻¹) flyby imaging at closest approach of 500 ± 50 km [97, 137]; DDA, developed by a University of Stuttgart team based on Cassini's Cosmic Dust Analyzer (CDA), will directly measure dynamical parameters and elemental abundances of dust particles near Phaethon [138].

DESTINY+'s scientific objectives are to investigate cosmic dust properties and origins, measuring physical properties and chemical composition of interplanetary and interstellar dust particles near 1 AU during deep space cruise; and to conduct geological observations of Phaethon to understand dust ejection mechanisms and surface composition changes in active asteroids. It will be launched by Epsilon launch vehicle into an elliptical Earth orbit, then use electric propulsion to raise orbit to reach the Moon. Subsequently, through multiple lunar gravity assists to escape Earth's gravitational field, then use electric propulsion to travel to Phaethon, ultimately achieving flyby observation [93]. At flyby, its geocentric distance is 1.72 AU, heliocentric distance is 0.87 AU; DESTINY+ may subsequently travel to 2005 UD [93].

Before flyby missions, detailed understanding of Phaethon's size, shape, albedo, and rotation state is needed to assess dust and debris environments near it. According to Masiero et al. [94] estimates, dust analyzers may encounter up to 10³ micron-sized dust particles within about 500 km of Phaethon. However, due to sparse dust, direct dust impact probability is low for detectors like PSP and DESTINY+ [126]. In-situ observation data of its dust activity will become important indicators for studying active asteroid properties and may reveal activity signs of 2005 UD. DESTINY+'s direct observation of Phaethon's surface will obtain more precise shape models and establish thermophysical models for geometric albedo, helping to better understand Phaethon's thermal history and formation reasons for current physical properties.

In small body exploration missions, China's Chang'e-2 successfully achieved a flyby of near-Earth asteroid (4179) Toutatis on December 13, 2012, at a closest approach of 770 ± 120 m from the asteroid surface [139], revealing its physical properties, rotation characteristics, internal structure, and formation mechanism, achieving major international impact. Future active asteroid missions include China's Tianwen-2 probe scheduled for launch in 2025, with active asteroid 311P as one of its targets, planning to achieve sampling return from near-Earth asteroid 2016 HO3 in a single launch; then conduct rendezvous探测with 311P, with size of 320–580 m, scientific探测focusing on active asteroid formation and evolution, gas activity mechanisms, etc. [140]. The Tianwen-2 mission for small bodies is a major national planetary exploration project and a landmark mission in China's aerospace power construction journey, laying important foundations for subsequent deep space exploration missions such as Jupiter.

References

[1] Whipple F L. IAUC, 1983, 3881: 1
[2] Green S, Kowal C. IAUC, 1983, 3878: 1
[3] Hanuš J, Delbo' M, Vokrouhlický D, et al. A&A, 2016, 592: A34
[4] Jewitt D, Li J, Agarwal J. ApJL, 2013, 771: L36
[5] Jewitt D, Mutchler M, Agarwal J, et al. AJ, 2018, 156: 238
[6] Kartashova A, Husárik M, Ivanova O, et al. CoSka, 2019, 49: 367
[7] Tedesco E F, Desert F X. AJ, 2002, 123: 2070
[8] Taylor P A, Rivera-Valentín E G, Benner L A M, et al. P&SS, 2019, 167: 1
[9] Kim M J, Lee H J, Lee S M, et al. AAP, 2018, 619: A123
[10] Arai T, Yoshida F, Hayamizu T, et al. LPSC, 2022, 1: 2916
[11] Hanuš J, Vokrouhlický D, Delbo' M, et al. A&A, 2018, 620: L8
[12] Ohtsuka K, Sekiguchi T, Kinoshita D, et al. AAP, 2006, 450: L25
[13] Ohtsuka K, Nakato A, Nakamura T, et al. PASJ, 2009, 61: 1375
[14] Kasuga T. EM&P, 2009, 105: 321
[15] Jewitt D, Hsieh H. AJ, 2006, 132: 1624
[16] Belton M J S, A'Hearn M F. AdSpR, 1999, 24: 1175
[17] Eyles C J, Harrison R A, Davis C J, et al. SoPh, 2009, 254: 387
[18] Battams K, Watson A. IAUC, 2009, 9054: 3
[19] Li J, Jewitt D. AJ, 2013, 145: 154
[20] Borisov G, Devogèle M, Cellino A, et al. MNRAS, 2018, 480: L131
[21] Kareta T, Reddy V, Hergenrother C, et al. AJ, 2018, 156: 287
[22] Tabeshian M, Wiegert P, Ye Q, et al. AJ, 2019, 158: 30
[23] Ohtsuka K, Ito T, Kinoshita D, et al. P&SS, 2020, 191: 104940
[24] Lin Z Y, Yoshida F, Lin Y C, et al. P&SS, 2020, 194: 105114
[25] ISAS. Space Science and Exploration Mission Progress. Japan: CAO, 2024, 1009: 11
[26] Jewitt D, Li J. AJ, 2010, 140: 1519
[27] Howard R A, Moses J D, Vourlidas A, et al. Space Sci. Rev., 2008, 136: 67
[28] Hui M T, Li J. AJ, 2017, 153: 23
[29] Schleicher D G, Bair A N. AJ, 2011, 141: 177
[30] Marcus J N. ICQ, 2007, 29: 39
[31] Hui M T. AJ, 2023, 165: 94
[32] Bewsher D, Brown D S, Eyles C J, et al. SoPh, 2010, 264: 433
[33] Schmidt C A. Journal of Geophysical Research (Space Physics), 2013, 118: 4564
[34] Domingo V, Fleck B, Poland A I. SoPh, 1995, 162: 1
[35] Brueckner G E, Howard R A, Koomen M J, et al. SoPh, 1995, 162: 357
[36] Zhang Q, Battams K, Ye Q, et al. PSJ, 2023, 4: 70
[37] Ye Q, Wiegert P A, Hui M T. ApJ, 2018, 864: L9
[38] Jewitt D, Asmus D, Yang B, et al. AJ, 2019, 157: 193
[39] Ye Q, Knight M M, Kelley M S P, et al. PSJ, 2021, 2: 23
[40] Yoshida F, Hayamizu T, Miyashita K, et al. PASJ, 2023, 75: 153
[41] Battams K, Knight M M, Kelley M S P, et al. ApJS, 2020, 246: 64
[42] Battams K, Gutarra-Leon A J, Gallagher B M, et al. ApJ, 2022, 936: 81
[43] Bach Y P, Ishiguro M. AAP, 2021, 654: A113
[44] Kim Y, Jewitt D, Agarwal J, et al. ApJ, 2022, 933: L15
[45] Jewitt D, Hsieh H, Agarwal J. Asteroids IV, 2015: 221
[46] Bolin B, Denneau L, Veres P, et al. CBET, 2013, 3736: 1
[47] Szalay J R, Pokorný P, Horányi M, et al. P&SS, 2019, 165: 194
[48] Murdoch N, Sánchez P, Schwartz S R, et al. Asteroids IV, 2015: 767
[49] Rozitis B, Maclennan E, Emery J P. Natur, 2014, 512: 174
[50] Ryabova G O. European Planetary Science Congress 2012, 2012: EPSC2012–171
[51] Takir D, Reddy V, Hanus J, et al. LPSC, 2018, 1: 2624
[52] Maclennan E, Granvik M. NatAs, 2023, 1: 60
[53] Yu L L, Ip W H, Spohn T. MNRAS, 2019, 482: 4243
[54] Jewitt D. AJ, 2012, 143: 66
[55] Colwell J E, Batiste S, Horányi M, et al. RvGeo, 2007, 45: RG2006
[56] Molaro J L, Byrne S, Langer S A. JGRE, 2015, 120: 255
[57] Hazeli K, El Mir C, Papanikolaou S, et al. Icar, 2018, 304: 172
[58] Molaro J, Byrne S. JGRE, 2012, 117: E10011
[59] Delbo M, Libourel G, Wilkerson J, et al. Nature, 2014, 508: 233
[60] Basilevsky A T, Head J W, Horz F, et al. LPSC, 2015, 1: 1440
[61] Krugly Y N, Belskaya I N, Shevchenko V G, et al. Icar, 2002, 158: 294
[62] MacLennan E, Toliou A, Granvik M. Icar, 2021, 366: 114535
[63] Blaauw R C. P&SS, 2017, 143: 83
[64] Takir D, Kareta T, Emery J P, et al. NatCo, 2020, 11: 2050
[65] Abe S, Ogawa T, Maeda K, et al. P&SS, 2020, 194: 105040
[66] Kasuga T, Watanabe J I, Sato M. MNRAS, 2006, 373: 1107
[67] Masiero J R, Davidsson B J R, Liu Y, et al. PSJ, 2021, 2: 165
[68] Davidsson B J R. MNRAS, 2021, 505: 5654
[69] Walsh K J, Richardson D C, Michel P. AAS/DPS, 2008, 40: 55.03
[70] Vokrouhlický D, Bottke W F, Chesley S R, et al. Asteroids IV, 2015: 509
[71] Ryabova G O. MNRAS, 2007, 375: 1371
[72] Bottke J, William F, Vokrouhlický D, Rubincam D P, et al. AREPS, 2006, 34: 157
[73] Nakano R, Hirabayashi M. ApJ, 2020, 892: L22
[74] Walsh K J, Richardson D, Michel P. AGU Fall Meeting Abstracts, 2012: P34A
[75] de Leon J, Campins H, Tsiganis K, et al. AAS/DPS, 2010, 42: 13.27
[76] Li J, Raymond J C, Acton L W, et al. ApJ, 1998, 506: 431
[77] Tholen D J. Asteroid Taxonomy from Cluster Analysis of Photometry. Tuscon: The University of Arizona, 1984: 1
[78] DeMeo F E, Binzel R P, Slivan S M, et al. Icar, 2009, 202: 160
[79] Devogèle M, Cellino A, Borisov G, et al. MNRAS, 2018, 479: 3498
[80] MacLennan E, Marshall S, Granvik M. Icar, 2022, 388: 115226
[81] Lee H J, Kim M J, Kim D H, et al. P&SS, 2019, 165: 296
[82] Licandro J, Campins H, Mothé-Diniz T, et al. A&A, 2007, 461: 751
[83] Clark B E, Ziffer J, Nesvorny D, et al. JGRE, 2010, 115: E06005
[84] de León J, Pinilla-Alonso N, Campins H, et al. Icar, 2012, 218: 196
[85] Madiedo J M, Trigo-Rodríguez J M, Castro-Tirado A J, et al. MNRAS, 2013, 436: 2818
[86] Clark B E, Binzel R P, Howell E S, et al. Icar, 2011, 216: 462
[87] Kareta T, Reddy V, Pearson N, et al. PSJ, 2021, 2: 190
[88] Ito T, Ishiguro M, Arai T, et al. NatCo, 2018, 9: 2486
[89] Johnson T V, Fanale F P. J. Geophys. Res., 1973, 78: 8507
[90] Lisse C M, Steckloff J K. Icar, 2022, 381: 114995
[91] Nesvorný D, Jedicke R, Whiteley R J, et al. Icar, 2005, 173: 132
[92] Lazzarin M, Marchi S, Moroz L V, et al. ApJL, 2006, 647: L179
[93] Arai T. 42nd COSPAR Scientific Assembly, 2018, 42: B1.1
[94] Masiero J R, Wright E L, Mainzer A K. AJ, 2019, 158: 97
[95] Zheltobryukhov M, Chornaya E, Kochergin A, et al. AAP, 2018, 620: A179
[96] Geem J, Ishiguro M, Takahashi J, et al. MNRAS, 2022, 516: L53
[97] Ishibashi K, Hong P, Okamoto T, et al. LPSC, 2022, 2678: 1729
[98] Shkuratov I G, Opanasenko N V. Icar, 1992, 99: 468
[99] Devogèle M, MacLennan E, Gustafsson A, et al. PSJ, 2020, 1: 15
[100] Okazaki R, Sekiguchi T, Ishiguro M, et al. P&SS, 2020, 180: 104774
[101] Chambers J E. MNRAS, 1999, 304: 793
[102] Lemaitre A, Morbidelli A. CeMDA, 1994, 60: 29
[103] Todorović N. MNRAS, 2018, 475: 601
[104] Delbo M, Mueller M, Emery J P, et al. Asteroids IV, 2015: 107
[105] Harris A W, Young J W, Bowell E, et al. Icar, 1989, 77: 171
[106] Rozitis B, Green S F. MNRAS, 2011, 415: 2042
[107] Davidsson B J R, Rickman H. Icar, 2014, 243: 58
[108] Yu L L, Ip W H. ApJ, 2021, 913: 96
[109] Durech J, Sidorin V, Kaasalainen M. A&A, 2010, 513: A46
[110] Lagerros J S V. AAP, 1996, 310: 1011
[111] Green S F, Meadows A J, Davies J K. MNRAS, 1985, 214: 29P
[112] Cutri R M, Wright E L, Conrow T, et al. https://ui.adsabs.harvard.edu/abs/2014yCat.2328....0C, 2025
[113] Bonsor A, Wyatt M C. MNRAS, 2012, 420: 2990
[114] Halliday I. Icar, 1988, 76: 279
[115] AlíLagoa V, Delbo' M. A&A, 2016, 603: A55
[116] Granvik M, Morbidelli A, Jedicke R, et al. Nature, 2016, 530: 303
[117] Morate D, de León J, De Prá M, et al. AAP, 2019, 630: A141
[118] Ryabova G O. MNRAS, 2016, 456: 78
[119] Sato J. NuPhS, 1997, 59: 262
[120] Alí-Lagoa V, de León J, Licandro J, et al. A&A, 2013, 554: A71
[121] Ryabova G O. SoSyR, 1999, 33: 224
[122] Beech M. MNRAS, 2002, 336: 559
[123] Arendt R G. AJ, 2014, 148: 135
[124] Ryabova G O. MNRAS, 2021, 507: 4481
[125] Kasuga T, Masiero J R. AJ, 2022, 164: 193
[126] Cukier W Z, Szalay J R. PSJ, 2023, 4: 109
[127] Kinoshita D, Ohtsuka K, Sekiguchi T, et al. A&A, 2007, 466: 1153
[128] Kasuga T, Jewitt D. AJ, 2008, 136: 881
[129] Pravec P, Vokrouhlický D, Polishook D, et al. Natural, 2010, 466: 1085
[130] Schunová E, Jedicke R, Walsh K J, et al. Icar, 2014, 238: 156
[131] Ohtsuka K, Arakida H, Ito T, et al. M&PSA, 2008, 43: 5055
[132] Ryabova G O, Avdyushev V A, Williams I P. MNRAS, 2019, 485: 3378
[133] Southworth R B, Hawkins G S. SCoA, 1963, 7: 261
[134] Schunová E, Granvik M, Jedicke R, et al. Icar, 2012, 220: 1050
[135] Arai T, Destiny+Science Team. LPSC, 2023, 2806: 3017
[136] Ozaki N, Yamamoto T, Gonzalez-Franquesa F, et al. AcAau, 2022, 196: 42
[137] Hong P K, Lee S R, Kim S S, et al. LPSC, 2022, 53: 1720
[138] Kobayashi M K, Kawai K K, Sakuma H S, et al. LPSC, 2018, 1: 1223
[139] Huang J, Ji J, Ye P, et al. Scientific Reports, 2013, 3: 3411
[140] Jewitt D, Weaver H, Mutchler M, et al. AJ, 2018, 155: 231

Active Asteroid (3200) Phaethon Physical Properties

ZHANG Xinyi¹;², JI Jianghui¹;², JIANG Haoxuan¹;³
(1. CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China;
2. University of Science and Technology of China, Hefei 230026, China;
3. Chuzhou College, Chuzhou 23909, China)

Abstract: This paper provides a comprehensive review of perihelion observations and significant observational events of the active asteroid (3200) Phaethon, analyzing mechanisms of activity near perihelion, particularly thermal fracturing and water ice and Na sublimation driving mechanisms. Based on spectral data, albedo, and polarization studies of Phaethon, we summarize research findings on its surface physical properties and composition, providing substantial evidence for comprehensive understanding of this celestial body. Phaethon-Geminid Complex (PGC) relationships, active asteroid classifications, and Phaethon traceability are thoroughly discussed. In studies of asteroid orbital evolution and thermophysical models, the MERCURY6 integrator was employed for millennial-scale inversion of Phaethon's orbital elements, preliminarily obtaining its motion patterns including perihelion distance. Additionally, based on the Advanced Thermophysical Model (ATPM), we performed integrated fitting of infrared multi-band observational data to derive Phaethon's thermal inertia, albedo, and diameter. Finally, we discuss JAXA's DESTINY+ mission and China's Tianwen-2 mission.

Key words: asteroid; activity mechanism; near-Earth asteroid