DESTINY+ is an upcoming JAXA Epsilon medium-class mission to fly by the Geminids meteor shower parent body (3200) Phaethon. It will be the world's first spacecraft to escape from a near-geostationary transfer orbit into deep space using a low-thrust propulsion system. In doing so, DESTINY+ will demonstrate a number of technologies that include a highly efficient ion engine system, lightweight solar array panels, and advanced asteroid flyby observation instruments. These demonstrations will pave the way for JAXA's envisioned low-cost, high-frequency space exploration plans. Following the Phaethon flyby observation, DESTINY+ will visit additional asteroids as its extended mission. The mission design is divided into three phases: a spiral-shaped apogee-raising phase, a multi-lunar-flyby phase to escape Earth, and an interplanetary and asteroids flyby phase. The main challenges include the optimization of the many-revolution low-thrust spiral phase under operational constraints; the design of a multi-lunar-flyby sequence in a multi-body environment; and the design of multiple asteroid flybys connected via Earth gravity assists. This paper shows a novel, practical approach to tackle these complex problems, and presents feasible solutions found within the mass budget and mission constraints. Among them, the baseline solution is shown and discussed in depth; DESTINY+ will spend two years raising its apogee with ion engines, followed by four lunar gravity assists, and a flyby of asteroids (3200) Phaethon and (155140) 2005 UD. Finally, the flight operations plan for the spiral phase and the asteroid flyby phase are presented in detail.
The JAXA/ISAS spacecraft DESTINY$^+$ will be launched to the active asteroid (3200) Phaethon in 2022. Among the proposed core payload is the DESTINY+ Dust Analyzer (DDA) which is an upgrade of the Cosmic Dust Analyzer flown on the Cassini spacecraft to Saturn (Srama et al. 2011). We use two up-to-date computer models, the ESA Interplanetary Meteoroid Engineering Model (IMEM, Dikarev et al. 2005), and the interstellar dust module of the Interplanetary Meteoroid environment for EXploration model (IMEX; Sterken2013 et al., Strub et al. 2019) to study the detection conditions and fluences of inter…
Thermophysical models allow for improved constraints on the physical and thermal surface properties of asteroids beyond what can be inferred from more simple thermal modeling, provided a sufficient number of observations is available. We present thermophysical modeling results of observations from the NEOWISE mission for two near-Earth asteroids which are the targets of the DESTINY+ flyby mission: (3200) Phaethon and (155140) 2005 UD. Our model assumes a rotating, cratered, spherical surface, and employs a Monte Carlo Markov Chain to explore the multi-dimensional parameter space of the fit. We…
Accurate estimation of brightness of (3200) Phaethon up to lower phase angles are essential for planning of the on-board camera of the DESTINY$^{+}$ mission. We have carried out intensive observations of Phaethon in the optical wavelength ($g$, $r$, and $i$) with the TriCCS camera on the Seimei 3.8 m telescope in October and November, 2021. We derived the absolute magnitude $H_\mathrm{V}$ and the slope parameter $G$ of Phaethon as $H_\mathrm{V}=14.23\pm0.02$ and $G=0.040\pm0.008$ from multiple photometric observations including lower phase angles down to $\sim$9$^{\circ}$ with the $H$-$G$ mode…
We report on the first polarimetric study of (3200) Phaethon, the target of JAXA's DESTINY$^+$ mission, in the negative branch to ensure its anhydrous nature and to derive an accurate geometric albedo. We conducted observations at low phase angles (Sun-target-observer angle, alpha = 8.8-32.4 deg) from 2021 October to 2022 January and found that Phaethon has a minimum polarization degree $P_{min}$ = -1.3 +- 0.1 %, a polarimetric slope h = 0.22 +- 0.02 % deg$^{-1}$, and an inversion angle alpha$_0$ = 19.9 +- 0.3 deg. The derived geometric albedo is $p_V$ = 0.11 (in the range of 0.08-0.13). These…
The polarimetric observations of asteroid 3200 Phaethon, the target of international observation campaign, did not cover a proper phase angle interval to provide estimating all the attributes of the asteroid polarization curve. Based on present discrete observation data for Phaethon, its full polarimetric curves in BVRI bandpasses were reproduced. The polarimetric properties of the asteroid correspond to a notion on surface structure as thermally altered regolith particles mixed with lager rock fragments like a coarse pebble.
The main aim of photometrical observations of the asteroid 3200 Phaethon was searching for its low-level cometary activity (possible coma and/or dust tail) in the pre-perihelion passage. We performed observational runs with telescopes ranging from 0.61-m to 2-m and BVR color imaging. Three longer photometric series were used for modeling of the 3D shape of Phaethon. The color indices and size of the asteroid were estimated.
Asteroid (3200) Phaethon is an active near-Earth asteroid and the parent body of the Geminid Meteor Shower. Because of its small perihelion distance, Phaethon's surface reaches temperatures sufficient to destabilize hydrated materials. We conducted rotationally resolved spectroscopic observations of this asteroid, mostly covering the northern hemisphere and the equatorial region, beyond 2.5-micron to search for evidence of hydration on its surface. Here we show that the observed part of Phaethon does not exhibit the 3-micron hydrated mineral absorption (within 2-sigma). These observations sugg…
Thermal infrared emission and thermophysical modeling techniques are powerful tools in deciphering the surface properties of asteroids. The near-Earth asteroid (3200) Phaethon is an active asteroid with a very small perihelion distance and is likely the source of the Geminid meteor shower. We estimate and interpret the thermal inertia of this extraordinary asteroid using observations that span ten distinct sightings. The variation in thermal inertia over these sightings is inconsistent with the expected temperature-dependent thermal inertia theorized from radiative heat transfer within the reg…
We report the discovery of a comet-like tail on asteroid (3200) Phaethon when imaged at optical wavelengths near perihelion. In both 2009 and 2012, the tail appears >=350" (2.5x10^8 m) in length and extends approximately in the projected anti-solar direction. We interpret the tail as being caused by dust particles accelerated by solar radiation pressure. The sudden appearance and the morphology of the tail indicate that the dust particles are small, with an effective radius ~1 micrometer and a combined mass ~3x10^5 kg. These particles are likely products of thermal fracture and/or desiccation …
(3200) Phaethon exhibits both comet- and asteroid-like properties, suggesting it could be a rare transitional object such as a dormant comet or previously volatile-rich asteroid. This justifies detailed study of (3200) Phaethon's physical properties, as a better understanding of asteroid-comet transition objects can provide insight into minor body evolution. We therefore acquired time-series photometry of (3200) Phaethon over 15 nights from 1994 to 2013, primarily using the Tektronix 2048x2048 pixel CCD on the University of Hawaii 2.2-m telescope. We utilized light curve inversion to: (1) refi…
We report on a multi-observatory campaign to examine asteroid 3200 Phaethon during its December 2017 close approach to Earth, in order to improve our measurements of its fundamental parameters, and to search for surface variations, cometary activity and fragmentation. The mean colors of Phaethon are B-V = 0.702 +/- 0.004, V-R = 0.309 +/- 0.003, R-I = 0.266 +/- 0.004, neutral to slightly blue, consistent with previous classifications of Phaethon as a F-type or B-type asteroid. Variations in Phaethon's B-V colors (but not V-R or R-I) with observer sub-latitude are seen and may be associated with…
We present results of polarimetric, photometric, and spectral observations of the near-Earth asteroid (3200) Phaethon carried out at the 6-m BTA telescope of the Special Astrophysical Observatory and the 2.6-m and 1.25-m telescopes of the Crimean Astrophysical Observatory over a wide range of phase angles during its close approach to the Earth at the end of 2017 (19-135 deg) and in 2020 at α= 52.2 deg. Using our and other available in literature data, we found that the maximum degree of linear polarization of Phaethon in the V band is 45% at the phase angle 124 deg. Using the dependence (polar…
Asteroid (3200) Phaethon, a B-type asteroid, has been active during its perihelion passages. This asteroid is considered to be a source of the Geminid meteor stream. It is reported that this asteroid is spinning at a rotation period of $3.60 \ hr$ and has a top shape (an oblate body with an equatorial ridge) with a mean equatorial diameter of $6.25 \ km$. Here, we report that Phaethon's rotation state may be close to or above its critical rotation period when the bulk density is $0.5 \ - \ 1.5 \ {g/cm^3}$ (a typical bulk density of a B-type asteroid). We found that in this condition, the struc…
A multi-colour phase-polarization curve of asteroid (3200)~Phaethon has been obtained during the December 2017 apparition by merging measurements taken at the observing station of Calern (France) and at the Rhozen observatory (Bulgaria). All the observations were obtained in the positive polarization branch, the phase angle ranging from 36$^\circ$ to 116$^\circ$. The measured values of linear polarization are among the highest ever observed for a Solar system body. The covered interval of phase angle was not sufficiently extended to derive a firm determination of the $P_{\rm max}$ parameter, b…
The asteroid (3200) Phaethon is known to be the parent body of the Geminids, although meteor showers are commonly associated with the activity of periodic comets. What is most peculiar to the asteroid is its comet-like activity in the ejection of micrometer-sized dust particles at every perihelion passage, while the activity of the asteroid has never been identified outside the near-perihelion zone at $0.14~\mathrm{au}$ from the Sun. From the theoretical point of view, we argue that the activity of the asteroid is well explained by the electrostatic lofting of micrometer-sized dust particles w…
We report Arecibo S-band (2380 MHz; 12.6 cm) radar observations of near-Earth asteroid (3200) Phaethon during the December 2017 apparition when Phaethon passed within 0.07 au of Earth. Radar images with a resolution of 75 m per pixel reveal a roughly spheroidal shape more than 6 km in diameter at the equator with several discernible surface features hundreds of meters in extent. These include a possible crater more than 1 km across located below 30$^{\circ}$ latitude and a roughly 600-m radar-dark region near one of the poles. Overall, the radar images of Phaethon are reminiscent of those of (…
Asteroid (3200) Phaethon is a Near-Earth Apollo asteroid with an unusual orbit that brings it closer to the Sun than any other known asteroid. Its last close approach to the Earth was in mid-December 2017 and the next one will be on October 2026. Previous rotationally time-resolved spectroscopy of Phaethon showed that its spectral slope is slightly bluish, in agreement with its B/F taxonomic classification, but at some rotational phases, it changes to slightly reddish. Motivated by this result we performed time-resolved imaging polarimetry of Phaethon during its recent close approach to the Ea…
We present space-based thermal infrared observations of the presumably Geminid-associated asteroids: (3200)Phaethon, 2005 UD and 1999 YC using WISE/NEOWISE. The images were taken at the four wavelength bands 3.4$μ$m(W1),4.6$μ$m(W2),12$μ$m(W3),and 22$μ$m(W4). We find no evidence of lasting mass-loss in the asteroids over the decadal multi-epoch datasets. We set an upper limit to the mass-loss rate in dust of Q<2kg s$^{-1}$ for Phaethon and <0.1kg s$^{-1}$ for both 2005 UD and 1999 YC, respectively, with little dependency over the observed heliocentric distances of R=1.0$-$2.3au. For Phaethon, e…
The near-Earth objects (NEOs) (3200) Phaethon and (155140) 2005 UD are thought to share a common origin, with the former exhibiting dust activity at perihelion that is thought to directly supply the Geminid meteor stream. Both of these objects currently have very small perihelion distances, which results in them having perihelion temperatures of or exceeding 1000 K. The current activity from Phaethon is relevant to the destruction of NEOs close to the Sun, which most likely has produced meteor streams linked to asteroids in the past. We model the past thermal characteristics of Phaethon and 20…
The linear polarization degree (referred to the scattering plane, P_r) as a function of the solar phase angle, α, of solar system objects is a good diagnostic to understand the scattering properties of their surface materials. We report Pr of Phaethon over a wide range of α from 19.1 deg to 114.3 deg in order to better understanding properties of its surface materials. The derived phase-polarization curve shows that the maximum of P_r, P_max, is >42.4% at α >114.3 deg, a value significantly larger than those of the moderate albedo asteroids (P_max ~9%). The phase-polarization curve classifies …
Time-series multi-band photometry and spectrometry were performed in Nov.-Dec. 2017 to investigate the homogeneity of the surface of asteroid (3200) Phaethon. We found that Phaethon is a B-type asteroid, in agreement with previous studies, and that it shows no evidence for rotational color variation. The sub-solar latitude during our observation period was approximately 55 degree S, which corresponded to the southern hemisphere of Phaethon. Thus, we found that the southern hemisphere of Phaethon has a homogeneous surface. We compared our spectra with existing spectral data to examine the latit…
The near-Earth asteroid (3200) Phaethon is an intriguing object: its perihelion is at only 0.14 au and is associated with the Geminid meteor stream. We aim to use all available disk-integrated optical data to derive a reliable convex shape model of Phaethon. By interpreting the available space- and ground-based thermal infrared data and Spitzer spectra using a thermophysical model, we also aim to further constrain its size, thermal inertia, and visible geometric albedo. We applied the convex inversion method to the new optical data obtained by six instruments and to previous observations. The …
In 2010 Jewitt and Li published a paper examining the behavior of comet-asteroid transition object 3200 Phaethon, arguing it was asteroid-like in its behavior throughout most of its orbit, but that near its perihelion, at a distance of only 0.165 AU from the sun, its dayside temperatures would be hot enough to vaporize rock (>1000 K, Hanus et al. 2016). Thus it would act like a "rock comet" as gases produced from evaporating rock were released from the body, in a manner similar to the more familiar sublimation of water ice into vacuum seen for comets coming within ~3 AU of the Sun. In this Not…
The recent close approach of the NEA (3200) Phaethon offered a rare opportunity to obtain high-quality observational data. We used the newly obtained optical light curves to improve the spin and shape model of Phaethon and to determine its surface physical properties derived by thermophysical modeling. We also used the available astrometric observations of Phaethon, including those obtained by the Arecibo radar and the Gaia spacecraft, to constrain the secular drift of the orbital semimajor axis. This constraint allowed us to estimate the bulk density by assuming that the drift is dominated by…
We report on optical polarimetric observations of two Apollo type near-Earth asteroids, (422699) 2000 PD3 and (3200) Phaethon, and BVRI photometric observations of 2000 PD3 using the 1.6m Pirka telescope in 2017. We derived the geometric albedo of pv = 0.22 +- 0.06 and the color indices (B-V = 0.282 +- 0.072, V-R = 0.198 +- 0.035 and V-I = 0.203 +- 0.022) for 2000 PD3 which are consistent with those of S-type asteroids (including Q-types). The effective diameter of 2000 PD3 was derived as 0.69 +- 0.15 km using our derived geometric albedo. We found that our polarimetric data of Phaethon in 201…
Solar system bodies with surface and sub-surface volatiles will show observational evidence of activity when they reach a temperature where those volatiles change from solid to gas and are released. This is most frequently seen in comets, where activity is driven by the sublimation of water, carbon dioxide, or carbon monoxide ices. However, some bodies (notably the asteroid (3200) Phaethon) show initiation of activity at very small heliocentric distances, long after they have reached the sublimation temperatures of these ices. We investigate whether the sodium present in the mineral matrix cou…
The recently discovered near-Earth asteroid 2021 PH27 has the shortest orbital period of all known asteroids. It cannot be excluded that 2021 PH27 is also an active asteroid, as (3200) Phaethon. We intend to estimate the consequences of this hypothesis, although testing is difficult with ground-based observations during perihelion passages, due to low solar elongation. Assuming a surface activity similar to that of Phaethon, an increase in brightness of about 1.4 mag can be estimated. Since it is an asteroid with a MOID of $0.014660 \pm 0.000034$ AU with Venus, 2021 PH27 could be the equivalen…
We present an analysis of asteroid (3200) Phaethon using coronagraphic observations from 2008 to 2022 by the COR2 cameras onboard the twin Solar TErrestrial RElations Observatory (STEREO) spacecraft. Although undetected in individual images, Phaethon was visible in stacks combined from the same perihelion observations, yet only at small ($\lesssim$30°) but not large ($\gtrsim$150°) phase angles. The observations are in line with the contribution from a bare nucleus, thereby seriously contradicting the interpretation based on HI-1 observations that attributes the perihelion activity to the ejec…
Context. Recent observations of dust ejections from active asteroids, including (3200) Phaethon, have drawn considerable interest from planetary astronomers studying the generation and removal of small dust particles on asteroids. Aims. In this work, we aim to investigate the importance of thermal radiation pressure from asteroid regolith (AR) acting on small dust particles over the surface of the AR. In particular, we aim to understand the role of thermal radiation in the near-Sun environment. Methods. We describe the acceleration of particles over the AR within the radiation fields (direct s…
The investigation of asteroids near the Sun is important for understanding the final evolutionary stage of primitive solar system objects. A near-Sun asteroid, (155140) 2005 UD, has orbital elements similar to those of (3200) Phaethon (the target asteroid for the JAXA's $DESTINY^+$ mission). We conducted photometric and polarimetric observations of 2005 UD and found that this asteroid exhibits a polarization phase curve similar to that of Phaethon over a wide range of observed solar phase angles ($ α= 20 - 105^\circ $) but different from those of (101955) Bennu and (162173) Ryugu (asteroids co…
We present an update to the first white-light detections of a dust trail observed closely following the orbit of asteroid (3200) Phaethon, as seen by the Wide-field Imager for Parker Solar Probe (WISPR) instrument on the NASA Parker Solar Probe (PSP) mission. Here we provide a summary and analysis of observations of the dust trail over nine separate mission encounters between October 2018 and August 2021 that saw the spacecraft approach to within 0.0277 au of the orbit of Phaethon. We find the photometric and estimated dust mass properties to be inline with those in the initial publication, wi…
Comets can exhibit non-gravitational accelerations caused by recoil forces due to anisotropic mass loss. So might active asteroids. We present an astrometric investigation of 18 active asteroids in search of non-gravitational acceleration. Statistically significant (signal-to-noise ratio (SNR) $> 3$) detections are obtained in three objects: 313P/Gibbs, 324P/La Sagra and (3200) Phaethon. The strongest and most convincing detection ($>$7$σ$ in each of three orthogonal components of the acceleration), is for the $\sim$1 km diameter nucleus of 324P/La Sagra. A 4.5$σ$ detection of the transverse c…
Asteroid exploration has been attracting more attention in recent years. Nevertheless, we have just visited tens of asteroids while we have discovered more than one million bodies. As our current observation and knowledge should be biased, it is essential to explore multiple asteroids directly to better understand the remains of planetary building materials. One of the mission design solutions is utilizing asteroid flyby cycler trajectories with multiple Earth gravity assists. An asteroid flyby cycler trajectory design problem is a subclass of global trajectory optimization problems with multi…
In May 2012 two asteroids made near-miss "grazing" passes at distances of a few Earth-radii: 2012 KP24 passed at nine Earth-radii and 2012 KT42 at only three Earth-radii. The latter passed inside the orbital distance of geosynchronous satellites. From spectral and imaging measurements using NASA's 3-m Infrared Telescope Facility (IRTF), we deduce taxonomic, rotational, and physical properties. Their spectral characteristics are somewhat atypical among near-Earth asteroids: C-complex for 2012 KP24 and B-type for 2012 KT42, from which we interpret the albedos of both asteroids to be between 0.10…
We have obtained polarimetric measurements of asteroid (101955) Bennu, a presumably primitive near-Earth object (NEO) that is the target of NASA's sample return mission OSIRIS-REx. During our observing campaign, Bennu was visible from Earth under a wide range of illumination conditions, with phase-angle in the range 16 to 57 degrees. Together with (3200) Phaethon and (152679) 1998 KU2, observed very recently, Bennu is the only existing example of a primitive NEO observed in polarimetric mode over a wide interval of phase angles. Based on our polarimetric data, we propose that Bennu belongs to …
Some asteroids eject dust, unexpectedly producing transient, comet-like comae and tails. First ascribed to the sublimation of near-surface water ice, mass losing asteroids (also called "main-belt comets") can in fact be driven by a surprising diversity of mechanisms. In this paper, we consider eleven dynamical asteroids losing mass, in nine of which the ejected material is spatially resolved. We address mechanisms for producing mass loss including rotational instability, impact ejection, electrostatic repulsion, radiation pressure sweeping, dehydration stresses and thermal fracture, in additio…
The Solar Probe Plus (SPP) mission will explore the Sun's corona and innermost solar wind starting in 2018. The spacecraft will also come close to a number of Mercury-crossing asteroids with perihelia less than 0.3 AU. At small heliocentric distances, these objects may begin to lose mass, thus becoming "active asteroids" with comet-like comae or tails. This paper assembles a database of 97 known Mercury-crossing asteroids that may be encountered by SPP, and it presents estimates of their time-dependent visible-light fluxes and mass loss rates. Assuming a similar efficiency of sky background su…
The Apollo-type near-Earth asteroid (155140) 2005 UD is thought to be a member of the Phaethon-Geminid meteor stream Complex (PGC). Its basic physical parameters are important for unveiling its origin and its relationship to the other PGC members as well as to the Geminid stream. Adopting the Lommel-Seeliger ellipsoid method and $H,G_1,G_2$ phase function, we carry out spin, shape, and phase curve inversion using the photometric data of 2005~UD. The data consists of 11 new lightcurves, 3 lightcurves downloaded from the Minor Planet Center, and 166 sparse data points downloaded from the Zwicky …
