Mon. Not. R. Astron. Soc. 373, 1107–1111 (2006)

doi:10.1111/j.1365-2966.2006.10862.x

Benefits of an impact mission to 3200 Phaethon: nature of the extinct
comet and artificial meteor shower
Toshihiro Kasuga, Jun-Ichi Watanabe and Mikiya Sato
National Astronomical Observatory of Japan (NAOJ), National Institute of Natural Science, Mitaka Tokyo 181-8588, Japan

Accepted 2006 July 24. Received 2006 July 22; in original form 2006 February 7

The asteroid 3200 Phaethon is suggested as a candidate for direct impact research. The object
is considered to be an extinct comet and the parent of the Geminid meteor shower. One could
say that this provides a possible argument for a space mission. Based on such a mission, thispaper proposes to investigate the nature of the extinct comet and the additional interesting
possibility of artificially generated meteor showers.
Dust trail theory can calculate the distribution of a bundle of trails and be used to show inwhich years artificial meteors would be expected. Results indicate that meteor showers will beseen on Earth about 200 yr after the event, on 2022 April 12.
Key words: comets: general – meteors, meteoroids – minor planets, asteroids.

1 INTRODUCTION
The nature of the asteroid 3200 Phaethon as the parent of the Geminid meteor shower is considered an open question (Hsieh & Jewitt2005). Parents of meteor showers are generally expected to becomets rather than asteroids (Williams 1985). However, unlike acomet, 3200 Phaethon seemed to have a rocky surface, which is characteristic of a B-type asteroid in Bus’s taxonomy (Green, Meadows& Davis 1985; Birkett et al. 1987; Bus & Binzel 2002). Coma activity of Phaethon and the dust trail associated with Geminid meteor
activity have still not been observed (Williams & Wu 1993; Hsieh &Jewitt 2005). This is a unique case for a parent of a meteor shower.
Phaethon’s nature is not conclusive as being that of just an asteroid
and another possibility is suggested.
Some Apollo-type asteroids, including Phaethon, might be nuclei of former comets deprived of their volatile components. The
degassed comets cannot be distinguished from asteroids. So called,they are extinct comets from the dynamical point of view. However, there is no direct physical evidence and thermal history forthe cometary nature of Phaethon from the beginning. The investigation of Phaethon will provide the first opportunity to prove thishypothesis.
Meteor spectroscopy is one of the best methods for exploring
pristine objects, through meteor phenomena, such as comets andasteroids in the solar system. The physical structure and chemicalcomposition of a Geminids meteor will provide important clues concerning both the origin and the evolution of Phaethon. For example,
derived metallic abundances of a 2004 Geminid meteor, especially
Na/Mg depletion and excess Ni/Mg, show different features fromother meteors whose parents are comets (e.g. Trigo-Rodrı̀guez et al.
 E-mail: kasugats@cc.nao.ac.jp
C 2006 The Authors. Journal compilation 
C 2006 RAS

2003; Kasuga et al. 2005a; Kasuga, Watanabe & Ebizuka 2005b).
Babadzhanov (2002) reported that the Geminid meteoroids have thelargest bulk density relative to that of other major meteor streams
of cometary origin. Meteor research is very valid for clarifying thedifferences in composition between some comets and Phaethon. Research may suggest a lack of volatiles in Phaethon, which may be
an asteroidal feature. It is quite essential to determine the mutual
evolutionary relationship between comets and asteroids. However,
the true nature of Phaethon, whether originally a comet or not, is
not conclusive only by observing the Geminid meteors.
For this purpose, we suggest the new type of scientific mission
to the asteroid 3200 Phaethon. This is an in situ investigation of
Phaethon and a first trial for producing artificial Geminid meteor
showers as a result of an impact with Phaethon. It was suggested
that the on-board measurements of Phaeton will be of particles ofmaterial thrown up by the impact along Phaeton’s trajectory. The
possibility of launching a space probe to Phaethon just before theyear 2000 was considered by Padevět, Lála & Bumba (1986). Our
new idea is applied to cause artificial meteor showers on Earth using
the idea of the Deep Impact mission in 2005 (A’Hearn et al. 2005).
After the Deep Impact with 9P/Tempel 1, ejection of dust in greatvolume was observed from the space probe using a flyby camera
and from the ground using telescopes such as Subaru (Sugita et al.
2005). A bundle of dust trails will be formed. In the case of the
impact with Phaethon, the same situation is predicted.
An in situ probe method is considered by the laboratory simulation of the capture of intact cometary and asteroidal dust particles(Fujiwara, Nakamura & Kadono 1994). This also will be rare andwill be the first occasion to improve on data for Geminid meteors,
leading to a worldwide campaign that will bring great advancesin meteor astronomy, such as the Leonid Multi-Instrument Aircraft Campaign (Leonid MAC; e.g. Jenniskens & Butow 1999) andspace-borne ultraviolet spectroscopy of a meteor for the discovery of

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ABSTRACTT. Kasuga, J.-I. Watanabe and M. Sato

1108

unknown elements related to astrobiology (Jenniskens et al. 2002;
Carbary et al. 2003). Increased public awareness, worldwide, is also
expected.
In this paper, a simulation of the artificial meteor shower broughtby the impact with Phaethon is described with a calculation of the
distribution of a bundle of trails. Dust trail theory can be used toshow specific years when extra meteor activity will occur and toshow that there are other years near the Phaethon nodal intersection
epoch. At present, we know that greater natural meteor activity
in the Geminid shower could occur every year for several years
around the Phaethon nodal intersection epoch. Similarly, we guessthat an impact could cause artificial meteors every year for several
years around the Phaethon nodal intersection epoch. Dust trail theory
was used first to show in which years artificial meteors would be
expected.

Table 2. The dates that Phaethon passes the ecliptic plane.
2017 December 23
2019 May 312020 November 52022 April 12
2023 September 182025 February 22
2026 July 31
2028 January 52029 June 12

2 S I M U L AT I O N O F T H E A RT I F I C I A L
M E T E O R S H OW E R
In this section we simulate the activity of the artificial Geminid meteor showers and their trails, which are associated with the ‘Phaethon
Impact’ mission. The most simple approach of dust trail theory(e.g. McNaught & Asher 2002) was applied (Sato 2003). The effectof the radiation pressure on the meteoroids is not taken into accountin our calculation, because meteoroid particles are characterized aslarge, compact, blackbody-like particles (Kasuga et al. 2006). The
applied orbital elements were obtained by the MPC Orbit Database(MPCORB; IAU Minor Planet Center).
First, an evolution of the orbital elements of Phaethon are summarized in Table 1. The long-term orbital motion of Phaethon has
already been clarified: for example, a large-amplitude q–i oscillation arises that lasts about 20 000 yr (Babadzhanov & Obrubov 1987,
1992; Ohtsuka et al. 2006). However, in the short term (for about
hundreds of years), the orbit of Phaethon is comparatively steady.
Because the orbit of Phaethon is not close to Jupiter, it is little affected by the perturbation of planets. With the passage of time, theperihelion, q, increases slightly, although the semimajor axis, a, isstable. On the other hand, the argument of the perihelion, ω, grows.
Then, as the descending node leaves the perihelion, the heliocentric
distance R of the descending node tends to increase gradually withthe effect of the evolution of the perihelion. The node will reach theorbit of the Earth around 2222.
Secondly, we calculated the distribution of a bundle of dust trails
formed at the epoch of the Phaethon Impact. To determine the dayof impact with Phaethon, we followed the ‘Deep Impact Mission
design’ (Blume 2005). In order to deliver a substantial science payload, the day that provided the best opportunity to impact 9P/Tempel
1 arose when the orbit of the comet was near its descending node,

Table 1. An evolution of the orbital elements of Phaethon. R is the heliocentric distance of the descending node (at the current epoch)and the node is seen to reach the orbit of the Earth around 2222. The epoch of ω,  and i is J2000.0.
T2022 May 15.18
2061 January 28.30
2101 March 23.22
2141 May 07.63
2181 June 17.68
2221 July 27.16
2260 March 27.06
2300 May 09.63

q

e

ω



i

a

R

0.14010.14070.14110.14170.14230.14270.14330.14410.88980.88940.88900.88850.88800.88770.88720.8866322.17322.83323.35323.87324.46324.97325.47325.96265.22264.57264.03263.52262.92262.41
261.89261.40

22.26
22.54
22.80
23.02
23.30
23.54
23.79
24.04

1.2714
1.2714
1.2712
1.27051.27071.2706
1.27091.27090.89100.9116
0.9284
0.94610.96690.98431.00251.0213


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which was very close to the perihelion, considering all the best
mission parameters. In the case of Phaethon, it is also important
to impact near the ecliptic plane on the Earth side to carry the
flyby and impactor. We have suggested in Table 2 ‘Phaethon impact days’ from 2017 onwards, which are limited chances, when
Phaethon passes the ecliptic plane on the Earth side. Hereafter,
we have focused on the impact day of 2022 April 12, because
other mission programmes are already projected until 2020 in Japan(see http://www.jaxa.jp/press/2005/04/20050406 sac vision j.pdf,
in Japanese, for a road map for exploration of the Solar system,from the Institute of Space and Astronautical Science, ISAS, theJapan Aerospace Exploration Agency, JAXA).
We show one of the results of a bundle of the trails caused by our
Phaethon Impact on 2022 April 12. Because of differential planetaryperturbations, the heliocentric distance R of the descending node isa function of the time when particles reach the node. Fig. 1 shows
the distributed trail formed due to the Phaethon Impact mission. In
2222, the descending node of Phaethon passes from the inside of
the orbit of the Earth to the outside. The trail is calculated using
test meteoroids ejected in the direction of the motion of Phaethon(which we define as positive ejection velocities) and in the opposite
direction (negative ejection velocities).
The overall orbital evolution is rather slow because the orbit ofPhaethon is a long way from Jupiter. Although the orbital evolutionof all the test meteoroids is slow for the same reason, the orbital
evolution of the dust that is ejected in the direction of the motion of
Phaethon is less slow because the orbits are slightly less far from
Jupiter. As a result, the descending node reaches the the orbit of the
Earth in the early phase in 2222. The negative ejected dust shows
the opposite result.
In the case of extreme high-speed dust ejection, for example+900 m s−1 , a bundle of dust trails will cross the orbit of the Earthafter the epoch of 2160. In contrast, in the event of dust ejection in
the negative direction (−900 m s−1 ), trails will cross the epoch of
2270. As shown by the results of high-speed dust ejection, meteor
showers will have been appearing from the period of 2160 to 2270.
However, the ejection velocity of a meteoroid is not high, as
revealed by Deep Impact 2005 (A’Hearn et al. 2005). A trail is apt
to be formed by much slower ejecta. Fig. 2 shows a period of time
when the trail crosses the orbit of the Earth. It is calculated using test

Scientific mission to 3200 Phaethon1109

meteoroids. The ejection velocity is chosen to be ±20 m s−1 because
trails evolve as the parent moves. In this case, meteor showers will
occur within around 10 yr.
Until 2214, the distance of trails whose ejection velocity of±20 m s−1 is more than 0.002 au from the orbit of the Earth, whichis too far to produce meteor showers. On the other hand, the bundle of trails between 2215 and 2225 are within 0.002 au, which is
close enough to produce meteor showers. Especially in 2218, 2219,
2222 and 2223, trails approach within 0.001 au, which results in agreat activity of meteor showers. The main situations when the trails
approach the Earth are summarized in Table 3. After 2224, meteor
activity will decrease as the trails occur more than 0.002 au outside
the orbit of the Earth.
Meteor showers on Earth are predicted with the Phaethon Impact.
In such a situation, meteoroids that are relatively large in size andhigh in velocity may cause tremendous damage to spacecrafts by
their impact, as seen in the 1993 Perseid meteor shower–Olympus
satellite anomaly (McDonnell 1993). For a reference, Yano et al.
(2003) derived a data set for satellite risk assessments with the relation of each zenith hourly rate (ZHR) in the 1998–2001 Leonid meteor storms. Consequently, Leonid storms in 1999 and 2001 seemed
to exceed the risk caused by sporadic meteors at 1 au by factors of∼2 to ∼4. In the future, meteoroid size, mass, population index andcumulative flux should be considered in order to estimate the significant levels of impact risk to Earth and spacecrafts. Vaubaillon,
Colas & Jorda (2006) derived the environment of the meteoroid sizeC 2006 The Authors. Journal compilation 
C 2006 RAS, MNRAS 373, 1107–1111

distribution, particle density and number of expected collisions forthe in situ flyby from the Deep Impact. These factors can be applied
to the preparation phase of any space mission to a comet. To preventthe impact hazard on Earth and spacecrafts related to the Phaethon
Impact mission, we must consider the environment of ejected meteoroids and estimate the hazard precisely as a serious future piece ofwork.

3 CONCLUSIONS
We suggested the scientific mission to Phaethon, together with thegreat activity of meteor showers on the basis of dust trail theory.
The Phaethon Impact on 2022 April 12 will show a great activityof meteor showers in 2218, 2219, 2222 and 2223. This will be rare
and first occasion to improve on data for Geminid meteors leading
to a worldwide campaign that will bring great advance in meteor
astronomy, such as the Leonid MAC. The true nature of Phaethon,
whether originally a comet or not, will be revealed by our scientificmission.

AC K N OW L E D G M E N T S
The authors are grateful to Dr A. Fujiwara (ISAS/JAXA) for hiskind support of this work. TK acknowledges support by the JSPS
Research Fellowships for young scientists.

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Figure 1. An evolution of the dust of Phaethon every 20 yr. Impact date is assumed to be on 2022 April 12. Ejected dust of ±20 m s−1 overlapped the parent
body because trails evolve as the parent moves.

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T. Kasuga, J.-I. Watanabe and M. Sato

Table 3. Main situations when the trails approach the Earth (within
0.001 au).
Year

Time (UT) R (au)Ejection velocity (m s−1 )2218

December 18 02:00+0.000 13+0.000 04+0.000 07−0.000 19

+10.8
+13.2
+15.9
+17.4

2219

December 18 08:00−0.000 65−0.000 69−0.000 86

−4.1
−3.9
−0.7

2222

December 18 03:00+0.000 57+0.000 46+0.000 37

−3.4
−1.8
+0.6

2223

December 18 09:00−0.000 29−15.3

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Figure 2. Distribution of a bundle of dust trails after the impact event on 2022 April 12. ‘+20’ (or ‘−20’) is the ejection velocity (m s−1 ) at impact. Note thatthe same particles return to the node at a time-spacing corresponding to their orbital period, with the nodal distance R being slightly shifted on each revolution
because of planetary perturbations. Meteor showers would be seen between 2215 and 2225 (approach distance 0.002 au). In 2218, 2219, 2222 and 2223,
great activity is predicted (approach distance 0.001 au).

Scientific mission to 3200 Phaethon
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C 2006 The Authors. Journal compilation 
C 2006 RAS, MNRAS 373, 1107–1111