| Back to DMS homepage |
 |
This page was last modified on June 30, 1998 by Peter Bus and Casper ter Kuile |
Meteor Outburst from Comet 103P/Hartley 2
GOOD PROSPECTS FOR A METEOR OUTBURST FROM COMET 103P/HARTLEY 2 =============================================================== P. Jenniskens NASA/Ames Research Center ---------------------------------------------------------------------------- On November 3, 1997, the Earth will pass close to the orbit of comet 103P/Hartley 2. At 00.9 h UT, the minimum distance will be only 0.003 AU and the Earth will lead the comet in passing that point by only 49.0 days. Those conditions compare favorably to previous times when meteor outbursts associated with short period comets were observed. The meteors should radiate from beta-Cygni and enter at a slow 17 km/s entry velocity. Without knowledge of the dust environment of this comet, we can not be certain about the time of the event (we pass the comet node already on Nov. 2 at 07.1 h UT) nor the magnitude of the event (calculations of dust ejection show that the Earth will pass somewhat in front of the main dust sheet). However, given the unique encounter conditions, this event does warrant monitoring on the nights of November 1/2 and 2/3. ----------------------------------------------------------------------------- Meteor storms are often associated with the Leonid shower and the return of the parent comet 55P/Tempel-Tuttle to perihelion. Indeed, the comet was discovered last March 10 and the Leonid storms are due in November of 1998 and 1999 [1,2]. In this paper, I would like to draw attention to yet another chance to see a meteor storm, one that may be caused by the dust of comet 103P/Hartley 2. The very favorable encounter conditions and the slow apparent speed of the meteors call our attention, although it is much less certain that a storm will occur. 103P/Hartley 2 was discovered on March 15, 1986, by Malcolm Hartley with the 1.2m UK Schmidt telescope at Siding Spring, Australia. Based on the preliminary orbit of 103P/Hartley 2, Robert McNaught immediately realised that two returns later, in 1997, the Earth would pass close to the comet itself, making it possible that some meteor activity might occur [3]. How favorable that encounter was going to be became known only after the successive observations during the return of 1991, the recent recovery of the comet, and the predicted orbit for the upcoming return of 1997. Recently, Mark Matney of NASA/Johnson Space Center drew my attention to that 1997 orbit, which was provided to him by Brian Marsden of the Minor Planet Center. 103P/Hartley 2 is one of those short period comets that continuously changes its orbit rather dramatically. Such comets usually stay only briefly close to Earth's orbit before moving on again and little is known about their dust environment. Indeed, no past meteor outbursts are known from 103P/Hartley 2. However, other such comets have given displays as good as a Perseid return in summer under somewhat similar encounter conditions, and impressive meteor storms were caused by Comets 21P/Giacobini-Zinner in 1933 and 1946 and 3D/Biela in 1872 and 1885. A close approach by Jupiter in December of 1993 (0.37 AU) has moved the perihelion distance of 103P/Hartley 2 from 0.95 AU during the previous return in 1991 to 1.03 AU during the upcoming December 22 1997 return. Such a change from just within to just outside the Earth's orbit is an important clue that something special may be happening. I asked Don Yeomans at JPL to further investigate. The Earth will pass closest to the comet's orbit near perihelion rather than at the comet's descending node on Nov. 3 at 0.9 hours UT (solar longitude 220.681 - J2000). "If there is a shower, this would be the time of shower maximum", according to Yeomans. Conditions are excellent, with only a sliver of a Moon early in the evening. Yeomans calculated an apparent radiant position at approximately RA = 295.6, DEC = +31.3 degrees (J2000), not far from beta-Cygnus. That is quite a bit higher in declination than a previous radiant calculated by Hasegawa for the 1985 orbit: RA = 290, DEC = +7 [4]. That is good, because it extends the period that the stream radiant is above the horizon for northern hemisphere locations. The meteors should enter at an apparent velocity of only about 17 km/s. Meteors do not get much slower than that, because 11.2 km/s is the limit set by Earth's escape velocity. What are the chances that there will be a meteor storm? Yeomans calculated that the Earth will lead the comet to this close approach point by 49 days and the separation distance will be 0.003 AU with the Earth just outside the comet's orbit. Those encounter conditions compare favorably to those at times when meteor storms were observed in the past. For example, the 1885 Andromedid storm happened when the Earth passed the nearest point only 10 days earlier at a distance of +0.0051 AU from the comet orbit. That time, Earth passed inside the comet orbit rather than outside [5]. During the Draconids storms of 1933 and 1946, the Earth followed the comet by a mere +80 and +15 days, with a minimum distance of +0.0054 and +0.0015 AU, respectively, again passing inside the comet's orbit. Recent outbursts of Pi-Puppids from comet P/Grigg-Skjellerup in 1977 and 1982 occurred when the Earth passed just outside the comet orbit, at times when the Earth was both leading (by 21 days) and following (by 12 days) the comet. In those occasions, the meteor activity rose to levels comparable to a display of the Perseid shower in summer [5]. It is almost certain that there will be some meteor activity from 103P/Hartley 2. That would be a novum, hence the activity would be called a "meteor outburst", even if rates stay low. The question remains if Earth will pass close enough to the comet to have a storm. Maybe not. Calculations by Mark Matney show that the most recent dust may not have spread far enough along the comet orbit for Earth to be able to meet it 49 days in front of the comet. This is not a certain conclusion, because the difference is only a factor of two in time. However, it is clear that we should not put our expectations too high. If an outburst occurs at 0.9 hours UT on Nov. 3th, as predicted by Yeomans, then the outburst can be seen in the early evening of November 2nd by observers on the east coast, while west coast observers may be able to see a tail of it. For observers in north-western parts of Europe, the outburst is expected to peak shortly after midnight. Best viewing is over the Atlantic Ocean. However, the problem is that the time of nearest passage does not need to be the time that we cross the dust sheet. The comet has an orbit at a shallow 13.6 degree angle with the ecliptic (was 9.3 degrees before the disturbance). A small deviation of the main dust sheet relative to the comet orbit can lead to a significantly different time of the event. In principle, the window of opportunity stretches from the time of passing the comet node on November 2 at 07.1h UT, until the point of nearest passage to the comet orbit on November 3 at 00.9h UT, which is a 17.8 hour interval. Best chances are perhaps a few hours prior to 00.9h UT, hence in the evening of November 3 for European observers. World-wide monitoring of meteor activity is needed on the nights of November 1/2 and 2/3 to increase the chances that the event will be observed. The duration of the outburst depends on the thickness of the dust sheet and the path of the Earth through the stream. If we only account for the inclination of the orbit, then the duration of the event, the time between activity levels of 14% of peak activity, will be of order 5 - 8 hours, based on the thickness of the dust sheets responsible for the Andromedids, iota-Draconids, October Draconids and Puppids, which are all very similar [5]. There may also be some low level activity that extends over many days. The occurrence of an outburst, even of relatively low intensity, has big implications. That is because this event is potentially most suited for collecting meteoroid debris in the stratosphere from a known source. Because of the slow entry velocity, even relatively large particles (50 micron) are expected to survive entry heating. Preparations have been made to collect particles by high-flying aircraft from NASA/Ames Research Center in November and December of 1997, if the shower manifests itself. The comet itself may reach magnitude +8 shortly after passing perihelion on December 21. Orbital elements calculated by Kenji Muraoka, finding charts and a predicted brightness evolution can be found at the website: http://www.info.waseda.ac.jp/muroaka/members/seiichi/comet/catalog/0103P/1997.html Finally, if the stream returns in the next year, then the shower maximum is expected to be about six hours later, hence on 1998 Nov. 3 at 7 hours UT, according to Yeomans, and the Earth would follow the comet to the close approach point by about 316 days. I would like to thank Mark Matney and Don Yeomans for their contributions to this paper and George Flynn of Plattsburgh University for calculations of the heating rate of P/Hartley 2 grains. Further information on the Leonid storms can be found at: http://www-space.arc.nasa.gov/~leonid/ [1] Jenniskens P., 1996, Meteoritics & Planetary Science 31, 177-184 [2] Yeomans D., Yau K., Weissman P., 1996, Icarus 124, 407-413 [3] McNaught R., 1986, in The Astronomer, No 266, June, p24 [4] Hasegawa I., 1990, Publ. Astron. Soc. Japan 42, 175-186 [5] Jenniskens P., 1995, Astron. Astrophys. 295, 206-235 Table 1: Orbital elements of 103P/Hartley 2 computed from observations in the period form 1985 until June 1997 (J2000), computed by Brian G. Marsden (top) and Donald K. Yeomans (bottom). The latter contain the most recent observations until July 8, 1997. ------------------------------------------------------------------------ 1985 1991 1997 ------------------------------------------------------------------------ Marsden: Epoch 1985, June 24.0 TT 1991, Sept. 21.0 TT 1997, Dec. 18.0 TT T 1985, June 4.8634 TT 1991, Sept. 11.6515 TT 1997, Dec. 22.0242 TT a 3.39680 3.39865 3.44357 AU e 0.719844 0.719502 0.700391 q 0.9516335 0.9533139 1.0317245 AU n 0.1574341 0.1573059 0.1542375 P 6.26 6.27 6.39 yr i 9.25279 9.25182 13.61908 o) (J2000) w 174.81006 174.89662 180.72400 o) Node 226.85420 226.78470 219.95471 o) ------------------------------------------------------------------------ Yeomans: Epoch 1985, June 24.0 TDB 1991, Sept. 21.0 1997, Dec. 18.0 TDB T 1985, June 4.8758 1991, Sept. 11.6517 1997, Dec. 22.0222 A 3.396700 3.39854 3.44349 AU E 0.719836 0.719494 0.700383 Q 0.9516320 0.9533128 1.0317265 AU I 9.25281 9.25183 13.61904 o) (J2000) W 174.81122 174.89667 180.72319 o) Node 226.85427 226.78478 219.95470 o) ------------------------------------------------------------------------ Peter Jenniskens The SETI Institute e-mail: peter@max.arc.nasa.gov NASA Ames Research Center tel: (415) 604-3086 Mail Stop 239-4 fax: (415) 604-1088 Moffett Field, CA 94035-1000 http://www-space.arc.nasa.gov/~leonid/
| Back to DMS homepage |
|
This page was last modified on June 30, 1998 by Peter Bus and Casper ter Kuile |