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:

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: 
[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 
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) 
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:       
   NASA Ames Research Center		tel: (415) 604-3086
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