[Logo showing SL9 collision with Jupiter]

The Impact

All 20 major impacts occurred at approximately the same position on Jupiter relative to the center of the planet, but because the planet is rotating the impacts occurred at different points in the atmosphere. The figure at the top of the page shows the viewing geometry from Earth at the time of impact. The impacts took place at approximately 45 degrees south latitude and 6.5 degrees of longitude from the limb, just out of view from Earth (approximately 15 degrees from the dawn terminator). Jupiter has a rotation period of 9.84 hours, or a rotation rate of about 0.01 degrees/sec, so the impacts occurred on the far side of the planet but the point of impact in the atmosphere rotated across the limb within about 11 minutes after the impact, and crossed the dawn terminator within about 25 minutes from the impact. The comet particles were moving almost exactly from (Jovian) south to north at the time of the impact (actually at an angle of 83 degrees to Jupiters equatorial plane), so they struck the planet at an angle of about 45 degrees to the surface. (The surface is defined for convenience as the Jovian cloud tops.) The impact velocity was Jovian escape velocity, 60 km/sec.
[Red bullet] Hubble press release on SL-9 collision results (29 September 1994)

Jupiter was approximately 5.7 AU (860 million km) from Earth, so the time for light to travel to the Earth was about 48 minutes. Below is a list of the collision times of the fragments as seen from Earth, as calculated by Chodas and Yeomans. Their methods of estimating the times are given following the list.


                                  Accepted
Fragment  Date    Prediction      Impact Time
          July    (HH:MM:SS)      & 1-sigma error

A         16      20:00:40        20:11:00 (3 min)   
B         17      02:54:13        02:50:00 (6 min)
C         17      07:02:14        07:12:00 (4 min)
D         17      11:47:00        11:54:00 (3 min)
E         17      15:05:31        15:11:00 (3 min)
F         18      00:29:21        00:33:00 (5 min)
G         18      07:28:32        07:32:00 (2 min)
H         18      19:25:53        19:31:59 (1 min)
J         19      02:40           Missing since 12/93
K         19      10:18:32        10:21:00 (4 min)
L         19      22:08:53        22:16:48 (1 min)
M         20      05:45           Missing since 7/93
N         20      10:20:02        10:31:00 (4 min)
P2        20      15:16:20        15:23:00 (7 min)
P1        20      16:30           Missing since 3/94 
Q2        20      19:47:11        19:44:00 (6 min)
Q1        20      20:04:09        20:12:00 (4 min)
R         21      05:28:50        05:33:00 (3 min)
S         21      15:12:49        15:15:00 (5 min)
T         21      18:03:45        18:10:00 (7 min)
U         21      21:48:30        21:55:00 (7 min)
V         22      04:16:53        04:22:00 (5 min)
W         22      07:59:45        08:05:30 (3 min)
                                          

Post-Crash Impact times for fragments of Comet Shoemaker-Levy 9
Don Yeomans and Paul Chodas 

There are several sources of information that can be used to 
estimate the actual impact times of the major fragments of 
Comet Shoemaker-Levy 9. Astrometric data has been used to 
determine updated orbits and these orbits have been used to 
determine the predicted impact times when the computed position 
of the fragment enters the 1 bar (atmospheric pressure) level of 
Jupiter's atmosphere. Values for Jupiter's mean radius and 
obliquity were taken from Reference 1. Because the astrometric 
data nearest the impact times themselves are the most powerful 
for reducing the error of the predicted impact times, we were 
particularly fortunate in receiving recent astrometric data from 
the European Southern Observatory (Richard West, Olivier Hainaut 
and colleagues) that were reduced with respect to the Hipparcos 
star catalog. Extremely valuable sets of astrometric data were 
received from the U.S. Naval Observatory in Flagstaff (Alice 
and Dave Monet), McDonald Observatory (A. Whipple, P. Shelus and 
colleagues), Spacewatch (J. Scotti and colleagues) and a number 
of other observatories. The final pre-crash impact predictions 
were sent out on July 16, 1994. As we received astrometric data 
from Dave Jewitt and Dave Tholen on a few of the trailing 
fragments on July 19, a revised subset of the July 16, 1994 
predictions were issued just before midnight on July 19, 1994. 
For completeness, the final predicted times of impact are given 
in column 1 in the table below. 

Impact times were determined by Andy Ingersoll and Reta Beebe 
using Hubble Space Telescope information on the location of the 
northwestern edge of the dark spots resulting from some of the 
impact events. The longitudes determined for these spots were 
compared to the longitude predictions given by Chodas and Yeomans 
and the differences in longitude were converted to time differences 
between actual and predicted impact times. The errors associated with 
this technique are a estimated to be a few minutes. For some fragments 
there are more than one impact time estimate determined from different 
frames from the Space Telescope. We put the most weight upon those 
determinations made from the frame taken closest to the impact time. 

For fragments H and L, the Galileo PPR instrument observed the flash 
phase of the bolide entry so that for these two cases, we have impact 
time data that are accurate to +/- 5 seconds. However, since we do not 
know whether the PPR times correspond to the initial impact or to a 
subsequent flash, we have assigned uncertainties of +/- 1 minute. 
There is also a hint of a signal in the PPR data corresponding to the 
Q1 impact. These data were provided by Terry Martin. By comparing the 
H and L impact times determined from the PPR data with the respective 
predicted impact times, we note that the PPR estimate is 6.1 minutes 
later than the ephemeris prediction for the H fragment and 7.9 minutes 
later than the prediction for the L fragment. The average of these two 
differences is 7.0 minutes and this average, when added to the predicted 
impact time, will give a rough determination of the true impact time. 

For fragments B, D, K, Q1, and R, we have estimates of both the initial 
flash times and the subsequent first plume observation. We only 
considered those plume observations seen in the 2-3 micron region. The 
time differences between initial flash times and first plume observations 
were respectively +6, +5, +6, +6, and +8 minutes. An analysis by Andy 
Ingersoll and John Clarke suggests that for fragment G, there was an 8 
minute lag between impact and the rise of the arc-shaped plume to where 
it could be seen in sunlight. In the absence of other information, the 
first plume observation minus an average of these values (+6.2 minutes) 
would give an estimated impact time. From the two fragment impacts 
observed by the GLL PPR, there is also evidence that the initial flash, 
as observed by ground-based telescopes, comes about 1 minute after the 
flash seen by the PPR instrument. 

The impact times are UTC times received at Earth (light time corrected). 

In setting forth the accepted impact times given in the final column of 
the following table, the priority of the various available techniques 
as as follows: 

1.  GLL PPR timing (Fragments H & L)
2.  When definitive flash times are available, with subsequent plume
    observations noted about 6 minutes later), we generally took the
    impact time as one minute before the flash time since the PPR
    instrument recorded its first signals about one minute before the
    reported flash times. (Fragments D,G,Q1,Q2,R,S,V, and W)
3.  Estimates determined from HST longitudes 
4.  Estimate determined from first plume observation minus 6.2 minutes
5.  Chodas/Yeomans prediction with empirical adjustment of + 7 minutes

The impact times for fragments A,C,E,K, and N were determined by 
considering the ephemeris prediction error (about 7 minutes early for 
most fragments), the times determined from the HST longitude estimates 
(uncertainty = 3-4 minutes or more) and the times determined from plume 
observation times (impact time = plume observation time less 5-8 minutes). 
An effort was made to consider and balance these three factors and the 
uncertainties on the estimated impact times reflect our confidence level. 
For fragment F, the impact time was determined using the ephemeris 
prediction and the Lowell Observatory estimate of when the F spot was seen 
on the terminator. In the absence of any quantitative impact time 
observations for fragments P2, T, and U, only the ephemeris prediction was 
used (plus 7 minutes). The impact time estimate for fragment B is based upon 
observatory reports and is relatively uncertain because the impact time 
occurs before the ephemeris prediction and well before the estimate 
determined from the HST longitude estimate. 


References:

1.      Explanatory Supplement to the Astronomical Almanac.  
        University Science Books, 1992, p. 404.

The rotational position of the Earth as seen from Jupiter at each of the impact times is shown in a plot by L. Wasserman of Lowell Observatory. Various models of this collision were hypothesized, and there was general agreement that a fragment would travel through the atmosphere to some depth and explode, creating a fireball which would rise back above the cloud tops. The explosion would also produce pressure waves in the atmosphere and "surface waves" at the cloud tops. The rising material may have consisted of a mixture of vaporized comet and Jovian atmosphere, but details about this, the depth of the explosion, the total amount of material ejected above the cloud tops, and almost all other effects of the impact are highly model dependent. Comparisons of the model results and the actual impact data are currently being done.

Other studies of the impact images are still ongoing. The following text was written before the impacts, but the information is still fairly accurate, and many of the results discussed await further detailed analysis of the data, so while the impacts themselves are all "past tense", the scientific results are still very much in the "future".

Reflections of the fireball off the Jovian satellites, the rings, and even the dust coma of the comet may be visible as a ~1% brightness increase. A particularly good opportunity to observe the effect may occur when certain satellites are eclipsed by Jupiter. There are a few impacts which may occur during these relatively infrequent (~4/day) and short-lived (1-4 hour) periods. The rings of Jupiter are always present as a source of reflection, a portion of the rings is always in Jupiter's shadow, and the rings are closer in to the impact site. Unfortunately, the rings are far less opaque and reflective than the satellites. The Jovian ring system consists of a main ring at 1.71 to 1.81 RJ (RJ = Jupiter radius = 71,400 km), a halo at 1.28 - 1.7 RJ, and a gossamer ring which extends from the surface to about 3 RJ. The main ring is a mixture of large and small particles, and the halo and gossamer rings consist of very small particles. There are also two known satellites, Metis and Adrastea, embedded in the rings.

The direct effects of the impact on the atmosphere of Jupiter are highly dependent on aspects of the collision, such as depth of explosion and amount of atmosphere displaced, which are not well constrained. Heating and transport of deeper atmospheric material is expected, which may have observable dynamical and chemical effects, especially in the ionosphere which could last from days to months. Minor comet components reacting with the atmosphere may also be observable and the collision is expected to cause traveling atmospheric waves at the cloud tops. There may also be production of vortices and hazes which could last on the order of weeks. Depending on the depth of explosion, the portion of energy directed downward, and the attenuation, "seismic" waves will be produced in the atmosphere which may be observable some distance from the impact point. These may tell something about the structure of the deeper atmosphere, and will have the effect of causing small motions in the troposphere and stratosphere which may be observable as temperature fluctations on the order of millidegrees Kelvin.

The dust cloud surrounding the fragment string may also have observable effects on the Jovian system, possibly starting months before the arrival of the fragments. The total mass of dust is only about 10 million kg or less, but a significant amount of this dust will not intersect the planet and may affect the rings, satellites, and magnetosphere. The dust will bombard the Jovian satellites, and may produce more dust for the rings, resulting in a noticeable increase in ring brightness. The magnetopause is at 85-100 RJ, so most of the dust will travel through the magnetosphere. Possible effects include aurorae, radio discharges, lightning, changes in the Io torus and surface of Io, a decrease in synchotron emissions, strong field aligned currents, and to a much lesser extent charging of the magnetosphere resulting in radiation. Again the effects of the dust are largely uncertain.


[NASA Logo]
Author/Curator:
Dr. David R. Williams, dave.williams@nasa.gov
NSSDC, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
+1-301-286-1258


NASA Official: Ed Grayzeck, edwin.j.grayzeck@nasa.gov
Last Updated: 30 December 2004, DRW