When comet Shoemaker Levy-9 collides with Jupiter next month, how will it affect the planet's atmosphere? An MIT scientist addressed this question and others as one of five panelists at a recent NASA media briefing on the coming event.
Heidi B. Hammel, a principal research scientist in the Department of Earth, Atmospheric and Planetary Sciences (EAPS), is the team leader for Hubble Space Telescope (HST) observations of how Jupiter's atmosphere will respond to the collision. (Four other Hubble teams are using the telescope to explore different phenomena such as the comet itself before impact.)
Comet Shoemaker Levy-9 broke into about 21 fragments during its last close encounter with Jupiter in July 1992. Those fragments, the largest of which are thought to be from about a half mile to a little over a mile across, are scheduled to hit Jupiter from July 16-22. In the process, huge amounts of energy will be dissipated, energy that may lead to fireballs, atmospheric waves, seismic waves and other perturbations of the atmosphere.
"The most obvious place that [the energy] can go is right back up the tube that the comet nucleus came down," creating a fireball. "We'll be looking for that with the HST, with the NASA Infrared Telescope Facility in Hawaii and with many other telescopes," said Dr. Hammel, who will monitor the Hubble observations from the Space Telescope Science Institute in Maryland.
Although the actual impacts won't be visible from Earth-the fragments will hit the planet on its far side-"we expect that the fireballs may be tall enough and bright enough to be seen sticking out from the side of the planet," Dr. Hammel said in an interview. The impact sites on Jupiter will rotate into view about 20 minutes later.
At the media briefing Dr. Hammel noted that the energy associated with the impacts will also go sideways, generating atmospheric waves. According to a simulation, "these waves basically look as if you threw a stone into a pond." She stressed, however, that "the ripples are not something that we're going to see looking through a telescope with our eyes. These ripples represent temperature variations in the atmosphere. The most likely way we'll see them is with an infrared telescope that measures temperature directly." (The simulation was produced by MIT's Joseph Harrington, Timothy E. Dowling and colleagues. Mr. Harrington is a graduate student in EAPS; Dr. Dowling is a professor in the department.)
Energy will also go down into Jupiter's atmosphere, creating seismic waves "that will be refracted back upwards and may become visible in the top of Jupiter's atmosphere," Dr. Hammel said. These waves, which will "probably look exactly like the ripples we see for the atmospheric waves," can be distinguished from the latter by the time scale involved. "The atmospheric waves are very slow," Dr. Hammel said. "It will take about a day and a half to two days for a ripple to be the size of [Jupiter's] Great Red Spot [which itself is larger than Earth]. In the case of seismic waves, it will only take about an hour."
In addition to the above perturbations, "we'll also be looking for cloud features, tiny little white clouds formed right at the impact sites themselves," Dr. Hammel said. She suggested that amateur astronomers hoping to see these clouds should "familiarize themselves with Jupiter's cloud structure now, before the impacts occur," because "there are a lot of other cloud features on Jupiter that could be confusing."
In conclusion, Dr. Hammel stressed that "we'll be looking for all these sorts of things, but we don't know for sure what we're going to see because everything depends on the size of the [comet] nuclei. All the energy calculations, all the brightness calculations, everything depends on the size of the body, and that's something that we just don't have a very good handle on."
In addition to Dr. Hammel, other MIT scientists will also be watching and studying the collisions.
Professor Richard P. Binzel of EAPS will make observations from the Michigan-Dartmouth-MIT 1.3 -meter telescope in Arizona, while graduate student Jeffrey A. Foust of EAPS will be MIT's representative at the Planetary Patrol Telescope in Hawaii, where he will use a high-speed CCD (charge coupled device) camera system to take images of Jupiter and of some of its moons. (The latter images are important because flashes from the comet impacts may be reflected off moons that are in the right place.)
Professor Dowling will be involved in analyzing the data resulting from the observations. He and Dr. Hammel are two of 14 scientists on a team for NASA's Hawaii telescope; the team determined "what science [with respect to the coming impact] will be done with the telescope, and what instrumentation will be needed to do that science," Dr. Hammel said. Mr. Harrington will be MIT's key person at the IRTF during the collisions.
Undergraduates here at MIT will also be monitoring the impacts using two different telescopes at MIT's Wallace Astrophysical Observatory in Westford, Mass. The 24-inch telescope there will have another high-speed CCD system, while the 16-inch telescope will feature a student-built spectrograph to look at the colors of the flashes.
A version of this article appeared in the June 15, 1994 issue of MIT Tech Talk (Volume 38, Number 36).