

JPL Comet Shoemaker-Levy Educators Guide



The Galileo spacecraft has the best vantage point from which to observe the impacts. It is on its way to Jupiter and will be only 246 million km away from the planet, less than a third the distance of Earth from Jupiter at that time. All of the impacts will occur directly in the field of view of its high resolution camera and 20-25 degrees of Jovian longitude from the limb. Images of Jupiter will be 60 picture elements (pixels) across, although the impact site will still be smaller than the resolution of the camera. Several instruments besides the camera have potential use, including an ultraviolet spectrometer, a near infrared mapping spectrometer, and a photopolarimeter radiometer. This last suite of instruments could acquire light curves (plots of intensity versus time) of the entry and fireball at many wavelengths from ultraviolet to thermal infrared (from wavelengths much shorter than visible light to much longer).

Using Galileo to make these observations will be challenging. The amount of data the spacecraft can transmit back to Earth is limited by the capability of its low-gain antenna and the time available on the receiving antennas of the National Aeronautics and Space Administration's (NASA's) Deep Space Network here on Earth. The "commands" that tell the spacecraft what to do must be sent up several weeks before the fact and before the impact times are known to better than about 20 minutes with 95% certainty. A later command that simply triggers the entire command sequence may be possible. A lot of data frames can be stored in the Galileo tape recorder, but only about 5% of them can be transmitted back to Earth, so the trick will be to decide which 5% of the data are likely to include the impacts and to have the greatest scientific value, without being able to look at any of them first! After the fact, the impact times should be known quite accurately. This knowledge can help to make the decisions about which data to return to Earth.

This was exerpted from the JPL Shoemaker-Levy Educators Guide. To see the entire guide, click here.