Gerhard Schwehm, the project scientist for ESA's cometary explorer Rosetta, was kind enough to share with me some preliminary details of the science team's plans for the upcoming Earth flyby. Closest approach will take place at 20:57 UTC on November 13. This flyby is a significant event in space history: Rosetta will be the first mission to have journeyed to Mars (on February 25, when it took that amazing image of Mars out the side window) and then return to Earth. Rosetta is not planning to stay here though! As with any mission's gravity-assist flyby, the primary reason for Rosetta to swing past Earth is to pick up exactly the right speed and change in direction to send it onward to its eventual destination. Therefore, if anything unusual happens during the flyby, the science plans, which are not critical to mission success, might be partially or fully scrapped. But provided that everything goes well during this important maneuver, the science team plans to take advantage of the rare opportunity to perform a few observations.
Many of the observations are done for calibration purposes. The spectral properties of Mars and Earth have been studied well from space, so the measurements performed by Rosetta's instruments can be compared to the known properties of Mars and Earth in order to investigate the performance and precision of Rosetta's instruments.
The first question on everybody's minds is probably what kind of images we can expect. It sounds like they have some fun plans. Schwehm mentioned that the OSIRIS wide-angle camera, which is a color framing camera with four times the pixels of Cassini, is planned to be pointed at the night side of Earth. (Rosetta is approaching Earth almost directly from the night side, as you can see if you visit the Solar System Simulator.) It'll be observing urban regions in Asia, Africa, and Europe at night. Also, Rosetta happens to be passing by Earth close to the cloud of dust left in the wake of comet Tempel-Tuttle that produces the annual Leonid shower (which peaks this year on November 18), so the nightside imaging might possibly yield some views of shooting stars from space. That would be awfully cool if it works out. The camera will also do lunar spectrophotometry of the Moon -- should produce some pretty Moon photos -- and take wide-angle shots of the Earth-Moon system from a distance after the flyby, when both are illuminated by the Sun. Again, let me emphasize that these are all only preliminary plans, which may suffer last-minute change or cancellation, but I wanted to give you an idea of what might result from the flyby.
There are, of course, lots of other instruments on Rosetta and its lander Philae besides the camera. ALICE, the ultraviolet spectrometer that is sister (or maybe aunt?) to the instrument of the same name aboard New Horizons, will be taking spectra of the sunlit side of Earth and the Moon for calibration purposes. MIRO, the microwave experiment, will be performing a dry run of an observation sequence intended for use during Rosetta's distant flybys of asteroids 2867 Steins on September 5, 2008 and 21 Lutetia on July 10, 2010. MIRO will be searching for four volatile molecular species at these targets. During these asteroid flybys, the spacecraft will have such a high velocity relative to the asteroids, as it first approaches and then recedes from them, that the microwave radiation from these volatile species will be Doppler-shifted out of the detection range of MIRO unless the MIRO team accounts correctly for this Doppler shift. They will be testing this correction during the Earth flyby.
Finally, several of the fields and particles instruments, including ROMAP (lander magnetometer), RPC (orbiter's plasma instrument), and SREM (orbiter's radiation monitoring experiment) will be operating, measuring Earth's magnetic field and plasma environment. It's valuable for deep-space missions flying by Earth to take these kinds of measurements, because they can tie measurements made by Earth orbiters to the space environment at greater distance from Earth. And, of course, radio science will be active, tracking the spacecraft's position and velocity in the sky.
Schwehm did not mention any plans to use the CIVA camera on Philae to repeat that out-the-side-window view that they accomplished at Mars, which is too bad; but we should be thankful for any science or imaging from these critically important navigational events.
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