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Planetary News: Asteroids and Comets (2008)

ESA's Rosetta Has Asteroid Steins in Sight

By Emily Lakdawalla
August 6, 2008

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Rosetta flies by an asteroid
Credit: ESA / AOES Medialab

ESA's flagship solar system mission Rosetta is fast approaching the next waypoint on its long journey to comet 67P/Churyumov-Gerasimenko. On September 5 at 18:37 UTC, the spacecraft will zoom past asteroid (2867) Steins, the spacecraft's first scientific target.

Steins has not before been visited by a spacecraft, so everything that is known about it is based on data from Earthbound telescopes. It is believed to be approximately 5 kilometers in diameter, and it is an E-type asteroid, a rare spectral class. The Rosetta team has planned a full slate of observations at Steins, aimed at refining calculations of its orbital characteristics, determining basic information about its shape, mass, density, surface composition, geomorphology, and its relationship to its nearby space environment. As valuable as these data will be to the scientific community, however, the real point of the flyby is to put ESA's most expensive planetary exploration spacecraft through its paces, an important step in preparations for its eventual orbital mission to a comet.

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The spacecraft has already completed a series of operations designed to ensure success in its encounter with Steins. In March of this year, a test simulated the complex series of maneuvers that Rosetta will have to perform as it passes by Steins at a relative speed of 8.6 kilometers per second (19,200 miles per hour) and attempts to keep its cameras focused on the tiny asteroid. This test was successful, and Rosetta was placed into a three-month hibernation on March 27. On July 1, Rosetta was awakened for tests and the delivery of new flight software for many of its instruments.

Approaching Steins

Since August 4, Rosetta's navigational cameras and OSIRIS imaging spectrometer have begun tracking the asteroid, still just a distant speck of light, 24 million kilometers away. The tracking operations are critical to the success of the flyby; knowledge of Steins' orbit is not yet precise enough to target Rosetta at exactly the desired distance as it passes by. Its position is only known to a precision of 100 kilometers. Data from the navigation camera and OSIRIS will be used to develop better models for the asteroid's orbit, improving the precision to about 2 kilometers. If they are needed, Rosetta can perform trajectory correction maneuvers at three opportunities, at 72, 36, and 12 hours before closest approach, to fine-tune the distance of the encounter.

The encounter distance -- 800 kilometers -- represents a balance between the desire for high-resolution imagery and the ability of Rosetta to track the fast-moving asteroid. At the high speed of the encounter, the asteroid will flash by, with the resolution of the cameras being 10 times poorer just 15 minutes before and after closest approach.

Science Plans

All of the instruments on the Rosetta orbiter will be active at some point during the encounter. The instruments which should provide the most easily interpretable data are the optical remote sensing instruments: the OSIRIS camera (the highest-resolution instrument on the orbiter) and the VIRTIS and ALICE spectrometers.

OSIRIS' narrow-angle camera has an angular resolution of 0.0186 milliradians (which is comparable to the resolution of New Horizons' MVIC imaging spectrometer, or about halfway between the resolutions of the wide-angle and narrow-angle cameras on Cassini). The navigational camera's resolution is about 5 times poorer than OSIRIS; it will not be able to spot Steins until about 11 days before the encounter.

High-resolution images of the asteroid taken by OSIRIS near closest approach will be among the first data returned from Rosetta following the flyby. Just two minutes before closest approach, Rosetta will observe Steins at "zero phase", meaning it will be fully lit by the Sun. The highest-resolution images could show features at about 15 meters per pixel; the asteroid should appear approximately 340 pixels across. OSIRIS data should help in the study of the shape, size, and topography of Steins, as well as with the properties of the surface.

The VIRTIS imaging spectrometer will obtain light curves and perform mineralogical mapping of the surface. A relatively low-resolution instrument, it will only be observing through the few hours around closest approach. Its data will be among the first to be transmitted back to Earth.

The ALICE ultraviolet spectrometer will search for an exosphere (a very, very thin atmosphere), and will obtain spectra of Steins' surface near closest approach. It will observe Steins from the day before the flyby through a few hours after.

A detailed timeline for the encounter is below.

Date/ Time (UTC)	Event
Aug 4 00:00	OSIRIS and NAVCAM begin optical navigation campaign
Sep 1 00:00	Other science instruments switched on
Sep 2 14:30	Trajectory Correction Maneuver This and the following two maneuvers may be canceled if optical navigation results indicate that the spacecraft is on the proper course for the encounter.
Sep 4 05:00	Trajectory Correction Maneuver
16:00	End optical navigation campaign The data must be returned to Earth quickly if the final Trajectory Correction Maneuver is to be performed.
Sep 5 05:00	Trajectory Correction Maneuver
08:00	Attempt to put cameras into tracking mode
17:57	Begin spacecraft flip The spacecraft must rotate into a particular orientation in order to track Steins throughout the flyby while also keeping sensitive parts of the Philae lander out of direct solar illumination. The flip takes about 20 minutes to complete; during this time, the cameras should still be tracking the asteroid.
18:18	Entry into Asteroid Fly-By Mode The spacecraft will now perform automatic tracking of the asteroid based upon information from the navigation cameras.
18:27	End telemetry from Rosetta The geometry of the flyby will result in the spacecraft's high-gain antenna pointing away from Earth. Earth will be out of communication with Rosetta for about an hour.
18:35	Rosetta views Steins at "zero phase" Rosetta will pass almost directly between the Sun and Steins, an unusual geometry that provides immensely valuable data on the way that the asteroid's surface reflects sunlight. The entire "globe" of Steins will be fully lit by the Sun.
18:37	Asteroid (2867) Steins Closest Approach (800 kilometers)
19:37	End Asteroid Fly-By Mode; start high-gain antenna rotation Control of the spacecraft's orientation will be handed back from the optical navigation system to the spacecraft's internal sequences. At the same time, the high-gain antenna will begin to rotate back towards Earth. The rotation will take 25 minutes to complete. It is possible that the spacecraft will remain out of communication with Earth for 22 of those minutes, until 22:02.
20:25	Resume telemetry transmission with Earth NASA's Goldstone radio antenna will receive Rosetta's communications.
21:25	Earliest possible start of scientific data reception on Earth The first five hours of downlink will contain data from the OSIRIS and VIRTIS instruments. Then two more hours of VIRTIS, followed by data from the rest of the science instruments.
Sep 6 14:01	End first downlink