Šteins Up Close

Hours after its successful flyby of “the jewel of the solar system”, the robotic traveler Rosetta continues to return data from the encounter. From a point of light discovered in 1969 into a world of new vistas, here is Asteroid (2867) Šteins:

Asteroid Steins seen from a distance of 800 km
Credit: ESA ©2008 MPS for OSIRIS Team MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/IDA - Asteroid Steins seen from a distance of 800 km

At a press conference to announce preliminary results and to show the first images of Šteins, European Space Agency (ESA) scientists compared the shape of the asteroid to that of a jewel and a diamond. The predicted shape model based on data obtained from Earth-based telescopes held up during the encounter, but the asteroid turned out to be nearly 10% bigger than expected. At its largest diameter Šteins measures 5.9 kilometers, and at its smallest 4.0 kilometers.

Asteroid Steins mosiac
Credits: ESA ©2008 MPS for OSIRIS Team MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/IDA - Asteroid Steins mosiac

Šteins is marked by a large 2.0 kilometer crater on its top hemisphere and a chain of seven craters that stretch from the rim of this large crater down the side of the asteroid. Whether or not these are secondary craters (fall back of material from an initial and larger impact) or a distinct primary impact event is unclear. Scientists counted 23 craters and believe they are of a variety of ages, suggesting a complex impact history.

To date, spacecraft have now explored eight asteroids up close. Šteins is different from the previously explored asteroids, however; it is an E-type body, a member of an asteroid family that is iron-poor and silicate rich compared to other families. Scientists will pour over the new data to determine the asteroid’s composition, mineralogy, topography, age, spin axis orientation, and impact history.

Anaglyph image of Steins
Credits: ESA ©2008 MPS for OSIRIS Team MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/IDA - Anaglyph image of Steins

Angioletta Coradini, Principal Investigator, announced that her team got a jump on calibration efforts using data from the VIRTIS (Visible and Infrared Mapping Spectrometer) instrument. She said that with quick input from the navigation team, a stable spacecraft platform, and extensive planning prior to the flyby, the team was able to provide nearly real-time calibration for the other instruments.

Gale Winters, ESA Director of Operations and Infrastrucuture, used the press conference to announce the Space Situational Awareness proposal. This proposed ESA and European program will conduct surveillance and tracking of a variety of objects in space, including Near-Earth asteroids that pose impact risk. The program will also support imaging of these objects and monitoring of space weather. Program researchers will use new data like those provided today from Rosetta to better understand these impact threats.

According to Andrea Accomazzo, Spacecraft operations Manager, the Šteins flyby pushed Rosetta to the limits of its capabilities. The complex flyby required thermal protection of the spacecraft’s back end, where the Philae lander awaits its future encounter with Comet 67P/Churyumov-Gerasimenko. In addition, poorly constrained orbital information based on monitoring from Earth-based instruments required autonomous fine-tuning from Rosetta itself. Rosetta began tracking Šteins on August 4, 2008 to better calculate its position and make the necessary course corrections to bring the spacecraft within 800 kilometers of the asteroid’s surface. The spacecraft came within two kilometers of this target distance.

Rosetta remains in good shape for the rest of its mission. The NAC (narrow-angle camera) went into safe mode just prior to the encounter due to conservative guidelines meant to protect the instrument for its primary mission at Comet 67P/Churyumov-Gerasimenko. The camera went back into operations not long after the encounter and successfully took more images as Šteins receded.

Next up for Rosetta: a flyby of Earth in November 2009 and a flyby of Asteroid (21) Lutetia on June 10, 2010. Rosetta will arrive at its final destination Comet 67P/Churyumov-Gerasimenko in May 2014.

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Richard Leis

Richard Leis is a writer and poet living in Tucson, Arizona. His poetry has been published in Impossible Archetype. His essays about fairy tales and technology have been published on Tiny Donkey and Fairy Tale Review’s “Fairy-Tale Files“.

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