Comet-chasing spacecraft has a flyby date with asteroid
Hurtling more than 33,500 miles an hour toward a comet coming from the far reaches of our solar system, a European spacecraft on Saturday will take a relative moment out of its long journey to rendezvous with a mysterious asteroid, 21 Lutetia.
Flying to within about 2,000 miles of Lutetia’s surface, the Rosetta orbiter will analyze the asteroid’s surface and composition to discover whether it’s metallic or filled with organic, carbon-based molecules.
Rosetta has been on course since 2004 to meet up with Comet Churyumov-Gerasimenko, which is approaching the sun from the outer solar system. Rosetta is set to meet up with the comet in 2014 and then escort it around the sun. So Saturday’s fly-by isn’t the spacecraft’s ultimate purpose, but that makes it all the more exciting to some astronomers.
It’s “basically a goodie — an-add-on we can do, which is very interesting,” said Rosetta mission manager Gerhard Schwehm. Not only might scientists finally be able to solve the “what’s-it-made-of” question, they’ll get close-up images of the asteroid.
Lutetia — one of the largest asteroids in the belt that sits roughly between Mars and Jupiter — was discovered more than 200 years ago, but much about the asteroid remains unknown. Estimates of its length have ranged from 59 to 83 miles. And its potential composition — metallic, organic or perhaps a combination — has proved puzzling. Previous scans using such tools as radar and spectroscopy have showed signatures of both carbonaceous and metallic asteroids, Schwehm said.
In the two hours that the comet-chaser will be watching the asteroid, Rosetta will try to determine Lutetia’s age, shape and whether the asteroid has a magnetic field and an exosphere. An exosphere is a very thin layer of gases between a celestial body’s surface and the rest of space; if it exists around Lutetia, analyzing it could help determine whether the asteroid is carbonaceous or metallic.
If Lutetia turns out to be metallic, that would make this fly-by the first to document a metallic asteroid. This would be of great help to astronomers, because metallic asteroids are thought to be shards from the cores of far larger bodies and thus could reveal much about long-gone giants.
Asteroids themselves are objects left over from the time when the planets were being formed, so any information about Lutetia’s composition will likely reveal information about the development of our early solar system, astronomers and planetary scientists said.
And given how few asteroids have been so closely visited, said Northern Arizona University astronomer David Trilling, every little bit of data helps.
“We’re not at the point where you have one asteroid and one more is boring … we have the potential to learn quite a lot,” said Trilling, who is not involved in the European Space Agency-led mission.
Rosetta conducted a similar fly-by of the much smaller main-belt asteroid Steins in 2008, and was able to determine its dimensions and describe its shape.
Pulling off such a maneuver is “really eye-of-the-needle amazing, because you’re trying to arrive at a location in space and time at the same time this asteroid is zipping by,” said Joseph Hora, an astronomer at the Harvard-Smithsonian Center for Astrophysics who is not involved in the mission.
Images of the asteroid will be sent back shortly after Lutetia comes into view — but since the probe is already two-thirds of its way to reaching the comet, there will still be a 25-minute delay each way as messages travel the great distance between Rosetta and Earth. (It takes light about eight minutes to reach the earth from the sun, by comparison.)
This fly-by presents astronomers with a unique opportunity before the main event, Hora said. “There’s not going to be another chance to do this, I don’t think, in the foreseeable future.”
A webcast of the fly-by will be available around 9:30 a.m. PDT on Saturday at the European Space Agency’s website.
