Comet-Chasing Spacecraft to Fly By Asteroid
ESA’s Rosetta spacecraft flew by asteroid (2867) Steins on 5 September 2008. Steins was Rosetta’s first nominal scientific target. The spacecraft encountered the asteroid in the course of its first incursion into the main asteroid belt located between the orbits of Mars and Jupiter, while on its way to Comet 67P/Churyumov-Gerasimenko.
Credit: ESA, image by C.Carreau

A European comet-chasing spacecraft will zip by a large asteroid next month, snapping pictures all the way and potentially unlocking some of the mysteries surrounding the space rock.

The Rosetta spacecraft, operated by the European Space Agency (ESA) is set to glide past asteroid Lutetia on July 10. At closest approach, Rosetta will come within close to 2,000 miles (3,200 km) of the space rock.

And while that sounds far, the flyby will actually give Rosetta an approximately two-hour window of opportunity to take the first up-close images of the space rock and immediately beam them back to Earth. [Photos: Asteroid probe returns to Earth.]

Rosetta has been taking navigational sightings of Lutetia since late May so its ground controllers could determine if any course corrections would be needed to achieve the intended flyby distance.

In 2008, Rosetta flew by a different asteroid, called Steins, and a few other space missions have also encountered asteroids. Each encounter has led to different findings, and scientists are hoping that observations from the Lutetia flyby will contribute to the relatively small body of knowledge about asteroids.

The truth about asteroid Lutetia

Currently, not much is known about Lutetia, including what it looks like.

The enigmatic space rock, which is from the main asteroid belt that orbits between Mars and Jupiter, simply appears as little more than a single point of light to ground-based telescopes.

Preliminary observations suggest that Lutetia's continuous variation in brightness indicate that it is rotating and has an uneven surface. These observations have allowed astronomers to estimate the asteroid's shape and size, but their determinations are still varied.

Initially, astronomers thought that Lutetia had a diameter measuring about 59 miles (95 km), and is only mildly elliptical. Yet, more recent estimates suggest that the diameter could be 83 miles (134 km), with a pronounced elongation.

Rosetta could provide more conclusive evidence about the asteroid's dimensions and composition ? another area with more questions than answers.

A carbon or metal asteroid?

Planetary scientists believe that Lutetia is a primitive asteroid that was left on the sidelines for billions of years, since no planet consumed during the formation of our solar system. In fact, most of the current measurements seem to support this hypothesis, placing the asteroid in the 'C-type' category, which contains primitive compounds of carbon.

Yet, some measurements seem to suggest that Lutetia could be a so-called 'M-type', which would mean that there are metals on its surface.

"If Lutetia is a metallic asteroid then we have found a real winner," said Rita Schulz, Rosetta's mission project scientist at ESA.

Although metallic asteroids do exist, they are thought to be fragments of the metallic cores of larger asteroids that were shattered to pieces. So, if Lutetia is made of metal, or even contains large amounts of metal, Schulz stated that the traditional asteroid classification scheme would need to be revamped.

"C-class asteroids should not have metals on their surfaces," Schulz said.

The data collected from the Rosetta flyby will provide valuable observations for asteroid science, and will at least give scientists preliminary information that can then be corroborated through ground-based observations. And, the findings will not only apply for Lutetia, but for other asteroids as well.

For 36 hours around the moment of closest approach, Rosetta will be in almost continuous contact with the ground. The only breaks will come as Earth rotates and engineers have to switch from one tracking station to another.

Good contact is essential because the uncertainties in the asteroid?s position and shape may demand last minute fine-tuning to keep it centered in Rosetta?s instruments during the flyby.

"The skeleton of the operation is in place, and we have the ability to update our plans at any time," said Andrea Accomazzo, ESA Rosetta spacecraft operations manager.