On to Lutetia at 54,000 km/h

On its way to the comet Churyumov-Gerasimenko, the European probe Rosetta is soon to flyby its second asteroid: Lutetia.

 

Artist view of the Rosetta comet chaser approaching an asteroid. Credit: ESA, C. Carreau

A date has been made for Saturday evening, 454 million kilometres from the Earth, Rosetta is to fly past the asteroid Lutetia at a distance of 3,162 km. The final trajectory correction successfully took place on 18 June and the European probe is now perfectly lined up.

About four hours before the flyby, the probe will be turned so that its instruments are pointing towards Lutetia. Three hours before the flyby, it will switch to an autonomous navigation mode that will keep it directly in line with the asteroid. The high bandwidth link with the Earth is to be cut-off for 40 minutes so that the probe can flip over and keep its instruments pointed at its target as it flies by the asteroid at about twice the speed of a space shuttle - 15 km/s! - without, at the same time, having to impose an overly fast correction of the angling of its high-gain antenna. Rosetta will pass closest to Lutetia at 17:44:46 CEST (16:44:46s GMT). Due to the distance, the probe’s signal will take 25 minutes and 21 seconds to reach the Earth which is why the flyby is officially set for 18:10:07 CEST.

High bandwidth communications will be re-established 35 minutes later and the downloading of scientific data will begin two hours after the flyby. Some instruments will not stop with the flyby but will continue their observations for several more hours.

These first results will constitute a special evening on Monday 12 July 2010 at 20:30 in the large IMAX auditorium at the Cité de l'Espace in Toulouse, organised in partnership with the CNES and the 3AF. A panel of scientists will explain what has been learnt from the Lutetia flyby and the issues of the mission to the public.

Lutetia is already in sight, as shown on this picture taken by one of Rosetta's navigation cameras on 31 May. Credit: ESA

The mysteries of Lutetia
Lutetia is the 21st asteroid to have been discovered. It was identified on 15 November 1852 by a German amateur astronomer, Hermann Goldschmidt, from his Parisian home on the “Rue de la Comédie”. Over the next nine years, he was to identify another 13 and even believed that he had discovered one of Saturn’s moons, which quickly proved not to exist.

As with all the asteroids identified at this time, Lutetia is relatively large: it is believed to be a 134 km long ellipsoid which is, therefore, twice the size of Mathilde, the biggest asteroid to have been flown by to date. Its exact type is much debated. Some measurements would appear to indicate a type C asteroid, that is to say rich in carbon, but others could lead one to think that it is actually a type M asteroid, with traces of metals. In the first case, it would probably be debris from the formation of the solar system, but in the other, we could be dealing with a fragment of a bigger asteroid, destroyed during a collision.

Much is expected of the Osiris spectroscopic camera regarding the identification of the composition of Lutetia’s terrain and the lifting of the curtain as regards its true nature.

Moreover, Lutetia completes its orbit in 8 hours and 10 minutes, turning around a “slanting” axis on the ecliptic plane like the planet Uranus.

A quick summary of all the asteroids and comet nuclei already visited by space probes to date. The pictures have been resized at the same scale. Credit: DR.

Last step before the comet
For Rosetta, this encounter is the last of its long journey to its final objective. Launched by Ariane 5 on 2 March 2004, the probe had to go the long way around with gravitational assistance gleaned by flying by the Earth three times and Mars once in order to be positioned on a trajectory which would enable it to accomplish its mission: to position itself in orbit around the nucleus of the comet Churyumov-Gerasimenko in 2014, to set down a landing module (equipped with a harpoon for anchoring itself firmly) and to study “the awakening” of the comet as it approaches the Sun.

This astonishing circuitous route has already enabled the probe to travel more than 5 billion kilometres (further than the distance that separates us from Neptune) and to fly by Steins, another asteroid in the Main Belt, in September 2008.

Following its flyby of Lutetia, the probe is to be put into hibernation for 3 and a half years. It will only be reactivated in January 2014 in order to prepare for its date with the comet.