Rosetta’s blind date with Lutetia

Artist’s impression of Rosetta about to rendezvous with Comet 67P/Churyumov-Gerasimenko in 2014.

Artist’s impression of Rosetta's rendezvous with Comet 67P/Churyumov-Gerasimenko in 2014.

  • Rosetta probe on its way to a comet
  • Will fly past an asteroid on July 10
  • Comet rendezvous due in 2014

ESA’s comet-chaser spacecraft Rosetta is heading for a blind date with asteroid Lutetia. Rosetta does not yet know what Lutetia looks like up-close, but beautiful or otherwise, the two will meet on July10.

Like many first dates, Rosetta will meet Lutetia on a Saturday night, flying to within 3,200 km of the space rock. Rosetta started taking navigational sightings of Lutetia at the end of May so that ground controllers can determine any course corrections required to achieve their intended flyby distance.

The close pass will enable around two hours of good imaging. The spacecraft will instantly begin beaming the data back to Earth and the first pictures will be released later that evening.

Rosetta flew by asteroid Steins in 2008, and other space missions have encountered a handful of asteroids. Each asteroid has proven to be an individual and Lutetia is expected to continue the trend.

An animation of asteroid (2867) Steins

An animation of asteroid (2867) Steins, which was visited by Rosetta in September 2008.

The mystery of Lutetia

Although recent high-resolution ground-based images have given some idea of the overall shape of Lutetia, astronomers no idea what it looks like in detail. Rosetta will tell us that.

Orbiting in the main belt of asteroids between Mars and Jupiter, initially it was thought that Lutetia is around 95 km in diameter but only mildly off-circular. Recent estimates suggest 134 km, with a pronounced elongated shape. Rosetta will tell us for certain and will also investigate the composition of the asteroid, wherein lies another mystery.

By any measure, Lutetia is quite large. Planetary scientists believe that it is a primitive asteroid, left on the shelf for billions of years because no planet consumed it as the Solar System formed. Indeed, most measurements appear to back this picture, making the asteroid out to be a ‘C-type’, which contains primitive compounds of carbon.

However, some measurements suggest that Lutetia is an ‘M-type’, which could mean there are metals in its surface. “If Lutetia is a metallic asteroid then we have found a real winner,” says Rita Schulz, ESA Rosetta Project Scientist.

That’s because although metallic asteroids do exist, they are thought to be fragments of the metallic core of larger asteroids that have since been shattered into pieces. If Lutetia is made of metal or even contains large amounts of metal, Dr Schulz says that the traditional asteroid classification scheme will need rethinking. “C-class asteroids should not have metals on their surfaces,” she says.

A busy fly-by

Asteroid science stands to gain once this observational conundrum is resolved because Rosetta’s data will provide a valuable collection of ‘ground truths’ that can be used to resolve conflicting ground-based observations not just 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.

Artist’s impression of Rosetta as it flies by asteroid Steins

Rosetta encountered asteroid Steins in 2008. Next stop is asteroid Lutetia on July 10, 2010.

Good contact is essential because the uncertainties in the asteroid’s position and shape may demand last minute fine-tuning to keep it centred 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,” says Andrea Accomazzo, ESA Rosetta Spacecraft Operations Manager.

Stay in touch with the flyby as it happens by visiting the Rosetta blog.

Mission to a comet

Rosetta’s 11-year expedition began in March 2004, with an Ariane 5 launch from Kourou in French Guiana, and the spacecraft was then sent towards the outer Solar System. The long journey includes three gravity assists at Earth (2004, 2007, 2009), one at Mars (2007), and two asteroid encounters: (2867) Steins (2008) and (21) Lutetia (2010).

After the third Earth-gravity assist and a large deep-space manoeuvre, the spacecraft will go into hibernation (July 2011 – January 2014). During this period, Rosetta will record its maximum distances from the Sun (about 800 million kilometres) and Earth (about 1 thousand million kilometres).

The spacecraft will be reactivated prior to the comet-rendezvous manoeuvre, during which the thrusters will fire for several hours to slow the relative drift rate between the spacecraft and comet to about 25 m/s.

Built by EADS Astrium, the Rosetta probe consists of a 3,065-kg spacecraft (1,578-kg dry mass) designed to enter orbit around the comet’s nucleus in August 2014 after a series of gravity assist manoeuvres to gain enough orbital energy, with three swing-bys at Earth (March 2005, November 2007 and November 2009) and one at Mars (February 2007).

The spacecraft carries 11 science instruments to probe the comet’s nucleus and map its surface in fine detail. It will also land a package of instruments (the Philae Lander) to study some of the most primitive, unprocessed material in the Solar System.

The mission will provide clues to the physical and chemical processes at work during the formation of planets, beginning 4.6 billion years ago.

Adapted from information issued by ESA / C.Carreau / AOES Medialab / J. Huart.

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