Rosetta, an 11 year trek to the heart of a comet!

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Comets are thought to represent icy fragments of part of the 'builder's rubble' left over from the construction of the solar system and as such their study can provide vital clues to the formation of the planets and the evolution of the Earth itself.

As a result several nations have launched a series of missions to study these 'dirty snowballs' including STARDUST, CONTOUR and the VEGA spacecraft. Building upon their experience of sending spacecraft to study comets, in particular the pioneering Giotto mission to Halley, the European Space Agency (ESA) has funded a new and ambitious probe, called ROSETTA, to fly out to another comet in early 2003.

The target of this mission is Comet 46P/Wirtanen, a short period comet with an orbital period of just under 6 years, which was discovered in 1948. This particular type of comet was chosen due to their predictable and relatively close orbits around the sun which are within range of present day launch systems and Wirtanen was chosen especially as it will be in the "right place at the right time" when the spacecraft is launched to complete the mission.

In contrast to Giotto, which was a close approach flyby mission, Rosetta will actually enter orbit around the target comet and dispatch a small lander onto its icy surface.

The Rosetta hardware actually consists of two main components, a box like main spacecraft orbiter bus which includes the communications, power and propulsion systems, as well as the orbital science payload, with a separate lander, called RoLand, which will descend to the surface of the comet and conduct in situ studies of the nucleus.

The main orbiter is very similar in design to the Mars Express spacecraft, the common design and manufacturing processes greatly reducing costs and build time. The lander is a nifty 100kg package of science instruments designed to return the first ever data direct from the surface of a comet's nucleus.

However there are also some significant differences in the spacecraft resulting from the much longer and more complex mission profile being undertaken by Rosetta. These include large solar panel arrays to provide adequate power for the spacecraft when it is far from the sun, at times up to 675 million km, where the sun is 20 times fainter than in Earth orbit. A highly sensitive and accurate communication system is required to maintain contact with the Earth at such long distances and to transmit the large volumes of scientific data from the 11 scientific instruments on board.

In addition the onboard computer system has to be smart enough to handle this complex suite of instruments, navigate the spacecraft when the communications time lag with Earth is too great to allow control from mission control and also to monitor the onboard systems while the probe is hibernating on its long cruise to Comet Wirtanen.

The main orbiter carries a set of 11 scientific instruments, from US as well as European research teams, divided into 6 suites dealing with different aspects of the science mission. The imaging and remote sensing instruments, which will study the comet in the visible, UV, IR and microwave spectral bands (ALICE, OSIRIS, VIRTIS and MIRO), are complemented by instruments which will measure the composition of the comet, its dust and gas (ROSINA, MODULUS,COSIMA and MIDAS).

The other instruments will look at the structure of the nucleus (CONSERT), the dust flux (GIADA), plasma environment (RPC) and carry out radio studies (RSI). All the instruments are fitted to the top deck of the orbiter which will constantly face the nucleus of the comet during the orbital phase of the mission. The lander is also carrying a suite of 10 instruments to study the surface and composition of the nucleus, including a small drill (SD2) to obtain samples from below the surface.

Rosetta will weigh a chunky 2.9 metric tons at launch in January 2003, with over 50% of the mass being onboard propellant to keep the spacecraft moving throughout its 11 year mission, particularly during the delicate and complex maneuvers in orbit around the comet nucleus.

In addition, Rosetta must also climb up out of the plane of the ecliptic to meet its target. As a result, ESA will use Europe's most powerful launcher, the Ariane 5 to boost Rosetta towards its destination. Even then the flight will also require a gravity assist flyby of Mars in 2005 and two similar flybys of Earth, one in 2005 and another in 2007 to put Rosetta on track for rendezvous with Wirtanen in November 2011.

The primary aim of the Rosetta mission is to study the evolution of the comet as it swings in towards perihelion, observing changes in the nucleus as it is heated by the sun. However, enroute to Comet Wirtanen, Rosetta will wake from its slumbers to observe two asteroids at close range, within 1000km. The first is a small 20km diameter body called 4979 Otawara in July 2006, followed by a flyby of the much larger, 110km diameter 120 Siwa in July 2008. To maximize the science return, Rosetta will first rendezvous with the comet in November 2011, matching velocities, and then drifting along carrying out observations until orbital insertion around the nucleus in the late summer of 2012.

Once in orbit, mission control will refine the course of the spacecraft using images sent back by the probe to move Rosetta into a tighter orbit, as close as 2km above the nucleus, to provide the most detailed views yet obtained of a comet. Once detailed mapping has been completed and a landing site selected, Rosetta will release the lander which will descend onto the surface and secure itself in position against the comets feeble gravity using a harpoon fired into the surface.

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The lander will operate for at least a month, transmitting its data back to Earth via the orbiter which will continue to observe the perihelion passage of the comet until the expected end of the mission in July 2013.

Named after the Rosetta Stone, the key to unlocking the secrets of ancient Egypt, it is expected that the modern Rosetta will prove instrumental in unraveling an even older mystery, the birth of the Solar System.

Update on Mission Progress:
Rosetta’s January 2003 launch was scrapped in favor of a search for a different destination. In May, the ESA Science Program Committee chose this new target. Launched at last on March 2, 2004, Rosetta’s launch delay is instead taking it to comet 67P/Churyumov-Gerasimenko in 2014. Rosetta was launched from Kourou, French Guiana, on an Ariane 5 G+.

Before arriving at its destination and releasing its rover Philae, Rosetta will fly by asteroids 2867 Steins on September 5, 2008, and 21 Lutetia July 10, 2010. Philae is named for an island on the Nile where a bilingually-inscribed obelisk was found that included the names of Egypt’s final rulers in hieroglyphs, helping a French historian decipher the Rosetta stone.