La mission Rosetta est l'une des grandes aventures spatiales des 15 années à venir, consistant à aller se poser sur la comète 46P WIRTANEN pour y étudier in situ ses caractéristiques et faire ainsi progresser les connaissances sur l'origine du système solaire. Les sites suivants renseignent sur les grandes lignes de cette mission:

Voici une compilation des explications utiles aux développements abordés dans les exercices qui suivent.

The Ariane-5 rocket lifts off from Kourou. After burnout of the lower stage, the spacecraft and upper stage remain in Earth parking orbit (4000 km x 200 km) for about two hours. Ariane's upper stage then ignites to boost Rosetta onto its interplanetary trajectory, before separating from the spacecraft. Sujet traité dans l'exercice I

3 - Mars Flyby (26 August 2005) : Objet de l'exercice II tremplin sur Mars
Rosetta flies past Mars at a distance of about 200 km, obtaining some science observations. An eclipse of the Earth by Mars lasts for about 37 minutes, causing a communications blackout.

5 - Otawara Flyby (11 July 2006):
The spacecraft goes into passive cruise mode on the way to asteroid Otawara. Rosetta observes the tiny asteroid from a distance of about 2200 km. Science data recorded onboard are transmitted to Earth after the flyby.

7 - Siwa Flyby (24 July 2008):
Once again, the spacecraft is put back into passive cruise mode on its way to the large asteroid Siwa. Flyby operations are similar to the Otawara flyby, although at a greater distance (about 3500 km).

8 - Deep Space Hibernation and Comet Rendezvous (June 2009 – November 2011):
After a large deep-space manoeuvre, the spacecraft goes into hibernation. During this period, Rosetta records its maximum distances from the Sun (about 800 million km) and Earth (about 1 billion km). The spacecraft is reactivated prior to the comet-rendezvous manoeuvre, during which the thrusters fire for several hours to slow the relative drift rate of the spacecraft and comet to about 25 m/s.

Rosetta's 10 year expedition will begin in January 2003, with an Ariane-5 launch from Kourou in French Guiana. The three-tonne spacecraft will first be inserted into a parking orbit, before being sent on its way towards the outer Solar System.

Unfortunately, no existing rocket, not even the powerful European-built Ariane-5, has the capability to send such a large spacecraft directly to Comet Wirtanen. Instead, Rosetta will bounce around the inner Solar System like a cosmic billiard ball, circling the Sun almost four times during its eight-year trek to Comet Wirtanen. Whilst following this roundabout route, Rosetta will enter the asteroid belt twice and gain velocity from gravitational 'kicks' provided by close flybys of Mars (2005) and the Earth (2005 and 2007).

The spacecraft will eventually arrive in the comet's vicinity in November 2011. Rosetta's thrusters will then brake the spacecraft, so that it can match Comet Wirtanen's orbit. Over the next six months, it will edge closer to the black, dormant nucleus until it is only a few dozen km away. The way will then be clear for the exciting transition to global mapping, Lander deployment and the comet chase towards the Sun.

5 September 2002
There will be greater tension than usual among engineers and scientists at Europe's spaceport at Kourou, French Guiana, in January 2003, as they gather to see ESA's comet-chasing spacecraft Rosetta departing on its long journey. If it is to keep its rendezvous with Comet Wirtanen in 2012, Rosetta must lift off on its Ariane-5 launcher no sooner than 03:40 CET on 13 January 2003 and no later than the end of that month.

This span of suitable dates is called a launch window. For interplanetary missions, such windows are much stricter than for satellites orbiting the Earth. To send a spacecraft from the ever-moving Earth to a planet or a comet following another course through space is highly complicated. Timing is everything.

Before it can meet Comet Wirtanen, far out in space, Rosetta first has a series of planetary appointments to keep. With each close fly-by of a planet, it receives an energy boost because of the planet's gravitational pull. The spacecraft is due to pass by Mars in August 2005, then do high-speed fly-bys of the Earth in November 2005 and November 2007.

In a way, Rosetta is like a passenger on a train journey involving several changes. Unless the first train leaves right on time, with the spacecraft on-board, it will miss the later connections. If it departed after 31 January 2003, Rosetta would be unable to reach the target comet.

"The cosmic clock of the Solar System fixed our launch date when Comet Wirtanen was selected as Rosetta's target ten years ago," comments John Ellwood, project manager for the mission. "Although there are risks in a precise, rather short launch window, it's had the advantage that everyone concerned knew there was no room for discussion - they had to be ready."

Besides the restricted span of launch dates, there is also a tight limit on the time of day at which Rosetta can leave Earth. Because the Earth rotates, Kourou must be correctly positioned in relation to the direction in which the spacecraft must head off, on the first leg of its interplanetary journey. The daily window is about 20 minutes, during which time the Earth rotates through 5 degrees.

In May 2003, similar concerns about a launch window will preoccupy the engineers and scientists of ESA's Mars Express mission, at the Baikonur Cosmodrome in Kazakhstan, in the former Soviet Union. There the launcher will be a Soyuz-Fregat rocket. Scientists have always planned to use the especially favourable relative positions of Earth and Mars occurring in mid-2003 (and not repeated until 2020) for Mars Express to have an express flight to the Red Planet.

Opportunities to fly to Mars occur every 26 months, but the travelling distance varies a lot because the orbit of Mars is elliptical, that is, egg-shaped. The 2003 opportunity coincides with a time when the Earth is about to overtake Mars, as the planets orbit around the Sun, and when Mars happens to be in the closest sector of its orbit. The Mars Express launch window opens at 20:41 CET on 23 May 2003 and closes at 17:47 CET on 21 June 2003.