The European Space Agency’s Rosetta spacecraft yesterday performed ESA’s closest-ever Earth fly-by, gaining an essential gravity boost in its ten-year, 7.1 billion kilometre flight to Comet 67P/Churyumov-Gerasimenko. At closest approach, at 22:09:14 GMT, Rosetta passed above the Pacific Ocean just west of Mexico at an altitude of 1954.74 km and a velocity relative to the Earth of 38 000 kph.
The passage through the Earth-Moon system allowed ground controllers to test Rosetta’s ‘asteroid fly-by mode’ (AFM) using the Moon as a ‘fake’ asteroid, rehearsing the fly-bys of asteroids Steins and Lutetia due in 2008 and 2010 respectively. The AFM test started at 23:01 GMT and ran for nine minutes during which the two onboard navigation cameras successfully tracked the Moon, allowing Rosetta’s attitude to be automatically adjusted.
Before and after closest approach, the navigation cameras also acquired a series of images of the Moon and Earth; these data will be downloaded early today for ground processing and are expected to be available by 8 March.
In addition, other onboard instruments were switched on, including ALICE (ultraviolet imaging spectrometer), VIRTIS (visible and infrared mapping spectrometer) and MIRO (microwave instrument for the Rosetta orbiter), for calibration and general testing using the Earth and Moon as targets.
The fly-by manoeuvre swung the three-tonne spacecraft around our planet and out towards Mars, where it will make a fly-by on 26 February 2007. Rosetta will return to Earth again in a series of four planet fly-bys (three times with Earth, once with Mars) before reaching Comet 67P/Churyumov-Gerasimenko in 2014, when it will enter orbit and deliver a lander, Philae, onto the surface.
The fly-bys are necessary to accelerate the spacecraft so as to eventually match the velocity of the target comet. They are a fuel-saving way to boost speed using planetary gravity.
Yesterday’s fly-by came one year and two days after launch and highlights the valuable opportunities for instrument calibration and data gathering available during the mission’s multi-year voyage.
In just three months, on 4 July, Rosetta will be in a good position to observe and gather data during NASA’s spectacular Deep Impact event, when the Deep Impact probe will hurl a 380 kg projectile into Comet Tempel 1, revealing data on the comet’s internal structure. Certain of Rosetta’s unique instruments, such as its ultraviolet light instrument ALICE, should be able to make critical contributions to the American mission.
Rosetta is the first mission designed to both orbit and land on a comet, and consists of an orbiter and a lander. The spacecraft carries 11 scientific experiments and will be the first mission to undertake long-term exploration of a comet at close quarters. After entering orbit around Comet 67P/Churyumov-Gerasimenko in 2014, the spacecraft will release a small lander onto the icy nucleus. Rosetta will orbit the comet for about a year as it heads towards the Sun, remaining in orbit for another half-year past perihelion (closest approach to the Sun). Comets hold essential information about the origin of our Solar System because they are the most primitive objects in the Solar System and their chemical composition has changed little since their formation. By orbiting and landing on Comet 67P/Churyumov-Gerasimenko, Rosetta will help us reconstruct the history of our own neighbourhood in space.