The Rosetta orbiter, which carries the lander Philae, has completed more than two thirds of its journey to the comet Churyumov-Gerasimenko. The most comprehensive cometary investigation ever, the mission will deliver the Philae lander to the comet’s surface for in situ studies. On their way to the comet, the spacecraft and lander are currently performing a close flyby of 21 Lutetia, a large Main-belt asteroid, on July 10. The Philae lander is operated by the German Aerospace Center (DLR) in Cologne, Germany.
Since launch, Rosetta has traveled roughly 3 billion miles. The solar-powered orbiter was launched on an Ariane 5 rocket in 2004. It has used several gravity-assist maneuvers—three from Earth and one from Mars—to gain the necessary momentum, refine its trajectory, and match the orbit of the comet once it reaches the outer Solar System. There, the orbiter will circle the comet and, after delivering the lander Philae to the surface, eventually escort the comet on its way to the Sun.
At about 60 miles in diameter, 21 Lutetia is one of the larger main belt asteroids. The lander will investigate whether this asteroid has a magnetic field and an atmosphere, and study their characteristics.
Three instruments on the lander will be switched on during the flyby:
(1) The Rosetta Lander Magnetometer and Plasma Monitor, ROMAP, a magnetometer and plasma monitor that will study the local magnetic field and monitor the interactions between the asteroid and the solar wind.
(2) MODULUS PTOLEMY, one of two evolved gas analyzers, which obtains accurate measurements of isotopic ratios of light elements by heating solid samples to release volatiles.
(3) The Cometary Sampling and Composition experiment, COSAC, which is also an evolved gas analyzer. It detects and identifies complex organic molecules from their elemental and molecular composition.
The ROMAP instrument will be measuring continuously while it is turned on, and will be looking for interactions between the asteroid’s magnetic field and the solar wind. COSAC and PTOLEMY will perform a series of “sniff” measurements (five by PTOLEMY and two by COSAC), which will be used to help determine whether or not the asteroid has any kind of atmosphere.
At closest approach (about 2,000 miles from the asteroid), Rosetta will be traveling past Lutetia at about 10 miles per second—or 34,000 miles per hour. This is comparable to sending a radio-controlled car down a freeway at roughly 60 miles per hour to take pictures of a stationary object it passes in the next lane (about 20 feet away), with the exact timing of the commands fixed a month in advance. If that doesn’t sound hard enough, the planning would also have to be done from so far away that the freeway would be located twice as far from the planner as the Moon is from Earth.