Messenger Rendez-Vous with Venus

In a rehearsal for the big event of meeting Mercury up close, the Messenger space probe is going to swing by Venus in a slingshot maneuver that will propel it towards the smallest planet in our solar system.

NASA's spacecraft Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging) is a mission launched August 3, 2004, designed to study the characteristics and environment of Mercury from orbit. Specifically, the scientific objectives of the mission are to characterize the chemical composition of Mercury's surface, the geologic history, the nature of the magnetic field, the size and state of the core, the volatile inventory at the poles, and the nature of Mercury's exosphere and magnetosphere over a nominal orbital mission of one Earth year.

Today, June 5th, the spacecraft will fly over Venus to use its gravity as a break, to slow down the probe, enough to enter Mercury's orbit. The deceleration will modify the present traveling speed of 22.7 miles per second and will make it reach 17 miles per second (36.5 to 27.8 km/s), just enough to place the craft on a stable orbit.

"This change in Messenger's velocity is the largest of the mission," said Messenger mission systems engineer Eric Finnegan, of the Applied Physics Laboratory at John Hopkins University.

The maneuver should be spectacular, as Messenger will be coming in close to the planet's surface, at an altitude of 200 miles (320 km) and a speed of 30,000 miles per hour (48,000 km/h) and will help it make some interesting scientific observations, that were impossible to make during the first flyby, in October 2006, during the planet's superior conjunction at the time.

"Because of superior conjunction, because we knew we were going into radio blackout, and we knew we had the second flyby coming up in June, we elected not to turn on any of the Messenger instruments at the time of that flyby," said the mission's principal investigator, Sean Solomon of the Carnegie Institution of Washington.

The second flyby will not only be a rehearsal for the actual rendez-vous with Mercury, but it will also be a test for the spacecraft's sensors and scientific instruments, which will be turned on simultaneously, allowing scientists to test and calibrate them before turning them onto their main planetary objective next January.