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Spacecraft Buzzes By Mercury
Riding with Robots writes "The robotic spacecraft MESSENGER is making its second fly-by of the first planet today, skimming just 200 kilometers above the surface. The fly-by will reveal portions of the planet that have never been seen before, but the main purpose of the maneuver is to prepare for an orbital insertion in 2011. The mission site offers extensive information, along with the first pictures that are already arriving on Earth, with many more expected in the coming hours and days."
Because it is energetically tough to get to Mercury they are trying to get into with as little fuel expenditure as possible, to send as much payload as possible. Since there is no atmosphere, aerobraking is not possible, and thus they are using gravity assists to help reduce the orbital insertion delta-v to a manageable number. Each flyby speeds up the spacecraft a little, to better match Mercury's orbital velocity, and they decided on 3 of these to get the performance they wanted. There is a synodic period (the orbital beat period) between each such opportunity, so it takes a while to complete three flyby gravity assists.
The mission FAQ has more information on this.
Ah, thanks! Very interesting.
FRY: This is a great, as long as you don't make me smell Uranus. Heh heh.
LEELA: I don't get it.
PROFESSOR FARNSWORTH: I'm sorry, Fry, but astronomers renamed Uranus in 2620 to end that stupid joke once and for all.
FRY: Oh. What's it called now?
PROFESSOR FARNSWORTH: Urectum.