NASA Spacecraft Set to Shine Spotlight on Mercury

coondoggie writes to tell us Network World is reporting that NASA will this month see the realization of a mission launched in 2004, sent to explore the planet Mercury. "MESSENGER, launched in 2004, is the first NASA mission sent to orbit Mercury, the planet closest to the sun. But on Jan. 14 it will pass close by the planet and use Mercury's gravity for a critical assist needed to keep the spacecraft on track for its ultimate orbit around the planet three years from now. Still, the spacecraft is also expected to throw back some never-before -seen images, NASA said. The flyby also will gather essential data for planning the overall mission. After flybys of Earth, Venus, and Mercury, it will start a year-long orbital study of Mercury in March 2011, NASA said. "

Comments

Why so long . . .

[StefanJ]

It's hinted at in the story, but the reason the probe is taking its sweet time to actually achieve an orbit is Mercury's high orbital velocity.

It's pretty easy to get into an elliptical orbit which stretches from Earth's orbit around the Sun to Mercury's orbit around the sun. But getting into a circular orbit means matching Mercury's velocity, and doing so in a way that lets a "burn" be made to actually enter into an orbit around the planet. As I recall, you need a total velocity change of 40 kps to get into orbit around Mercury. That more than twice the change required to get into an orbit around Mars.

It's pretty impressive that NASA figured out a way to do this with a gravity assist. A proposed European probe would have used an ion rocket to make the velocity change.

Re:Why so long . . .

[evanbd]

Well, delta-v is usually treated as a positive scalar value in orbital mechanics. The propellant needed to change your velocity by (say) 3 km/s is independent of whether you're speeding up, slowing down, or changing direction. So, while velocity is a vector quantity with direction and magnitude, delta-v is usually treated as a simple positive-valued scalar. (At least when the impulse comes from a high-thrust rocket engine; for very low thrust things like ion engines, or weird things like solar sails, the problem changes somewhat.)

Not the first mission to Mercury

[cusco]

This isn't the first mission to Mercury, just the first mission to ORBIT the planet. Mariner 10 swung by the planet several times.

"http://www.jpl.nasa.gov/missions/past/mariner10.html"

It was also the first mission to use a gravity assist. At the time of launch the rotation period of Mercury was unknown. By an amazing coincidence, every pass of the spacecraft photographed the SAME FACE of the planet, as its rotation period matched exactly the interval of Mariner 10's return.

Re:Refrence for Mecury day

[Technician]

Mercuries rotation is synchronized with its' orbit in such a fashion that the same portion always faces towards/away from the sun.

http://www.enchantedlearning.com/subjects/astronomy/planets/mercury/

"Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that same face to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years. Mercury is the only body in the solar system known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonances at all)."