Standard Mercury Orbit, Mr. Sulu

Mercury Has a Long-Term Visitor

MESSENGER at Mercury (artist's conception). Courtesy MESSENGER Mission.

Last night I went down to the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado for a briefing and “wait-it-out” event for the MESSENGER spacecraft’s orbital insertion. The lab built one of the instruments onboard the spacecraft — the Mercury Atmospheric and Surface Composition Spectrometer (MASCS). As I was sitting there watching the folks in the control room at the Johns Hopkins Applied Physics Laboratory (by remote link), I thought about all the times we’ve seen “standard parking” orbit commands applied in shows like Star Trek. They make it sound so easy. The captain just says “Standard orbit” to the helm, the officer punches a few buttons or slides a fader or waves a hand over the console, and the ship slides into the correct spot.  What we don’t see are the ship’s thrusters firing to nudge the ship — a massive behemoth compared to a small planetary probe like MESSENGER —  from its previous course into the parking orbit around the world it’s visiting.

We didn’t actually see MESSENGER’s thruster fire either last night. What we heard was a stream of announcements that told us what the Doppler readings were from the spacecraft as its fuel tanks fed the thruster that nudged it into a parking orbit (highly elliptical at that!) around Mercury.  You can see an animation of how it might have looked here.

To see the kind of orbital trip MESSENGER has taken since its launch on August 3, 2004 , go here for an animation showing its long journey. As of today, the spacecraft has traveled 4,902,668,000 miles. That’s 7,890,000,000 kilometers, or 52.7 astronomical units. If Messenger had traveled in a straight line for that distance, it would be well beyond the most distant point of Pluto’s orbit!

MESSENGER's orbit around Mercury is very elliptical, meaning its lowest point is only about 124 miles (200 km) above the surface and gets as far asway as 9,300 miles (15,000 km) at its most distant. The closest approach of the orbit will take it high over the north pole, where there is some radar evidence of something -- possibly water ice -- hidden on crater walls. Courtesy NASA/JHUAPL.

MESSENGER is small, only about the size of a minivan, so it wouldn’t take nearly as much power to put it into “standard orbit” as a giant starship would need.  But, the principles are the same no matter what size of object you’re trying to put in orbit around another one.  The incoming object has a certain path it’s following, and it’s going at a certain speed. If you want the spacecraft to merely fly by, you wouldn’t need to deviate the probe from its path.  But, if you want it to go into orbit, you have to slow it down at the right place, the right time, and at the right rate. Once you do that, you’ve got your spacecraft (regardless of its size) at a point where the gravitational pull of the planet it’s orbiting is EXACTLY matched by the spacecraft’s speed around the planet. It’s a constant tug of war that must be balanced correctly.

Now that MESSENGER is safely orbiting Mercury, scientists are testing its systems to make sure they’re doing okay in the hot, harsh environment around the planet.  The spacecraft is subject to solar heating eleven times hotter than we experience here on Earth, and it is protected with a sunshade to protect the delicate instruments inside. As soon as they’re satisfied that all systems are working on the nominal, they’ll turn the instruments on and commence the next phase of mission science.  The first images should start streaming to Earth in the first part of April, so stay tuned!