Mercury to Get Its Best Close-Up Yet

Jan. 11, 2008 -- For a planet in first position, Mercury has been fairly ignored.

It's not that astronomers haven't wanted to find out more about the small, strange world. Actually they believe Mercury holds important clues to understanding how the solar system ended up a veritable cosmic zoo.

Just look at the four rocky planets, for example -- Mercury, Venus, Earth and Mars, which set up house closest to the sun -- and there is little family resemblance.

"Mercury … is a real oddball," said long-time Mercury researcher Sean Solomon, with the Carnegie Institution for Science of Washington D.C.

So far, only one spacecraft has ever visited Mercury, the 1970s-era Mariner 10. The best scientists could manage at the time was a series of three flybys that returned basically everything that is known about the first rock from the sun.

But Mercury isn't expected to hang on to its secrets much longer: a new robotic probe is on the way and this time, engineers have figured out how to get the spacecraft into orbit for a good, long look around.

The trick to settling so close to the sun is to slingshot off planets' gravity fields. The technique can be used to speed up a spacecraft's momentum, such as what the Pluto-bound New Horizons spacecraft did during a pass by Jupiter last year. It can also slow it down, which is the goal of the new Mercury probe, named MESSENGER. The moniker is an acronym for a mouthful: MErcury Surface, Space ENvironment, GEochemistry, and Ranging, which reflects the tall order riding on this diminutive but sophisticated probe.

Surfing planets' gravity fields may be an effective way to travel the solar system, but it's slow. Messenger departed for Mercury in 2004 and won't arrive for another 3.5 years. To arrive at the innermost planet and drop into orbit, six flybys are needed as well as a truckload of fuel to tweak the path and finally apply the brakes.

But after sweeping once by Earth and twice past Venus, all MESSENGER's remaining passes will be at Mercury, itself, offering scientists tantalizing appetizers of what is to come.

Mercury exploration 2.0 begins this weekend as Messenger prepares to fly by the hemisphere completely missed by the Mariner mission.

"We're expecting some pretty major surprises out of this," said Faith Vilas, director of the MMT Observatory in Arizona and a MESSENGER participating scientist.

"The first thing most of us want to see is what the other side … looks like. We can't get cocky about this and say 'Oh it's going to look like this,' because every solar system body looks very different from every other solar system body," she said.