NASA’s Messenger exploratory vehicle achieved orbit around planet Mercury on March 17, and has now sent back the first close-up image of the planet.
From the image, Mercury looks much like our moon. A continuous mapping program will begin April 4. You can keep track of progress here. Messenger’s orbit is highly elliptical around Mercury, varying from 124 miles to 9,300 miles above the surface.
Why explore Mercury? According to NASA:
The importance of Mercury, beyond that we know so little about this planet, is that it offers a chance to examine another outcome of the processes that also produced Earth, Venus, and Mars. Learning how Mercury ended up the densest planet (after correcting for internal pressures) will tell us much about planetary formation. Discovering how Mercury has sustained a magnetic field while larger bodies either have lost an earlier field (as Mars did) or have no present field or a record of a past field (Venus) will help us to understand magnetic field generation in our own planet.
Mercury also has the thinnest atmosphere among all the terrestrial planets and an incredibly wide temperature range. In fact, temperatures vary from nearly the highest in the solar system (at the equator) to among the coldest (in the permanently shadowed areas where ice deposits seem to lurk). Documenting the nature of Mercury’s tenuous and changeable atmosphere and the composition of its mysterious polar deposits – thought by many to consist of water ice – will give us new insight into the volatile materials in the inner solar system.
On its sunny side, Mercury can reach a scorching 700 K (about 800°F). But because it has virtually no atmosphere to hold in that heat, temperatures drop to about 90 K (about -300°F) on the dark side.