MESSENGER successfully flies by Mercury

The MESSENGER probe zipped past Mercury yesterday in the first of three flybys needed before it settles into a regular orbit. NASA scientists say the probe survived the encounter, which brought it to within 200 km (124 miles) of the planet’s surface.

MESSENGER is the first probe to visit Mercury in 30 years, so planetary scientists are excited to see what new data the new probe returns. MESSENGER (short for “MErcury Surface Space ENvironment GEochemistry and Ranging” — NASA must have a special office to devise names like this one) will investigate Mercury in ways that were not possible when Mariner 10 paid it two flyby visits in 1974. Mariner 10 was able to image only one side of Mercury.

Here’s an image of Mercury taken by MESSENGER on its approach Sunday, from 760,000 kilometers away (about twice the distance from the Earth to the Moon). (Click on the image to see a larger version.)


After two more flybys this year and next, MESSENGER will settle into a regular orbit in 2011 when it will begin a longterm study of the innermost planet, the first in its history. The peculiar trajectory is a fuel-saving measure, to enable the probe to “catch up” to Mercury in its fast orbit around the Sun. [Earth travels at an average of 30 km/s; Mercury at 48 km/s. A more direct route would require burning a lot of fuel to speed the probe up and change its orbit.]

Mercury is a bit of a puzzle, so scientists hope MESSENGER can answer a few questions about it. Why is Mercury so dense for its size? Why do Earth and Mercury have magnetic fields, but Venus and Mars do not? What creates Mercury’s magnetic field, since its interior structure is different from Earth’s? Is the shiny stuff seen in craters at the poles ice? What does the unimaged side look like? Can we understand something about the geology of Mercury, and can it help us understand more about Earth’s formation and geology?

The smallest of the four terrestrial planets (Mars, Venus and Earth are larger, in that order), Mercury zips around the Sun in 88 days. It rotates around its axis three times for every two revolutions around the Sun, so daylight on Mercury lasts for 176 Earth days. There is no atmosphere to speak of, and the terrain is rugged and desolate, resembling that of the Moon.

Sending people to visit Mercury would not be impossible, but they would need to plan their excursions carefully. Daytime temperatures average around 350° C (662° F), and the ultraviolet radiation would be fierce (no atmosphere means no protective ozone layer), so walking around the surface during daytime would be inadvisable. Nighttime temps average -170° C (-274° F), so any visitors would probably explore the surface at night, properly bundled up of course.

Of the nine planets, we know least about Mercury and Pluto. [Yes, I know Pluto has been reclassified as a dwarf planet. So what?] So anything MESSENGER returns is bound to be interesting. NASA may have some more details about this recent flyby later in the week.

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