Not Yet in Orbit, Messenger Returns Important Mercury Data

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NEW YORK — A NASA spacecraft aimed at Mercury has already returned valuable observations despite still being months away from entering orbit around the small, rocky world, the mission’s lead scientist said.

Sean Solomon, principal investigator for NASA’s Messenger mission, said the spacecraft is poised to enter orbit around Mercury next March to build the most detailed maps ever made of the planet.

“Mercury is not what we thought it was even two-and-a-half years ago,” Solomon said during a public lecture in late July at the American Museum of Natural History here as part of the 73rd annual meeting of the Meteoritical Society.

After three flybys of Mercury, Messenger has beamed home stunning views of the cratered world and provided a fresh look into its volcanic past and tenuous atmosphere. But the best is still to come, Solomon said.

NASA’s Messenger probe, short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, is the first spacecraft to examine Mercury up close since the space agency’s Mariner 10 mission in the mid-1970s. In addition to the new global maps, Messenger scientists hope the mission will find new clues on how Mercury was formed, how it evolved, and how it generated its magnetic field.

“We’re trying to learn how a planet very near its host star differs from others that are more far out and more massive,” Solomon said. “Exploring the inner part of our solar system is to understand our place in the solar system.”

While Messenger’s main mission is to orbit Mercury, the probe had to perform six different flybys that took it past Earth once, Venus twice and Mercury three times. Each time, the spacecraft used the gravity of each planet flyby to refine its flight path through space.

The gravitational tugs from the flybys also work to slow the spacecraft’s speed enough so that Messenger’s propulsion system can successfully execute its scheduled “orbital insertion” in March 2011, putting the spacecraft in an elliptical orbit around Mercury.

“We could have gotten to Mercury in four or five months, as Mariner 10 did, but we can’t get into orbit flying that fast,” Solomon said. “We have to be going by Mercury slowly enough so that the propulsion system can perform the orbit insertion.”

Three of the six flybys had Messenger swooping by Mercury itself — in January and October 2008, then again in September 2009.

The flyby events allowed the probe to snap pictures of Mercury’s surface, enabling scientists to map the planet in unprecedented detail. The first flyby alone returned 1,317 images.

In one of the flybys, Messenger also was able to take measurements of the planet’s magnetic field over one hemisphere. Mercury was found to have a dominantly dipolar magnetic field — with opposite magnetic poles — with the dipole being closely aligned with the planet’s spin axis, Solomon said.

This seems to suggest that Mercury’s magnetic field is similar to Earth’s, and was possibly generated in the same way. Once Messenger enters the orbital stage of its mission, researchers will be able to study this in greater detail. Scientists also will be interested in looking for evidence of volcanic processes on Mercury, Solomon said.

Messenger is expected to enter Mercury’s orbit on March 18, 2011. The propulsion system will fire for 15 minutes, placing the spacecraft in an initial orbit around Mercury at an altitude of 200 kilometers.

Each elliptical orbit will last 12 hours, meaning the probe will circle the planet twice per Earth day. At the time of orbit insertion, Messenger will be 46 million kilometers from the sun, and 155 million kilometers away from Earth.

The $446 million Messenger spacecraft, built by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., launched in August 2004 and is funded to orbit Mercury for one Earth year, which is equivalent to just over four of Mercury’s 88-day years. Solomon is currently writing a proposal to extend the mission beyond the initial timeframe.

“We think we have ample propellant not only for that one year, but probably for a year or two after that,” Solomon said. “But we’ll eventually run out of propellant or NASA dollars, and that will cause the probe to impact the surface.”

Still, the Messenger probe has already yielded important results, and has paved the way for future missions to our innermost planet — including a joint European Space Agency and Japan Aerospace Exploration Agency mission that will launch two spacecraft in 2014 to study Mercury.

And beyond that, Solomon has his sights set on a future sample-return mission to Mercury.

“It’ll probably not happen nearly as soon as one to an asteroid and eventually Mars, but I think it’s the next thing to do,” Solomon said.