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NASA probe set for close encounter with Mercury

Posted: September 28, 2009

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The MESSENGER probe is rapidly closing in on Mercury for a fleeting pass just 142 miles over its jagged surface Tuesday, a maneuver that will simultaneously serve scientific goals and bend the spacecraft's trajectory for its 2011 return to the innermost planet.

An image of Mercury captured by MESSENGER on Sunday as the spacecraft was about 418,000 miles from the planet. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
On Tuesday, the probe will use Mercury's gravity to slow its velocity by about 6,000 mph and reduce the time it takes to circle the sun by 13 days, according to Eric Finnegan, MESSENGER's mission systems engineer at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

"The maneuver itself is absolutely critical to the rest of our mission. We must fly by Mercury hitting the right aimpoint so that the rest of the trajectory is the one we want to be on for orbit insertion in March 2011," said Sean Solomon, principal investigator at the Carnegie Institution of Washington.

During the 2011 encounter, MESSENGER will fire its engines to enter orbit around the rocky planet for at least one year of extensive science investigations.

Tuesday's flyby is the last of six visits of Earth, Venus and Mercury since MESSENGER's launch in August 2004.

"After more than five years of exciting and scientifically rewarding flight, MESSENGER is now nearing its third flyby of Mercury," said Anthony Carro, the mission's program executive at NASA Headquarters.

Scientists are taking advantage of the swing by Mercury to conduct the $446 million mission's most ambitious science program since launch.

"A planetary flyby is very much like Christmas morning to the science team. We're looking forward to this flyby. We know there are presents under the tree. We don't know, in detail, what's inside each one of those boxes," Solomon said.

All eight instruments aboard MESSENGER will be active during the hours surrounding the spacecraft's closest approach at about 2155 GMT (5:55 p.m. EDT) Tuesday, Finnegan said.

Unlike MESSENGER's last two visits to Mercury, the probe's science package will be commanded to sweep across the planet to focus on several interesting features researchers identified from previous imagery.

Tuesday's flyby will be an important rehearsal for the spacecraft's planned orbit operations because the commanding sequence will be similar to science activities during the primary mission.

The maneuver will also bring MESSENGER closer to Mercury's equatorial surface than it will fly during its orbital mission. The craft's final orbit around Mercury will be closer to the planet's polar regions.

"It's our closest look at the equatorial surface, so all of our instruments are going to be taking optimum advantage of this last opportunity to be so close to this part of the planet," Solomon said.

Mercury is the least explored of the solar system's four inner planets.

"The one thing we know about Mercury is it's made out of denser materials than any of the other inner planets, including Earth," Solomon said. "And that speaks to some unusual aspects of how Mercury was assembled. It ended up much richer in metal than the Earth or any of the other inner planets."

About 90 percent of Mercury's cratered surface has now been imaged by visits from the Mariner 10 mission and MESSENGER. Another sliver of the planet will be unveiled Tuesday, bringing the revealed surface area to about 95 percent.

"Even though this is a mission that is going to come of age when we're in orbit...we've learned an awful lot about Mercury from these encounters. It has reawakened scientific interest in this innermost planet," Solomon said.

Mariner 10's three flybys of the planet in the 1970s studied less than half of the planet. MESSENGER's two previous visits filled in the glaring blanks and captured high-resolution images, analyzed the components of Mercury's surface and tenuous atmosphere, studied the dynamics of its strong magnetic field, and mapped the planet's terrain with unprecedented detail, according to Solomon.

A projection Mercury's surface showing the swath to be covered on Tuesday's flyby by MESSENGER's main camera. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
"We now have a nearly global view. We'll be missing only some polar areas that we'll capture in the orbital phase, and that is why...we have elected to use this flyby to do a lot of targeted observations," Solomon said.

Scientists have picked nine locations to study with MESSENGER's primary wide-angle camera and atmospheric and surface spectrometer.

"These are areas where the spacecraft is actually going to pause and stare at a single spot for about 35 seconds," said Noam Izenberg, instrument scientist for the Mercury Atmospheric and Surface Composition Spectrometer.

The targets include craters and other mysterious surface markings. Officials believe the full color imagery and high-resolution spectral data will reveal new details about the planet's formation and history.

"Each of these targets is going to get this finer full set of spectral data, which is going to be a tremendously exciting data set examining terrain types that we have not been able to see with this kind of fidelity on any of the previous flybys," Izenberg said.

Another focus during Tuesday's encounter will be additional observations of the planet's atmosphere and an attached comet-like tail of atoms being stripped away by pressure from solar radiation.

MESSENGER is already carrying out 7,000 stored commands for the flyby sequence.

The spacecraft will return 1,559 images during the core portion of the close approach. Finnegan said the instruments will collect more than 50 megabytes of data during the most intensive 37 hours of the maneuver.

MESSENGER turned its main communications antenna away from Earth at around 1430 GMT (10:30 a.m. EDT) Monday to begin its primary imaging phase for the flyby. Ground stations will only receive a tracking beacon signal from the spacecraft until it establishes high-speed communications again at about 0134 GMT Wednesday (9:34 p.m. EDT Tuesday).

The beacon signal will fall silent for about 51 minutes around MESSENGER's closest approach as the probe flies behind Mercury.

Data recorders will store information from the science instruments as MESSENGER buzzes Mercury at a relative velocity of more than 12,000 mph. The stored data will be played back to receivers on Earth beginning late Tuesday night.

Finnegan said the team's attention will next turn to preparations for MESSENGER's last deep space engine firing in November to tweak its path after the flyby.

After that burn, engineers and scientists will ready the spacecraft for the crucial orbit insertion maneuver after it arrives back in the vicinity of Mercury in March 2011.