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.
Studies of Mercury from Earth are difficult because from our planet's vantage point, Mercury never strays more than 28 degrees from the sun. That's too close even for orbiting telescopes like Hubble to image for fear of damaging the sensitive optical equipment by overexposure to harmful solar rays.
Ground-based observatories are developing new techniques to work around Mercury's tough neighborhood, but the science planned for MESSENGER's year-long mission has the potential to open entirely new perspectives on how the planets formed.
For example, one of the most debated questions about Mercury is why it is about as dense as Earth, though it has just 40 percent of the real estate. Scientists estimate Mercury's core must be about 75 percent iron, or iron mixed with other metals. Yet the planet also has a magnetic field, which on Earth is generated by a conductive molten core.
In a planet as small as Mercury, however, a core should have cooled and solidified a long time ago.
One possible explanation is that Mercury wasn't always number one in the parade of planets. Some astronomers suspect Mercury migrated to its present position after a cosmic collision by the Mars freeway. The solar system was a nasty place back then, an over-crowded autobahn before Jupiter arrived, with its great massive gravity, to impose order like a traffic cop.
The crash may have cost Mercury dearly, arriving at its new location stripped of crust and missing mantle, a fraction of its former size and mass. The heavy metal core remaining would have then found balance in one of the harshest environments imaginable, so close to the sun that an average day on the Tropic of Capricorn registers more than 400 degrees Fahrenheit.
MESSENGER may solve the mystery. Its science instruments include mineral detectors and chemical analyzers scientists can use to determine, for example, how materials on Mercury's surface formed and if they have been ground cover since inception or if they bear signs of a more internal origin.
During Monday's close approach, MESSENGER will soar 146 miles over the planet's equator, closer than Mariner ever flew and in position to study the 55 percent of Mercury never seen. Scientists will have to wait about a week to get all the data collected during the flyby relayed back to Earth.
A second pass is expected in October and a third and final flyby in September 2009. When MESSENGER returns in March 2011, it will be going slow enough to be captured by Mercury's relatively puny gravity and slip into orbit to become its first artificial satellite. A joint European/Japanese mission slated to launch in 2013 will add a second companion six years later.
NASA is spending about $446 million for MESSENGER, which is designed to last about a year.
"It is an understatement to say that the science team is extremely excited," said Solomon, the lead MESSENGER researcher. "It's our first glimpse of Mercury in 33 years. It's been a really long road and we're about to see some payoff."
Related Links:
Irene Klotz's blog: Space Diary
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