The long and winding road to a hot planet

Move over, Mars. Stand aside, Saturn. After 30 years orbiting in the scientific shadows, Mercury is about to get its day in the sun.

On Monday, the National Aeronautics and Space Administration is set to launch one of the most technically demanding planetary missions in its history - the Messenger mission to Mercury.

Not much bigger than Earth's moon, Mercury once was written off as a barren world too boring to bother with. Now, many researchers argue that Mercury holds the key to understanding the conditions from which the diverse range of inner planets formed some 4.6 billion years ago. And Mercury is expected to shed light on other star-hugging terrestrial planets that are expected to pop into view when a new generation of space-based telescopes is sent into orbit over the next 10 to 15 years.

"A mission to Mercury is really a mission to the innermost part of the nebula out of which planets formed," says Sean Solomon, director of the terrestrial magnetism department at the Carnegie Institution of Washington and the mission's lead investigator.

Until now, the US space program has largely ignored Mercury, notes Robert Strom, a University of Arizona planetary scientist who with colleague Ann Sprague published a book about the planet last year. "Mercury looked too much like the moon to a lot of people, and sending an orbiter there was extremely difficult," he says.

How difficult? Take heat, for instance. Mercury traces an elliptical orbit that ranges from 28.5 million miles to 43.3 million miles from the sun, compared with Earth's average distance of about 93 million miles. The planet's "day" lasts for two Mercury years - or 176 Earth days. To a hypothetical observer standing on the surface, that works out to 88 Earth days basking under a blazing sun roughly 11 times as hot as it appears at Earth. The temperature difference between Mercury's day side and night side spans 1,100 degrees F. With a tenuous atmosphere - dubbed an exosphere - that fails to trap heat, the surface radiates virtually all its heat back into space.

Thus, engineers need not only to devise a suitable shield to protect a spacecraft from the sun, but also to design instruments that can survive stresses imposed by staring at a surface that cycles from torrid to frigid and back on each orbit.

In addition, the energy demands to reach and circle Mercury are enormous. It's possible - and fastest - to reach the planet by making a beeline for it, as Mariner 10 did the last time NASA flew by 30 years ago. But a craft would gain speed relentlessly because of the sun's gravitational pull, requiring a braking rocket as big and expensive as the one that launched the spacecraft from Earth in the first place, says mission manager Robert Farquhar of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

These days, however, new lightweight materials, innovative instrument designs, and the creative use of gravity en route have dramatically cut the cost of sending a long-duration orbiter there and have given it a high probability of surviving in Mercury's demanding environment.

The trade-off: That three-month beeline to Mercury stretches into a 6-1/2 year trip when using the gravity of Earth, Venus, and Mercury itself to bleed enough speed from Messenger to allow Mercury to capture it. Messenger is expected to begin orbiting the planet in 2011.

Messenger carries a suite of instruments covering virtually as wide a range of scientific interests as those carried on behemoths such as the $3 billion Cassini spacecraft now orbiting Saturn. And while its sensors may not be as capable as those on Cassini - for size and weight reasons - it's tackling Cassini-size questions for one-seventh the cost. "We're getting a Mercedes for the price of a Volkswagen," says Dr. Strom, a Mariner 10 veteran who is part of Messenger's core science team.

Yet if changes in technology and a deeper understanding of orbital mechanics now make a visit to Mercury possible, Mariner 10 results and observations over the past 15 years have strengthened the scientific case for returning there.

"Mercury is an enormous gap in our knowledge of the solar system," Strom notes. "It's less well-known than any other planet."

To start with, more than half the planet is missing from earthlings' maps. Mariner 10 flew by Mercury three times. But it returned images of only 45 percent of the planet's surface. Earth-based radar slowly has started to fill some of the gaps. However, researchers say they need Messenger's detailed global images to more easily read the geological history written in its craters, faults, and other surface features.

Page: 1 | 2