At 2:04 p.m. Eastern time, Messenger will flit past the planet 124 miles above its surface at a blink-and-you-miss-it speed of 141,000 miles an hour. It's the first of three Mercury flybys the craft will perform in preparation for orbiting the planet, beginning in March 2011.
During the pass, the craft's camera is set to grab more than 1,200 exquisitely detailed images of the surface, covering large swaths of the planet that no human has ever seen before. Other instruments will provide the first assay of minerals and chemical elements on the surface, as well as measure Mercury's gravity and magnetic fields.
It's all in service to the question: What does this oddball planet tell us about how the solar system, especially the rocky inner planets, evolved?
"Mercury has been the Cinderella of the solar system for a long time, caught in the shadow of stepsisters Venus and Mars," says Sean Solomon, who heads the Department of Terrestrial Magnetism at the Carnegie Institution of Washington and is the mission's lead investigator. Between NASA's Messenger mission and missions to Mercury that Europe and Japan are pursuing, "that's about to change."
Mercury's evolutionary story is key to understanding the inner moons and inner planets of the solar system, notes Daniel Baker, who heads the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder and is a member of the Messenger science team. "These close neighbors of the Earth are really important for us to understand our own origins."
Mariner 10 logged three Mercury flybys in 1974 and '75 and mapped 45 percent of the planet's surface. Since then, scientists have had to rely on ground-based optical and radio telescopes to pursue the mysteries Mariner left behind. Mercury is a forbidden target for the Hubble Space Telescope; the planet orbits too close to the sun.
"Probably the first thing most of us want to see is what the other 55 percent of Mercury's surface looks like," says Faith Vilas, director of the MMT Observatory on Mt. Hopkins in Arizona and a Messenger scientist. At first, it wouldn't seem as though there would be much difference. But Dr. Vilas recalls two Mariner flybys of Mars, which showed a crater-pocked surface similar to the moon's. It took the third flyby to reveal large volcanoes and yawning canyons and chasms, she says, adding: "We can't get cocky about this and say: Oh, it's going to look like this."
Over the years, scientists have used observations and Mariner data to build an ambitious Messenger agenda. One thread involves the planet's magnetic field. Mercury is the only inner planet besides Earth with a global magnetic field, although it's weak compared with Earth's. Mercury's high density points to an iron core that scientists estimate accounts for 65 percent of the planet's mass and fills about 75 percent of the planet's interior. An active magnetic field would require at least part of that core to be molten but the planet's surface shows no sign of tectonic activity that testifies to the action of a molten outer core, as it does on Earth.
Some say because the planet appears to have cooled rapidly early in its history, its core has solidified and any magnetic field is a remnant captured in rocks.
But last May, a team of astronomers using radio-telescopes in Puerto Rico, West Virginia, and California reported that they had measured tiny changes in the planet's rate of spin. This suggests at least a partially molten core perhaps kept molten by sulfur, which lowers iron's melting temperature.
Mercury's high density, small size, and other aspects of its structure make it possible that it formed elsewhere in the solar system, some say. The idea is driven in large part by what astronomers have seen in other solar systems: Gas-giant planets that orbit so close to their parent star that there is no plausible explanation for how they could have formed there.
Mercury could have formed out near the orbit of Mars, then gotten smacked by another large object. The impact would have knocked off much of Mercury's original surface. And it would have triggered Mercury's migration to its current orbit.
"What we see could be residual mantle and core," Dr. Vilas says.