Mercury nets spacecraft companion

Mercury was the fleetest of the Roman gods, the messenger of heaven. A tough guy to catch in a race. Catching up to Mercury, the god's planetary namesake, was no easy feat either.

That's why the engineers whose spacecraft this week caught up to the world named for the wing-footed deity, the fleetest planet in our solar system, perhaps deserve some victory laurels.

"We hit it right on all the orbital numbers. It went perfect," says MESSENGER mission engineer Eric Finnegan of the Applied Physics Laboratory in Laurel, Md.

Consummating a six-and-a-half year pursuit, NASA's $446 millionMESSENGER spacecraft fell into orbit around Mercury on Thursday evening, following a 15-minute rocket firing. (MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, surely one of the most contrived acronyms in the space agency's Inquisition-like history of torturing names.) To hit its mark, the rocket burn only exceeded its planned time by 4 seconds, Finnegan says. "After six-and-a-half years, we think that's pretty close."

But that's not all. The trip also required one flyby of Earth (in 2005), two flybys of Venus (2006 and 2007) and three flybys (2008, and two in 2009) of Mercury itself, all to set up the spacecraft for its capture by the planet's gravitational pull on Thursday night. The trips were more than just opportunities to glimpse the scenery, they made it possible for the spacecraft to make its trip to the inner reaches of our solar system.

Mercury zips around the sun at 107,000 miles-per-hour, finishing a year in 58.7 days, while Earth circles at a more stately 66,600 mile-per-hour, completing a year in 365 days. Which might make you think that getting your spacecraft to speed up is the ticket to catching Mercury. But it is not so simple.

"When you are falling toward the sun, which wants to pull you in, you are picking up speed and you have to slow down," Finnegan says. Each flyby provided a "gravity assist" in aerospace engineering parlance, a way to pick up or drop some speed from a planet while circling around the sun, without burning any fuel.

Essentially, a gravity assist is a gravitational tug-of-war between a planet and a spacecraft, one that the big guy in the contest always wins in space, just like in life. Spacecraft mission designers have taken advantage of this inevitability since Mariner 10's 1974 visit to Venus. Here's how it works :

•Pass behind a planet as it circles the sun, and the planet's gravity will tug the spacecraft in the direction that both are headed, giving it a speed boost, or gravity slingshot.

•Pass ahead of a planet, and its gravity will pull back against the direction of the spacecraft, slowing the mission down.

NASA's Voyager 1 and 2 spacecraft, now headed right out of the solar system, used the gravity slingshot trick, for 35,700 mph speed boosts from Jupiter on their missions. MESSENGER's braking maneuvers on the other hand were much smaller (Finnegan compares them to "speed bumps") but they were absolutely necessary for the mission. "The majority of the energy we used to navigate was from the gravity assists," Finnegan says. "We could not have loaded enough fuel and still built the spacecraft, it would have been way too heavy, to just use rockets to get to Mercury."

So MESSENGER went like a stone skipping off the surface of a pond, slowing down with each gravity assist as it circled the sun, and shedding enough speed with each pass to put it into perfect position for settling into a gravitationally-balanced orbit around Mercury. In a sense, MESSENGER's final entry into orbit around Mercury was one last gravity assist, where it passed ahead of the planet, then fired its rockets to slow down and most important, line up the spacecraft into a successful planetary orbit.

"Through all the flybys, we effectively matched the speed of the planet," Finnegan says. "We wanted to slow just enough (with the rocket burn) for the planet to gently pull us into orbit."

Not too slow either, or the spacecraft would crash into Mercury. Instead, the planet essentially captured the spacecraft with its gravity, setting it on an egg-shaped orbit that takes MESSENGER to within 120 miles of Mercury's surface, but also sends it farther out in space, about 9,445 miles above the cool nighttime surface of the world, where the craft can cool off. The o rbit also follows an 80-degree tilt, with respect to Mercury's equator, unlike most planetary orbiters, which either circle the equator or poles of a world.

"A lot of the orbit is arranged so we can turn the spacecraft to Earth and transmit everything we find," Finnegan says. And a lot of it is arranged to keep a ceramic sunshade between the sun's 660-degree Fahrenheit temperatures and the spacecraft. "It might be room temperature behind the sunshade," he says.

In one romantic poem of the Middle Ages, Mercury, the Roman god, famously captured a nymph (named Cloris) with a magical net. Just so, Mercury, the planet, has now captured its own attendant. For at least the next year, the spacecraft will plumb the secrets of Mercury on a mission made possible through gravity, a magical net no matter the era.