NASA spacecraft now circling Mercury — a first

Messenger probe enters orbit more than six years after launch

A well-traveled NASA probe made history Thursday, becoming the first spacecraft ever to enter into orbit around Mercury.

NASA's Messenger spacecraft fired its main thruster in a 15-minute orbital insertion burn Thursday night to slow down by about 1,900 mph (3,058 kilometers), enough to enter Mercury's gravitational influence and settle into orbit around the planet.

NASA is still analyzing the spacecraft's trajectory, but a preliminary look suggests everything went as planned to put Messenger into its looping, 12-hour circuit around the solar system's innermost planet.

Mercury now has an artificial satellite, for the first time ever.

Messenger will soon begin mapping Mercury and studying the planet's composition, geology and tenuous atmosphere from its orbital perch. The mission's name is actually an acronym, standing for MErcury Surface, Space ENvironment, GEochemistry and Ranging.

"The spacecraft is ready, and the team is ready," Messenger mission operations manager Andy Calloway, of Johns Hopkins University's Applied Physics Laboratory, told reporters before the insertion burn. "On April 4, we'll begin prime science." [Photos of Mercury From Messenger Probe]

Messenger's arrival at Mercury marks the end of a long, circuitous journey for the probe, and the start of its main science operations. The Messenger mission first began development about 15 years ago, and the probe launched in August 2004.

Over the past six and a half years, Messenger has been a solar system wanderer, completing 15 orbits of the sun and traveling about 4.9 billion miles (7.9 billion kilometers). During this time, it made one flyby of Earth, two flybys of Venus, and three of Mercury, primarily to slow the probe down in preparation for Thursday's orbital insertion maneuver.

Messenger also took pictures and did some science work during these close encounters. Its Mercury observations were the first spacecraft data returned from the planet since NASA's Mariner 10 probe made three flybys in the mid-1970s.

But the Mercury flybys were a prelude to Messenger's chief mission — scrutinizing the planet from orbit for the next 12 months, researchers said.

Success wasn't guaranteed
Mission scientists planned for Messenger to slip into a highly eccentric orbit around Mercury. The spacecraft should come as close as 124 miles (200 kilometers) to the planet at times and retreat to more than 9,300 miles (15,000 kilometers) away at others, researchers said.

The mission team should begin learning soon — within the next day or so — how well Messenger's actual orbit matches up with the planned one. But just achieving Mercury orbit is a success, and not something to take for granted.

Japan's $300 million Akatsuki probe, for example, failed to enter into Venus orbit in December 2010 when its thrusters conked out early in its insertion burn. Akatsuki is still circling the sun, with another possible shot at Venus coming in late 2016 or early 2017.

Science work begins soon
Over the next 12 months, Messenger will map Mercury's surface in unprecedented detail and investigate the planet's composition, magnetic field, geologic history and its thin, tenuous atmosphere, among other features. Scientists hope the probe helps them better understand what makes the tiny planet tick.

"Mercury has been comparatively unexplored, considering its proximity in our solar system," said Messenger principal investigator Sean Solomon of the Carnegie Institution of Washington.

The overall goal of Messenger's mission is to use an increased understanding of Mercury to learn more about how our solar system — and solar systems in general — formed and evolved, scientists have said.

Over the next few weeks, the mission team will turn on and check out Messenger's suite of seven science instruments, making sure everything is working properly.

Messenger's first pictures of Mercury from orbit should start trickling out in about two weeks, Solomon said, but th