NASA's Messenger spacecraft successfully achieved orbit around Mercury at 9pm Eastern Thursday night, the first time a spacecraft has accomplished this feat, NASA announced.
While in orbit, Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging) will be as close as 124 miles from the surface of Mercury. It is now 28.67 million miles from the sun and 96.35 million miles from Earth.
Around 8:45pm, the spacecraft's main thruster was fired in order to slow it down enough to be "captured" by Mercury, NASA said. It fired for about 15 minutes and slowed Messenger by 1,929 miles per hour, a process that consumed 31 percent of the propellent that the spacecraft had at launch. It had about 9.5 percent of its fuel left after the orbit insertion process, but NASA said it will have plenty of propellant left to complete its mission.
Data will be collected by Deep Space Network antennas and transferred to the Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.
In the next few weeks, APL engineers will make sure the spacecraft's systems are functioning in Mercury's harsh thermal environment. Mercury is close enough to the sun that it's 11 times brighter on the planet than it is on Earth, and surface temperatures can reach about 840 degrees Fareheit. There are pockets of the planet, however, that are constantly shaded, so temperatures there can drop to minus 350 degrees Farenheit. But Messenger was built using a composite material that makes the spacecraft relatively light, and it will stay cool via a sunshade made of heat-resistant ceramic cloth, NASA said.
If all goes well, the spacecraft's instruments will be activated on March 23 and the primary science phase will begin on April 4.
Messenger has been prepping for this journey for quite some time. It launched in August 2004, and completed an Earth flyby in August 2005. Over the next several years, it completed two flybys of Venus and three Mercury flybys before entering the planet's orbit last night. Only one other spacecraft - the Mariner 10 - has visited Mercury. That was back in 1974 and 1975, when Mariner 10 made three flybys to take photos of the planet's surface. Unfortunately, Mariner 10 ended up capturing images of the same side of Mercury each time, so Messenger is intended to explore the sides we've never seen before.
There are six key questions NASA hopes to answer during Messenger's year-long orbit of Mercury: why is Mercury so dense; what is the planet's geological history; what is the nature of Mercury's magnetic field; what is the structure of Mercury's core; what are the unusual materials (ice?) at Mercury's poles; and what volatiles are important at Mercury?