Out of fuel, Messenger spacecraft will slam into Mercury

Gravity soon will kill the Messenger.

The NASA spacecraft that launched in 2004 from Cape Canaveral, Fla., and became the first to orbit Mercury in 2011 is on course to crash into the planet's surface April 30 at more than 8,700 mph.

"Messenger is going to create a new crater on Mercury at some point in the very near future," said John Grunsfeld, head of NASA's Science Mission Directorate. "Rather than be sad about that, we really are celebrating just a fantastic mission."

Messenger is short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging.

Mission scientists today said the spacecraft's mapping of the planet and discoveries about its makeup have reshaped understanding of Mercury and ideas about how early planets formed.

For example, Messenger confirmed the existence of ice deposits made of water at the poles of the planet closest to the sun.

Those deposits are covered by a mysterious dark layer of what could be organic material delivered by the same objects that brought the water ice.

The material offers "a record in these poles of the delivery, probably from the outer solar system, not only of water ice but of what on our planet were once some of the building blocks of organic chemistry and life, here on the planet closest to the sun," said Sean Solomon, the mission's principal investigator from Columbia University. "So Mercury therefore preserves a record of some very interesting processes in solar system history."

Solomon said findings about Mercury's surface composition would make scientists "reject most of the ideas for how Mercury was assembled as a planet at the beginning of the history of the solar system" and come up with new ideas to explain the chemistry.

Messenger already has run out of fuel, but engineers have come up with clever ways to extend its life, including using helium intended to pressurize propellant tanks to provide an extra boost.

One more orbital maneuver is planned April 24. Data collection will continue until 10 or 15 minutes before impact and will continue to be analyzed for years.

"We should be studying Mercury science long past when our crater is created," said Helene Winters, project manager from Johns Hopkins University's Applied Physics Laboratory.

NASA concluded a "celebration" of the mission with a Twitter message attributed to the spacecraft.

"Please give my thanks to the team," it said. "It's been a wonderful and exciting trip."

Mercury basics

NASA has had only two missions to Mercury, Messenger and a flyby of Mariner 10 in 1974 and '75. Still those who watched the sky have known of the planet since ancient times.

• Closest. It orbits about 36 million miles from the sun, the closest of the planets.

• Smallest. It is the smallest planet in our solar system, slightly larger than Earth's moon.

• Long. One of its days is the equivalent of 59 Earth days.

• Short. One of its years is the equivalent of 88 Earth days.

• Extreme. Daytime temperatures can reach 800 degrees Fahrenheit; night can be minus-290 degrees.

• Scarred. Mercury's atmosphere isn't thick enough to burn off meteors before they hit the planet's rocky surface, so it has many craters like our moon.

• Alone. It has no moon, and if it were in a direct line with Venus, it still would be more than 31 million miles away.

Source: NASA