From Near Sun Comes a Probe's-Eye View of Earth and Moon

A new photo shows the Earth and moon as seen from deep space by a NASA probe near the sun.

The picture was captured by the Messenger spacecraft May 6 while it was making its way toward Mercury. Both Earth and its only natural satellite are visible in the lower left of the photo; the moon sits to the right of Earth.

When it snapped the picture, Messenger was about 114 million miles (183 million km) from Earth. By comparison, the average distance between the Earth and sun is about 93 million miles (150 million km).

The moon, separated from Earth by a mere 238,000 miles (384,400 km), appears tiny in this photo, but in the solar system it's actually the largest natural satellite relative to the size of its planet. The moon is 25 percent as wide as Earth and about 1.2 percent as massive.

The Earth-moon relationship is special in other ways, too. For example, it is unlikely Earth captured a fully formed moon through its gravitational pull. Instead, astronomers think a Mars-size body slammed into Earth about 4.5 billion years ago, blasting a huge chunk off our planet. This ejected material then accreted and compacted into the moon.

Messenger has been snapping nice photos of Earth and Venus from time to time since its 2004 launch, but its primary mission is to scope out Mercury. The name "Messenger" is a NASA acronym for Mercury Surface, Space Environment, Geochemistry and Ranging.

Mercury — both the smallest planet in the solar system and the one closest to the sun — remains relatively little-known. The only spacecraft to have visited it was NASA's Mariner 10 in the 1970s.

Astronomers have tasked Messenger with helping them answer a few key questions about Mercury. They want to know why, for example, the planet is so dense. Mercury is the densest world in the solar system, and measurements suggest a metal-rich core makes up more than 60 percent of its mass. Messenger will aim to determine the size of the core and whether or not it is molten.

Astronomers also hope the probe will help them understand Mercury's magnetic field and its geological history.

Because Mariner 10 photographed less than half the planet's surface, Messenger is showing scientists parts they haven't seen before. The probe has already made three flybys of the planet from its solar orbit, photographing most of the planet's surface.

A month ago, Messenger was at perihelion — its closest approach to the sun during its solar orbit. That put it in a good spot for another of its assignments: to look for the small asteroids thought to be in the space between Mercury and the sun. These asteroids — called vulcanoids — may or may not exist; astronomers hope Messenger can help settle the question.

So far, Messenger has been studying Mercury from afar. In March 2011, though, it is scheduled to move into orbit around Mercury and stay there for about a year. Astronomers hope the probe's data will yield clues not just about Mercury's nature but about the formation of the solar system's other rocky planets: Venus, Mars and Earth.

• Video: Messenger Probe Views Earth in Flyby
• Top Ten New Mysteries of Mercury
• Top Ten Cool Moon Facts