As the World Turns:  MESSENGER's Home Movie of Earth
This image, taken while MESSENGER was 34,692 miles (55,831 kilometers) above Earth, shows the Galapagos Islands as tiny specks peeking through clouds. The line dividing day and night cuts through South America, with night about to fall on the western half of the continent. The large bright spot to the west of South America is the Sun’s light scattering off ocean waves.

A probe bound for the planet Mercury took one parting look at its home world, recording images during an Earth flyby that mission scientists have assembled into an eye-catching film.

During the first of a series of planetary flybys - though the only one to visit Earth - NASA's MESSENGER spacecraft photographed Earth with enough detail to pick out the Galapagos Islands from 34,692 miles (55,831 kilometers) away.

"The movie was kind of a bonus, really," Louise Prockter, MESSENGER's deputy instrument scientist, told "We were just very pleased everything went exactly as planned."

In addition to the Galapagos Islands, MESSENGER's flyby film shows a crisp Earth and the Sun's reflection off the Pacific Ocean, which moves as the planet rotates through one full turn. The spacecraft also photographed Earth in the near-infrared wavelengths, giving a different view on otherwise familiar continents.

Scientists compiled a movie out of MESSENGER's images of Earth during its Aug. 2, 2005 flyby. Credit: NASA/JHU/APL. Click to view.

"When we do these flybys, we need everything to work," said Prockter, who planned the MDIS camera sequences for the flyby, adding that the first science MESSENGER will do at Mercury occurs during a flyby.

The Earth flyby was the first of six planned for MESSENGER as it winds its way Sunward towards Mercury. The probe will fly past Venus twice and Mercury three times before finally settling into orbit around the rocky planet in 2011. The gravity assists the probe receives during each planetary pass help push it deeper into the inner solar system.

Launched on Aug. 3, 2004, MESSENGER swung past Earth almost one year later on Aug. 2, 2005. But it took weeks for mission scientists to compile the images taken by the spacecraft's Mercury Dual Imaging System (MDIS) into the final movie.

The process is one-part test and one-part practice. On one hand, mission scientists used the Earth flyby to calibrate MESSENGER's instruments and make sure they were in working order. But the flyby also gave researchers their first opportunity to work with in-flight images - such as MDIS photographs of Earth's moon also taken during the rendezvous - that resemble what they hope to see at Mercury.

"We do a lot of testing on the ground, but there's only so much we could do before launch," said Prockter. "There are always some little things that you didn't expect."

Unlike the two upcoming Venus flybys, only the Earth pass allowed researchers to make full use of MESSENGER's MDIS camera, as well as several of its six other instruments.

"Our instrument is not designed to look at Venus." Procktor said. "All we're going to see is a sort of bright, fuzzy blob."

MESSENGER also used its atmospheric and surface composition spectrometer to scan the moon, as well as particle and magnetic field instruments to study Earth's magnetosphere, researchers said.

"It's certainly performing to our highest expectations," Prockter said of MDIS, the probe's wide-angle camera. "We're going to a lot of effort to improve what we can."

MESSENGER is set to swing past Venus in October 2006, then again in June 2007 before finally reaching Mercury. But before the probe enters orbit around the small, rocky planet, it will execute three flybys - the first in January 2008, then again in October 2008, and finally in September 2009. The spacecraft is expected to enter orbit around Mercury in March 2011 as it passes over planet's dark side, providing the first-ever up close look at the region.

"For me, it's amazing that we've never seen the back side of Mercury close up," Prockter said, adding that she's looking forward to it. "We don't expect it to be [that] different from the rest of Mercury, but we still don't know. It certainly is quite mysterious."