MESSENGER’s First Look at Mercury’s
Previously Unseen Side

This image was taken by NASA’s MESSENGER spacecraft at a distance of approximately 17,000 miles following the spacecraft’s closest approach to Mercury. The image shows features as small as 6 miles in size. Similar to previously mapped portions of Mercury, this hemisphere appears heavily cratered. It also reveals some unique and distinctive features.

Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

(JPL/NASA) When Mariner 10 flew past Mercury three times in 1974 and 1975, the same hemisphere was in sunlight during each encounter. As a consequence, Mariner 10 was able to image less than half the planet. Planetary scientists have wondered for more than 30 years about what spacecraft images might reveal about the hemisphere of Mercury that Mariner 10 never viewed.

The MESSENGER spacecraft observed about half of the hemisphere missed by Mariner 10. This image was snapped by the Wide Angle Camera, part of the Mercury Dual Imaging System (MDIS) instrument, about 80 minutes after MESSENGER’s closest approach to Mercury (2:04 pm EST), when the spacecraft was at a distance of about 27,000 kilometers (about 17,000 miles). The image shows features as small as 10 kilometers (6 miles) in size. This image was taken through a filter sensitive to light near the red end of the visible spectrum (750 nm), one of a sequence of images taken through each of MDIS’s 11 filters.

Like the previously mapped portion of Mercury, this hemisphere appears heavily cratered. It also reveals some unique and distinctive features. On the upper right is the giant Caloris basin, including its western portions never before seen by spacecraft. Formed by the impact of a large asteroid or comet, Caloris is one of the largest, and perhaps one of the youngest, basins in the Solar System. The new image shows the complete basin interior and reveals that it is brighter than the surrounding regions and may therefore have a different composition. Darker smooth plains completely surround Caloris, and many unusual dark-rimmed craters are observed inside the basin. Several other multi-ringed basins are seen in this image for the first time. Prominent fault scarps (large ridges) lace the newly viewed region.

Other images obtained during the flyby will reveal surface features in color and in much more detail. Collectively, these images and measurements made by other MESSENGER instruments will soon provide a detailed global view of the surface of Mercury, yielding key information for understanding the formation and geologic history of the innermost planet.

Messenger Flies Past Mercury in
Preparation for Permanent Mission

By Jessica Berman

(VOA) The U.S. space agency spacecraft Messenger swooped within 199-kilometers of the planet Mercury, Monday, in preparation for a permanent orbit, beginning in 2011. Messenger, which is operated by remote control from Earth, will soon begin beaming data back which scientists hope will answer questions about the planet closest to the Sun. Jessica Berman reports.

The unmanned spacecraft was launched by NASA in August, 2004. Messenger, stands for Mercury Surface, Space Environment, Geochemistry and Ranging.

Eric Finnegan is systems engineer for NASA’s Messenger Mission. Finnegan says Messenger made a nighttime approach toward Mercury at nearly 26,000 kilometers per hour. The planet’s gravitational pull slowed the spacecraft by 8,000 kilometers per hour for its eventual descent into orbit around Mercury in 2011.

Finnegan says Messenger sent back preliminary photos of the approach. He says the spacecraft will begin beam back more detailed imagery and data within the next day or two.

"We’re very, very excited. We had a very successful flyby," he said. "We’ll still still be waiting for the next day or so to get down imagery but our initial indications flying by the planet using just radio beacon information looks good."

Planetary scientists hope the $450 million mission will help them answer key questions about Mercury, a tiny planet with temperatures that swing between 315 degrees in the daytime to minus 180 degrees at night.

Astronomers say the planet is heavily cratered and has a large iron core.

"Mercury is really an oddball," said Louis Friedman who heads the Planetary Society. "It is a very dense solar system object. It’s very small. It’s in toward the Sun. It’s only about the size of the Earth’s moon, a little larger. And, as such, how did it form? And however it formed is going to tell us something about planet formation."

Friedman says the Milky Way is like a jigsaw puzzle; he says only when all the pieces are in place, and all the planets are explored, will astronomers understand how the galaxy was formed.

"Mercury is a very, very dense and very heavy object, and therefore getting a really good handle on that density and its mass and associated size will help us in trying to determine what happened at the time of planetary formation," he said. "How did it reach that size, how did it evolve and how did assume its final orbit?"

Messenger is about halfway through its journey to put it in permanent orbit around Mercury in 2011.

But, until then, there will be another flyby this October and again in September 2009.