WASHINGTON, Sept. 24 (UPI) -- NASA's latest mission to scan Mercury's surface for key rock features and mineral resources is set to fly by the planet next Tuesday as part of a plan to enter Mercury's orbit by 2011.
The Messenger spacecraft has flown past Mercury, the innermost and smallest planet in the Earth's solar system, twice before during its five-year mission.
On Sept. 29, Messenger is scheduled to pass within 142 miles of Mercury's surface to photograph its rock features -- and whatever else becomes visible. NASA said the craft will collect data vital for determining the planet's composition and its mineral content.
Photographs have shown Mercury as similar in appearance to the Moon, interspersed with craters and smooth plains.
Huge iron and titanium have been discovered on Mercury, which has a heavy iron core. Scientists have yet to determine how Mercury's rarefied atmosphere reacts to activity on the sun. Mercury temperatures range from about minus 297 degrees Fahrenheit to 801 degrees Fahrenheit.
NASA scientists also plan to look closely at low-energy neutrons that get absorbed in vast quantities as they strike surfaces packed with iron and titanium.
Messenger already has imaged more than 90 percent of the Mercury's surface, but this time instruments on board the spacecraft will attempt to view and photograph specific features that may reveal more information about the planet, its gravity and its mysterious comet-like tail, NASA said.
Noam Izenberg, scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., said, "Scans of the planet's comet-like tail will provide important clues regarding the processes that maintain the atmosphere and tail."
An atmospheric and surface composition spectrometer on board the Messenger "will give us a snapshot of how the distribution of sodium and calcium vary with solar and planetary conditions," Izenberg said.
"In addition, we will target the north and south polar regions for detailed observations and look for several new atmospheric constituents," he added.
The last Messenger flyby in October 2008 concentrated its imaging on Mercury's northern hemisphere. This time around, scientists expect to have more than 1,500 pictures of the southern hemisphere at the end of the flyby mission.
With a successful flyby next Tuesday, NASA hopes the entire surface of Mercury will have been photographed. The next mission in March 2011, when Messenger enters the planet's orbit, will focus on a much closer look at the surface and its mineral content.
The first flyby took the spacecraft over the eastern hemisphere in January 2008, and the second flyby took it over western side in October 2008.
"We are going to collect high resolution, color images of scientifically interesting targets that we identified from the second flyby," said Ralph McNutt, a project scientist at APL. "The spectrometer also will make measurements of those targets at the same time," he added.
Recorded sightings of Mercury date back to the first millennium B.C., although ancient cultures have references that may also refer to the planet. The ancient Greeks mistook it for two space objects, alternating between sunset and sunrise, and made it into the gods Hermes and Apollo. The planet is named after the messenger god ancient Romans equated with Hermes.