Previous passes of Phobos by Mars Express have raised as many questions as they've answered.
NASA
THE GIST:
- The Mars Express will pass as close as 50 kilometers (31 miles) above Phobos, one of Mars' two moons, on Wednesday.
- Scientists hope to be able to learn more about the origin of Phobos.
- Russia, Europe and China are collaborating on a mission that includes returns soil samples from Phobos to Earth.
A European space probe is on track for a close encounter with the Martian moon Phobos, an odd, potato-shaped satellite -- origins unknown -- that may be partly hollow.
Mapping Phobos' gravity is among scientists' top priorities when the Mars Express spacecraft soars as close as 50 kilometers (31 miles) above the moon Wednesday night.
Previous passes of Phobos by Mars Express have raised as many questions as they've answered. For example, calculations of the moon's density led scientists to the surprising theory that parts of Phobos may be hollow. Minute changes in the probe's flight path -- tracked by a radio signal -- as it passes over the moon Wednesday will be closely monitored in an attempt to correlate Phobos' gravitational tugs with internal structural variations.
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The information should help fill in some significant holes in the story of where Phobos came from.
"It will not be definite proof, but this will shed new light on the origin of Phobos," said Mars Express project scientist Olivier Witasse.
There are two principal competing hypotheses about how Phobos, as well as a sister moon Deimos, came to exist, notes NASA Mars scientist Pascal Lee and colleagues, who are presenting a paper at the Lunar and Planetary Science Conference in Houston this week outlining a proposed NASA-led mission to retrieve samples from Phobos and Deimos and return them to Earth.
The moons may be asteroids that became trapped by Mars' gravity, or they could have formed along with Mars, possibly even from remnants from a meteorite crash.
"The new data from Mars Express may improve our knowledge of the porosity of Phobos. We will see if this is consistent with what we know about asteroids," Witasse told Discovery News.
The mystery likely will not be resolved until samples of the moon can be chemically analyzed. Lee's proposed mission, named Hall after astronomer Asaph Hall who discovered the two moons of Mars in 1877, likely will not be the first to arrive. A joint European, Russian and Chinese mission, slated to launch in late 2011 or early 2012, includes a Phobos lander that will pluck a 0.2- to 0.4-pound sample of soil and return it to Earth.
The Hall mission proposes to return up to a kilogram (2.2 pounds) of rock and soil samples from several locations to provide a more complete history of the moons. If the probe is launched in 2015, the samples could be back on Earth by 2020, Lee said.
Mars Express arrived at Mars in December 2003 to survey the planet for sub-surface water. The spacecraft's highly elliptical orbit brings it close to Phobos every five months. Mars Express is halfway through a series of 12 passes of Phobos, with its closest approach expected on Wednesday.
Tags: Asteroids, Gravity, Moons, Moons of Mars, NASA,
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