Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2010 March 17
See Explanation. Go online for the highest resolution available.

Phobos from Mars Express
Credit: G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA

Explanation: Why is this small object orbiting Mars? The origin of Phobos, the larger of the two moons orbiting Mars, remains unknown. Phobos and Deimos appear very similar to C-type asteroids, yet gravitationally capturing such asteroids, circularizing their orbits, and dragging them into Mars' equatorial plane seems unlikely. Pictured above is Phobos as it appeared during last week's flyby of ESA's Mars Express, a robotic spacecraft that began orbiting Mars in 2003. Visible in great detail is Phobos' irregular shape, strangely dark terrain, numerous unusual grooves, and a spectacular chain of craters crossing the image center. Phobos spans only about 25 kilometers in length and does not have enough gravity to compress it into a ball. Phobos orbits so close to Mars that sometime in the next 20 million years, tidal deceleration will break up the rubble moon into a ring whose pieces will slowly spiral down and crash onto the red planet. The Russian mission Phobos-Grunt is scheduled to launch and land on Phobos next year.

View this page (with external links) online at
http://apod.gsfc.nasa.gov/apod/ap100317.html.


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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Official: Phillip Newman. Specific rights apply.
A service of: ASD at NASA/GSFC
& Michigan Tech. U.

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