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Kepler's first view of planet hunting territory

...NASA's Kepler spacecraft has opened its eyes and blinked at the rich star field where it will search for extraterrestrial planets like Earth...

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Four-way cosmic

pile up

...Combining images from space- and ground-based telescopes, astronomers have revealed the first cosmic collision of four separate galaxy clusters...

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Hubble witnesses flaring in black

hole jet

...A flare of matter blasting out from a monster black hole is outshining even the core of its host galaxy, M87...

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Video archive

STS-120 day 2 highlights

Flight Day 2 of Discovery's mission focused on heat shield inspections. This movie shows the day's highlights.


STS-120 day 1 highlights

The highlights from shuttle Discovery's launch day are packaged into this movie.


STS-118: Highlights

The STS-118 crew, including Barbara Morgan, narrates its mission highlights film and answers questions in this post-flight presentation.

 Full presentation
 Mission film

STS-120: Rollout to pad

Space shuttle Discovery rolls out of the Vehicle Assembly Building and travels to launch pad 39A for its STS-120 mission.


Dawn leaves Earth

NASA's Dawn space probe launches aboard a Delta 2-Heavy rocket from Cape Canaveral to explore two worlds in the asteroid belt.

 Full coverage

Dawn: Launch preview

These briefings preview the launch and science objectives of NASA's Dawn asteroid orbiter.

 Launch | Science

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Solar wind tans young asteroids



Posted: 27 April, 2009

After a collision, the colour of an asteroid is modified rapidly by the solar wind so that it resembles the colour of much older asteroids. Image: ESO/M. Martins.

Unlike human skin which is damaged by prolonged exposure to sunlight over a lifetime, an asteroid's surface is aged in the first instances of its life.

Of course, the time scales of the exposure are much different: for an asteroid the damage is done over a period of one million years, but this is still a very short timeframe compared with the 4.6 billion year age of the Solar System itself. "Asteroids seem to get a 'sun tan' very quickly," says Pierre Vernazza, lead author of a paper published in this week's edition of the journal Nature. "But not, as for people, from an overdose of the Sun's ultraviolet radiation, but from the effects of its powerful wind."

It is already well known that asteroid surfaces change over time thanks to space weathering reddening their surfaces, but the actual processes and timescales involved were controversial.

Thanks to new observations conducted using ESO's New Technology Telescope at La Silla and the Very Large Telescope at Paranal, astronomers have shed some light on this mystery. The astronomers looked at freshly exposed asteroid surfaces (caused by the collision of two asteroids) and noticed that they change colour in less than a million years.

"The charged, fast moving particles in the solar wind damage the asteroid's surface at an amazing rate," says Vernazza. The solar wind contains highly energetic particles that bombard the exposed surfaces of asteroids, eroding the molecules and crystals on the surface and rearranging them into different configurations with distinct colours and properties.

By studying different families of asteroids, the team also demonstrated that an asteroid's surface composition can influence the rate of reddening after the initial one million year burst of exposure. The observations also suggested that collisions may not be the main mechanism behind the high proportion of fresh surfaces seen among near-Earth asteroids. Instead, they may be caused by gravitational interaction with a planet, where the tug of a planet has "shaken" the asteroid, exposing unaltered material.

This fundamental work will enable astronomers to better relate the surface of an asteroid to its history.