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Did life need asteroid bombardment?

...A period 3.9 billion years ago when Earth was peppered with impacts by large asteroids may have created an environment in which primitive life could take hold, rather than destroying that life...

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Mystery of millisecond pulsars solved

...Astronomers have watched a pulsar be spun up in real time by its companion star, turning it into an incredibly fast millisecond pulsar rotating a breakneck 592 times per second...

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Doomed planet may have been drenched in water

...A small exoplanet full of water may have been swallowed up by a dead white dwarf star, according to anomalous readings of hydrogen in the star's helium-rich atmosphere...

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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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How to search for alien seas



Posted: 27 May, 2009

The search for habitable planets around other stars has taken a step forward by, ironically, looking at our own planet. To test whether we would be able to detect oceans on exoplanets, researchers used the Deep Impact spacecraft to observe Earth, “as if we were aliens looking at Earth with the tools we might have in ten years,” according to Nicolas Cowan of the University of Washington.

How many worlds that possess oceans like Earth are there in the Galaxy? We may be on the cusp of finding out. Image: NASA/JPL–Caltech/R Hurt (SSC–Caltech).

The Deep Impact spacecraft hit the headlines in July 2005 when it fired a copper projectile into the comet 9P/Tempel 1, creating a huge outburst on the comet’s icy surface. Since it still has fuel and power, astronomers have since been employing it in other jobs. For instance, at the beginning of this year it began using its cameras to search several stars for the telltale dips in light that indicate transiting planets.

Now it has turned its camera back towards Earth, observing our planet over two 24-hour periods in seven wavelengths of visible light from vantage points of 17 million and 33 million kilometres. As the Earth rotated, different oceans and clouds and continents came into view. As they did, they created small deviations in the average colour of Earth, with two dominant colours: blue at short wavelengths and red at long wavelengths, which were inferred as oceans and landmasses. This was confirmed when compared with a map of the Earth.

“You could tell that there were liquid oceans on the planet,” says Cowan, who led the research. “Liquid water on the surface of a planet is the gold standard that people are looking for.”

Over the last year astronomers have taken the first snapshots of planets orbiting other stars, but these have all been hot, young gas giants orbiting very far from their parent star. Meanwhile, infrared observations of exoplanets by NASA’s Spitzer Space Telescope have mapped variations in their atmospheric temperature. Within ten years the capability to be able to image a rocky world in the habitable zone around its star (where temperatures are appropriate for liquid water on the surface, crucial to life as we know it) should be with us. The images will be mere pixels in size, a far cry from taking a proper Earth-like portrait, but the study led by Cowan shows that it will be possible to use the slight deviations in their overall colour as they rotate to figure out which ones are most likely to have water and support life.

These temperature maps of the exoplanet HD 80606b, made by the Spitzer Space Telescope, prove that it is possible to map some of the details of alien worlds. Note that these are simulated images based on variations in infrared emission as the planet moved around its highly elliptical orbit, and are not real pictures of the planet.

Image: NASA/JPL–Caltech/J Langton (UC Santa Cruz).