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Mars polar cap mystery solved

...the Martian weather system and a giant impact crater could explain why the residual ice cap at the south pole of Mars is offset by several degrees...

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Hubble's ANGST reveals diversity of galaxies a survey of 14 million stars in 69 galaxies from the ACS Nearby Galaxy Survey Treasury (ANGST), Hubble reveals the true diversity of galaxies...

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Young galaxy's magnetism surprises astronomers

...the first direct measurement of the magnetic field in a young, distant galaxy, reveals it to be 10 times stronger than that of our own Milky Way...

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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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A deserted outer Solar System

Posted: October 06, 2008

After clocking up over 200 hours of observations as part of the Taiwanese-American Occultation Survey (TAOS) to map the distribution of small Kuiper Belt objects in the outer Solar System, astronomers have come up empty-handed.

TAOS dedicated two years to periodically photograph portions of the sky to look for small chunks of rock and ice orbiting beyond Neptune, in a region of the Solar System called the Kuiper Belt. The survey targeted Kuiper Belt objects (KBOs) with sizes between 3 and 28 kilometres and watched for stars to dim as these KBOs passed in front of them, since such objects are too small to see directly. Out of 200 hours of observations, TAOS did not spot any such occultations.

The New Horizons mission is currently en route to the Kuiper Belt system and will reveal more about the types of bodies that reside there. The Kuiper Belt extends outwards beyond the orbit of Neptune. The map above shows the dates of various landmarks in the New Horizons mission. Image: NASA.

The Kuiper Belt contains objects in a range of sizes from a handful of large dwarf planets Pluto, Eris, Makemake and Haumea, to many more smaller ones. The commonness of a given size of KBO gives information about the history of planet formation and dynamics, in terms of the history of colliding and planetary building events as well as destructive collisions to shatter rocky bodies.

Astronomers questioned whether they would find more and more KBOs as sizes decreased further, or whether the distribution levelled out. The fact that no occultations were seen has set a stringent upper limit on the number density of KBOs between 3 and 28 kilometres in diameter, revealing the outer Solar System as less crowded as some theories suggest. Astronomers speculate that this is either because small KBOs have already stuck together to form larger bodies, or frequent collisions have ground down small KBOs into even smaller fragments below the threshold of the survey.

The paper announcing this result appears in the 1 October issue of the Astrophysical Journal Letters.