The Rosetta spacecraft swung by asteroid 21 Lutetia on 10 July 2010 (Image: ESA 2010 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA)
Asteroids are generally regarded as the solar system's scrap heap, the battered bits that broke off and were left behind when the planets were forming but the lumpy asteroid 21 Lutetia may be a whole, unbroken building block left nearly untouched since the solar system's birth.
"We think planets were built of things like Lutetia," says Ben Weiss of the Massachusetts Institute of Technology in Cambridge. "We're getting a chance to see one of the building blocks of the solar system up close."
The European spacecraft Rosetta zipped past Lutetia at 50,000 kilometres per hour in July 2010, snapping photos of a cratered world about 121 kilometres long.
That makes it the second-largest asteroid ever visited by a spacecraft, next to 560-kilometre-wide Vesta.
Violent mêlée
Most of the asteroids to get visits from spacecraft are rubble piles, chunks of debris that were loosely held together by gravity. But Lutetia is so dense that it appears to have survived the violent mêlée of collisions in the early solar system intact.
"The real new thing is that it's not a rubble pile, it's a solid block of rock," said Holger Sierks of the Max-Planck Institute for Solar System Research in Germany, lead author of a new paper reporting the observations. "It's really a remnant from the early days."
Even more surprisingly, the spacecraft's measurements suggested the rock has one of the highest asteroid densities ever measured, at 3.4 tonnes per cubic metre.
That is denser than granite and suggests Lutetia might have heavy metals in its core.
Asteroids are generally regarded as the solar system's scrap heap, the battered bits that broke off and were left behind when the planets were forming but the lumpy asteroid 21 Lutetia may be a whole, unbroken building block left nearly untouched since the solar system's birth.
"We think planets were built of things like Lutetia," says Ben Weiss of the Massachusetts Institute of Technology in Cambridge. "We're getting a chance to see one of the building blocks of the solar system up close."
The European spacecraft Rosetta zipped past Lutetia at 50,000 kilometres per hour in July 2010, snapping photos of a cratered world about 121 kilometres long.
That makes it the second-largest asteroid ever visited by a spacecraft, next to 560-kilometre-wide Vesta.
Violent mêlée
Most of the asteroids to get visits from spacecraft are rubble piles, chunks of debris that were loosely held together by gravity. But Lutetia is so dense that it appears to have survived the violent mêlée of collisions in the early solar system intact.
"The real new thing is that it's not a rubble pile, it's a solid block of rock," said Holger Sierks of the Max-Planck Institute for Solar System Research in Germany, lead author of a new paper reporting the observations. "It's really a remnant from the early days."
Even more surprisingly, the spacecraft's measurements suggested the rock has one of the highest asteroid densities ever measured, at 3.4 tonnes per cubic metre.
That is denser than granite and suggests Lutetia might have heavy metals in its core.
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