Sightseeing on its way to a comet, Europe's Rosetta spacecraft snapped pictures of an oddly shaped asteroid named Steins that turns out to be not a solid body, but a pile of rubble spun up, in part, by sunlight.
The findings have implications for scientists and engineers studying ways to deflect an asteroid that might be on a collision course with Earth.
Scientists studying the images used a mathematical technique to reconstruct the shape and spin rate of Steins, which is located in the main asteroid belt between Mars and Jupiter.
They determined that Steins, which is shaped like a cut diamond, has undergone a transformation, most likely from a phenomenon known as the YORP effect.
YORP is an acronym of the last names of scientists Ivan Yarkovsky, John O'Keefe, V.V. Radzievskii and Stephen Paddack, who all came up with the theory that solar radiation will increase or decrease the rate of an asteroid's spin. The effect is due to the gentle recoil as the asteroid emits heat.
"Whether an asteroid is porous or solid makes a big difference ... in devising a strategy to mitigate these asteroids," University of Maryland astronomer Michael A'Hearn told Discovery News.
A rubble pile asteroid, for example, would not respond to a nuclear detonation the same way that a solid body would. In addition, the YORP effect itself might be tapped to change an asteroid's orbit by engineering a darker or lighter surface on the object.
"Knowing the exact effects of sunlight on an asteroid's dynamics is important," Mikko Kaasalainen, a mathematician at Tampere University of Technology in Finland, wrote in an email to Discovery News. "It could even be used (in the long run) as a means to deflect near-Earth asteroids by suitably altering the surface properties."
Based on Steins' shape and bulge of material at its equator, scientists believe YORP effects long ago changed the asteroid's rotational period from about two to 2.5 hours to its current six-hour cycle.
"A plausible scenario is that Steins was spun up by YORP, leading to material sliding toward the equator to form the typical cone shape. Such reshaping requires that Steins has a rubble pile structure," wrote lead researcher Uwe Keller, senior scientist with the Max Planck Institute for Solar System Research in Germany, in a report published in this week's Science.
The YORP effect on Steins is not a direct observation, added Kassalainen, lead author of a 2007 paper in Nature about the YORP effects on another asteroid known as 1682 Apollo.
"There are no direct signs of YORP working on Steins at the moment," Kaasalainen said. "The spin-up effect must have happened much earlier in its history."
"The shape of an asteroid is important because it tells a lot about the asteroid's history and structure," he noted.
"As we go to more and more small bodies, we keep finding new questions to ask about how these bodies work," added A'Hearn. "We keep discovering surprising things."
Rosetta came as close as 803 kilometers to Asteroid Steins on Sept. 5, 2008. The probe was launched in 2004 to rendezvous comet 67P/Churyumov-Gerasimenko in 2014.
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