Incoming: How Sunlight Nudges an Asteroid's Path

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Asteroids have collided with Earth in the past and may do so again, which is why NASA is keen on tracking them, learning as much as they can about their orbits, mass, and other properties.

Now research is showing that even sunlight can alter an asteroid's path and must be taken into consideration.

NASA has a mission that specifically targets one asteroid in particular — 1999 RQ36. The mission, OSIRIS-REx, is intended to rendezvous with the space rock and take a sample before returning to Earth.

Members of the OSIRIS-REx team are already learning new things about 1999 RQ36 from a distance — not just determining its mass, but also its orbit, including a slight drift resulting from the absorption and re-emission of heat from the sun. Steve Chesley of NASA's Jet Propulsion Lab reported his findings in May at a conference in Japan.

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In order to calculate the precise mass of 1999 RQ36, Chesley needed to factor in its orbit and anything that might affect that orbit, such as nearby objects or propulsive force, however minute. That meant taking into account the effects of sunlight.

As unlikely as it may seem, photons carry momentum just like any other particle. That means that when light strikes something, its photons transfer energy just like hitting the object with tiny pellets, producing a recoil effect.

Solar sail propulsion for spacecraft exploits this fundamental principle.

It's also at the heart of the "Yarkovsky effect," proposed by a Russian engineer named Ivan Yarkovsky over 100 years ago. He hypothesized that rotating asteroids could experience tiny changes in their orbits as they absorbed heat from the sun's light on one side, radiating it back as they turned around.

The effect would be small, just a slight imbalance, but over time it would add up, significantly altering the orbit. And it would act more strongly on smaller objects as opposed to larger ones.

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Just how small a force are we talking about? Chesney estimates that when the asteroid is nearest the sun, the force on 1999 RQ36 would be roughly half an ounce — the weight of three grapes, compared to a rock that weighs millions of tons.

It takes years before any noticeable orbital changes can be measured, but in this case, his colleague, Michael Nolan of the Arecibo Observatory, provided a dozen years' worth of data.

Chesney combined that data with similar data from the Goldstone Solar System Radar observatory, figuring in the gravitational effects of the sun, moon, planets and other asteroids.

It turns out that the orbit of 1999 RQ36 has deviated from the predictions of the mathematical model by about 100 miles in the last 12 years. The only explanation for this would be the Yarkovsky effect.

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Based on this, Chesley and his colleagues were able to determine any particularly close approaches to Earth that the asteroid would make. Its closest approach should occur in 2135, when it will pass within 220,000 miles of Earth — closer than the moon, which orbits at 240,000 miles from Earth. Chesley insists, however, that the odds of an actual collision with Earth in the 22nd century remain about one in a few thousand.

Josh Emery of the University of Tennessee, Knoxville, also contributed to the project, by using NASA's Spitzer Space Telescope back in 2007 to study the asteroid's thermal emissions, from which he could derive its temperature.

Once the orbit, size, thermal characteristics and Yarkovsky effect were taken into consideration, the rest was relatively easy: Chesney basically solved for "x" and calculated the asteroid's bulk density. The verdict: 1999 RQ36 measures half a kilometer across and weighs around 60 million metric tons, similar to the density of water. Chesley thinks this means it's likely the asteroid is "a very porous jumble of rocks and dust."

I guess we'll know more when OSIRIS-REx returns from its mission with a sample of 1999 RQ36, although that will take about 10 years. It is scheduled to launch n 2016 and won't reach the asteroid intil 2019, with a slated return to Earth in 2023.

Images: (top) Series of radar images of asteroid 1999 RQ36, obtained by NASA's Deep Space Network antenna in Goldstone, Calif. on Sept 23, 1999. Credit: NASA/JPL-Caltech. (bottom) Illustration of the Yarkovsky effect. Credit: NASA/OSIRIS-REx.