A computer generated imaged of asteroid 1999 RQ36, using radar data from the Arecibo Observatory. Image: NASA/NSF/Cornell/Nolan.

Scientists have used an innovative technique that combines radar and infrared measurements in order to weigh the asteroid 1999 RQ36, the target for an ambitious NASA mission to return a sample of its regolith.

Celestial bodies can be weighed if there is gravitational interplay between them and another body; however this method cannot be implemented for asteroid 1999 RQ36. "This asteroid is not a binary system (as far as we know), and it is not big enough to perturb other natural bodies," explains Steve Chesley from JPL's Near-Earth Object Program Office. "Thus there is no way to use gravity perturbations to estimate the mass in this case."

1999 RQ36 is the target for NASA's OSIRIS-Rex, a sample return mission due to launch in 2016. When OSIRIS-Rex reaches its destination in 2019, it will be able to measure the space rock's mass from the way in which the gravity tugs on the spacecraft. Such a technique has already been implemented many times, and can even detect subtle variations in mass on a body. NASA's dual GRAIL craft are currently using this method to produce a high resolution gravity map of the Moon.

However, not all bodies in the Solar System have the luxury of a spacecraft visit in order to determine their mass, and even though 1999 RQ36 has one on the cards, it is still useful to know the mass prior to the arrival of OSIRIS-Rex.

Artist's impression of the OSIRIS-REx spacecraft at asteroid 1999 RQ36. Image: NASA/GSFC/UAan.

In order to calculate the mass of 1999 RQ36, an intricate knowledge of the asteroid's orbit had to be obtained using twelve years of radar data from the Goldstone Solar System Radar in California and the Arecibo Observatory in Puerto Rico. However, the path of the asteroid was not quite where it was predicted to be; in fact it had deviated from its calculated orbit by 160 kilometres over twelve years.

The cause of the asteroid's detour is an additional force acting upon it, known as the Yarkovsky effect. As an asteroid absorbs light from the Sun and re-emits it as heat, a small propulsive force is generated that give the asteroid a little extra kick. "The Yarkovsky effect relies on the fact that sunlight warms the surface during the day and then the surface cools during the night. So the evening temperatures are warmer than morning, just as they are on Earth," Chesley tells Astronomy Now. "The surface material essentially stores energy during the day and re-radiates it later, and the Yarkovsky effect depends critically upon the 'insulation' qualities of the surface material."

If the asteroid is swathed in a blanket of dust, heating and cooling will occur rapidly and thus the Yarkovsky effect will be weak. A solid rock surface, on the other hand, will retain heat for longer, therefore providing more fuel for the Yarkovsky effect.

In 2007, observations were made by Josh Emery from the University of Tennessee using NASA's Spitzer Space Telescope in order to gather information on the thermal properties of 1999 RQ36. The infrared emissions of the asteroid revealed details on how efficient the insulation is, and thus allowed a thorough understanding of the Yarkovsky effect for 1999 RQ36.

Once the orbit, size, and the Yarkovsky effect had been accounted for, calculating the density, and thus the mass, of the asteroid were made possible. The density of the asteroid is similar to that of water, and shows that it is likely a very porous ensemble of rocks and dust. This is good news for OSIRIS-Rex as it will make the collection of samples an easier task.