Just over a year from now, we’ll be anticipating the launch of the OSIRIS-REx mission, scheduled to rendezvous with the asteroid Bennu in 2018. This will be the first American mission to sample an asteroid, and it’s interesting to note that the materials scientists hope to return will constitute the largest sample from space since the days of Apollo. As with recent comet studies, asteroid investigations may give us information about the origin of the Solar System, and perhaps tell us something about sources of early water and organic materials. This NASA Goddard animation offers a fine overview of the target and the overall mission.

But OSIRIS-REx is about more than the early Solar System. Recent scare stories have compelled NASA to state that a different asteroid, sometimes identified as 2012 TT5, will not impact our planet in September of this year. As Colin Johnston points out in Astronotes (the blog of Armagh Planetarium), 2012 TT5 will, on the 24th of September of this year, pass within 0.055 AU, or roughly 8.25 million kilometers of the Earth, a rather comfortable miss in anyone’s book. We’re talking about a distance twenty times further than the Moon is from the Earth. [NOTE: I originally mis-stated the kilometer equivalent of 0.055 AU, now fixed thanks to readers who spotted the mistake in the comments].

In other words, this asteroid poses no problem whatsoever when it passes by in September. Let me just quote the NASA news release briefly to get this out of the way:

“There is no scientific basis — not one shred of evidence — that an asteroid or any other celestial object will impact Earth on those dates,” said Paul Chodas, manager of NASA’s Near-Earth Object office at the Jet Propulsion Laboratory in Pasadena, California.

In fact, NASA’s Near-Earth Object Observations Program says there have been no asteroids or comets observed that would impact Earth anytime in the foreseeable future. All known Potentially Hazardous Asteroids have less than a 0.01% chance of impacting Earth in the next 100 years.

The longer-term picture is that assessing asteroids that could be a problem — astronomers call these Potentially Hazardous Asteroids, or PHAs — is an ongoing effort and a wise one. We know that asteroid impacts have had a role to play in the history of our planet, and it would be folly to ignore the potential. Fortunately, the OSIRIS-REx mission plays into that effort, because Bennu is an object with a chance (about 1 in 2500) of impacting the Earth late in the next century. Getting samples here will help us understand how to mitigate any future impact.


Image: An artist’s concept of NASA’s OSIRIS-REx asteroid-sample-return spacecraft arriving at the asteroid Bennu. Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab.

An extremely dark object, Bennu absorbs most incoming sunlight and radiates it away as heat. This brings the so-called Yarkovsky effect into play, gradually changing the orbit of the asteroid over time. Clearly, understanding the asteroid’s trajectory involves anticipating what this tenuous effect can do. Edward Beshore (University of Arizona), who is deputy principal investigator for OSIRIS-REx, explains the mission’s role:

“We’ll get accurate measurements of the Yarkovsky effect on Bennu by precisely tracking OSIRIS-REx as it orbits the asteroid. In addition, the instrument suite the spacecraft is carrying is perfectly suited to measure all the things that contribute to the Yarkovsky effect, such as composition, energy transport through the surface, temperature, and Bennu’s topography. If astronomers someday identify an asteroid that presents a significant impact hazard to Earth, the first step will be to gather more information about that asteroid. Fortunately, the OSIRIS-REx mission will have given us the experience and tools needed to do the job.”

When Bennu was selected as the target in 2008, there were over 7000 known Near-Earth Objects (NEOs), of which fewer than 200 had orbits with the low eccentricity and inclination best suited for the mission. At 492 meters in diameter, Bennu is large enough to offer a stable target for the lander, with a carbon-rich composition believed to include the organic molecules, volatiles and amino acids that could be considered life’s precursors. Of the list of 7000, only 5 NEOs met all these criteria. Bennu was the final pick, and we’ll track OSIRIS-REx’s operations there with great interest. The spacecraft is to arrive at the asteroid in August of 2018.