|Set for launch in 2016, the OSIRIS-REx spacecraft will orbit asteroid 1999RQ36 for a year, mapping its surface and making detailed measurements. (Illustration: NASA/GSFC/UA)|
Astronomers working on the U.S.' first asteroid-sample return mission – the NASA mission named OSIRIS-REx – have begun a months-long observing campaign that is the last chance to study their target asteroid from Earth before the OSIRIS-REx spacecraft launches in 2016.
OSIRIS-REx is a quest to bring back to Earth a good-sized sample of an asteroid unaltered since solar system formation – a sample that very well could contain molecules that seeded life.
Discovered in 1999, the OSIRIS-REx target asteroid, designated 1999 RQ36, nears Earth once every six years. During the 2011 closest approach in early September, it will be 10.9 million miles (17.5 million kilometers) away. In 1999, closest approach was 1.4 million miles (2.3 million kilometers).
"Six years sets the whole cadence for our mission," said Dante Lauretta of the University of Arizona Lunar and Planetary Laboratory, deputy principal investigator for the OSIRIS-REx mission.
"The next chance for ground-based telescopes to see this asteroid will be in 2017, when it again nears Earth. Our spacecraft performs a gravity-assist at this time, giving it the kick it needs to rendezvous with the asteroid in 2019-20. The next chance for ground-based astronomy is 2023, the year the spacecraft returns a sample of the asteroid to Earth."
1999 RQ36 last attracted astronomers' attention in 2005, when it passed 3.1 million miles (5 million kilometers) from Earth and appeared 30 times brighter than it does this year.
In 2005, Carl Hergenrother of the UA Lunar and Planetary Laboratory was searching with the 61-inch Kuiper telescope on Mt. Bigelow north of Tucson for exciting targets for the proposed asteroid sample-return mission. He observed 1999 RQ36.
"Looking at my data, I saw this was a B-type asteroid, which is carbonaceous and related to unusual outer main-belt asteroids that act like comets by outgassing volatiles," Hergenrother, who heads the OSIRIS-REx asteroid astronomy working group, said.
After a quick search of the scientific literature, which turned up nothing on the object, he did a Google search. Jackpot.
"Astronomers had been observing this asteroid, just not formally publishing about it," Hergenrother said. "Their results were sitting on their personal Web pages. They had radar images of it, visible and near-infrared observations, confirmed it was a B-type (bluish) asteroid, got a pretty good light curve and a rotation period, although the rotation period was wrong."
Michael Drake of the UA Lunar and Planetary Laboratory, principal investigator for OSIRIS-REx, urged Josh Emery, one of Drake's former students, now of the University of Tennessee and a co-investigator on OSIRIS-REx, to observe 1999 RQ36 with the Spitzer Space Telescope. Emery won the telescope time, providing first observations of the asteroid at thermal infrared wavelengths.
"Coming out of 2006-07, 1999 RQ36 was probably the best-studied near-Earth asteroid out there that hadn't already been visited by a spacecraft," Hergenrother said. "We lucked out in that not only is this an asteroid that's relatively easy to get to, it is extremely interesting, exactly the kind of object that we want for this mission."
The international team of astronomers collaborating in the fall 2011-spring 2012 observing campaign for 1999 RQ36 have time or are applying for time on a network of telescopes operating in Arizona, the Canary Islands, Chile, Puerto Rico and space.
The new observations will not only influence mission planning and development, but will directly address two key OSIRIS-REx mission goals, Lauretta said.
One goal is to check results from ground-based observations against results from OSIRIS-REx spacecraft observations that will be made in 2019-20 as the spacecraft circles the asteroid for about 500 days.
Another goal is to measure a slight force called the "Yarkovsky effect" to better understand the likelihood that potentially hazardous near-Earth asteroids, such as 1999 RQ36, will strike our planet, and when.