Why mount a mission to an asteroid? For one thing, some of them cross the Earth’s orbit, and that makes gathering knowledge about their composition essential to any future trajectory-altering operation. For another, the science return could be immense. These are unprepossessing objects, no more than chunks of rock and dust, but they can tell us much about the early Solar System. Moreover, getting to an asteroid, as NASA GSFC is now proposing to do with a mission called OSIRIS-REx, would allow us to examine samples in situ, something mission proponent Bill Cutlip finds more valuable than studying chunks of asteroids that fall to Earth in the form of meteorites:
“[Meteorites] are toasted on their way through Earth’s atmosphere. Once they land, they then soak up the microbes and chemicals from the environment around them.”
No, pristine is better, for we’re trying to learn about the earliest days of our system, the period of planetary formation and the origins of the organic compounds that resulted in life. An asteroid like 1999 RQ36, some 580 meters in diameter and the target of OSIRIS-REx, is debris from the solar nebula that gave birth to the Sun and planets about 4.5 billion years ago. Usefully, it seems to have undergone little alteration since that era, unlike asteroids that have suffered from collisions or grown large enough that their interiors became molten. Moreover, RQ36 is rich in carbon. Does it also contain organic molecules of the kind found in other meteorite and comet samples?
If the mission is approved, we’ll learn the answer to this and more, generating a complete map of the asteroid and its topography. The mission will include two infrared spectrometers, a light detection and ranging (LIDAR) instrument to bounce laser pulses off the surface, a mass spectrometer to separate and identify atoms and molecules, and a laser altimeter. Following evaluation from orbit, OSIRIS-REx will collect a surface sample that will be returned to Earth. The plan is to orbit the asteroid for about a year before selecting the sample site, allowing a thorough study of the surface. Sampling an asteroid could be tricky because of the fast rotation, making the operation more like the meeting of two spacecraft:
“Gravity on this asteroid is so weak, if you were on the surface, held your arm out straight and dropped a rock, it would take about half an hour for it to hit the ground,” says Joseph Nuth (NASA GSFC). “Pressure from the sun’s radiation and the solar wind on the spacecraft and the solar panels is about 20 percent of the gravitational attraction from RQ36. It will be more like docking than landing.”
But learning how to maneuver in proximity to an asteroid could be useful for future dealings with near-Earth objects. RQ36 has a slight but real chance of striking the Earth in the year 2170. One motivation for OSIRIS-REx is to measure the Yarkovsky effect, the slight push that occurs when an asteroid soaks up sunlight and emits heat. The uneven surface and variation in asteroid composition makes the Yarkovsky effect difficult to calculate, but if we ever need to change the trajectory of such an object, we’ll need to know how the effect changes its orbit. Adds Nuth: “It’s like trying to make a complex, banking shot in a game of pool with someone shaking the table and kicking the legs.”
If selected by NASA, OSIRIS-REx would be launched no later than December of 2018, but right now it’s one of three proposals chosen in late 2009 under NASA’s New Frontiers program. As for the acronym, it’s torturous indeed. O stands for origins (i.e., the origin of life), SI for spectral interpretation, RI for resource identification, S for security, and REx for regolith explorer. All of that in a single package, but hey, it’s not easy to choose a mission name, and once cobbled together, this one has a nice ring to it.