Astronomers shared a collective sigh of relief last week when a 100-foot asteroid hurtling toward Earth missed by 50,000 miles—just a fifth of the distance to the moon. As comforting as the avoided terrestrial calamity was, what remains disturbing is that no one knew it was coming.
The near miss came just days before NASA plans to launch an $800 million probe that will land on a much larger asteroid, a remnant from the beginning of the solar system that should provide clues to Earth’s origins.
The mission OSIRIS-REx, which actually stands for something (Origins Spectral Interpretation Resource Identification Security—Regolith Explorer), is slated to blast off on Sept. 8, from Cape Canaveral, Fla.
The mission “advances our more practical goals of understanding the resources of the near earth Solar System—as well as the hazards,” Jeffrey Grossman, a mission scientist, said at a press conference last month.
The probe will visit a “near-Earth object” that traces an orbit around the Sun similar to the Earth’s. The asteroid, called Bennu—named by a nine year-old from North Carolina—recommended itself for several reasons. It’s old—basically leftover pizza dough from the beginning of the solar system. As a result, it might contain some of the chemical secrets about how the Earth was seeded with the potential for life.
Being a near-Earth object, however, doesn’t make Bennu a pal of our home planet. By being in the neighborhood, it passes Earth every six years, coming so close that scientists give it a 1-in-2,700 chance of hitting us over the next two centuries.
It's tempting to believe that Kepler's laws of planetary motion describes heavenly objects, including asteroids and comets, as taking immutable, precisely calculable tours around the sun. But there's more to it than that. OSIRIS-REx will be measuring a phenomenon known by the spy-novel sounding moniker, “the Yarkovsky effect.” A big chunk of rock can pick up speed as sunlight heats it up and the blackness of space cools it off. This acceleration can nudge its heading slightly. The effect “acts like a thruster and changes the trajectory of the asteroid,” Dante Lauretta, the mission's principal investigator and a professor at the University of Arizona, said last month. “So if you want to predict where an object like Bennu is going to be in the future, you have to account for this phenomenon.”
What it means is that Bennu, which was discovered in 1999, might still surprise astronomers when its orbit starts to more closely track Earth's in 160 years. By collecting precise data on its composition, shape, and surface features, NASA hopes it will be able to document the Yarkovsky effect in greater detail, and consequently get a better sense of the risk asteroids pose to Earth—like "2016 QA2," the recent near-miss.
|Scientists and engineers gather around Lockheed Martin’s thermal vacuum chamber to watch the OSIRIS-REx spacecraft enter one of its last phases of testing.
Photo: University of Arizona/ Symeon Platts
NASA, directed by Congress, takes impact-risk seriously, maintaining a database of possible hazards and a scientific scale for categorizing their threats. The National Research Council in 2010 published a report about asteroid risks and what to do about them. That research suggests that a rock the size of 2016 QA2 might have had some serious local impacts:
Many dozens of people work for years to launch a mission that has the complexity of OSIRIS-REx. Often, it’s a lifelong dream. As Canadian Space Agency’s Tim Haltigin said in this NASA video, “I grew up playing video games about shooting lasers at asteroids, and now it’s my job to shoot lasers at asteroids. It never stops amazing me.”
The sample-return spacecraft can carry up to 4.4 pounds of space rock. The minimum amount scientists expect is about 2 ounces. It's a lot of work for what seems like a little material—and yet a much better idea than waiting to see if the whole 1,600-foot-wide asteroid slams into earth at the end of the next century.