This week, a kilometer-wide asteroid whizzed by within about a million miles of this planet—about four and half times the distance between the Earth and the moon. A near miss? Not really. The odds of 2014 JO25 actually hitting Earth were around one in a million.
The safer bet is on science. As in, how much of it astronomers were able to gather from the close pass of such a huge space rock.
Size, speed, spin, shape, structure … an asteroid’s attributes are clues about where it came from, and where it’s going. Measure enough of them, and you can start making better assumptions about how the solar system works. But because they are essentially lightless flecks zipping around the vacuum, first you have to find them. That’s largely the job of two big telescopes high in the Arizona hill country.
“We discovered 2014 JO25 on the night of May 4th, 2014 using our 1.5 meter survey scope on top of Mt. Lemmon,” says Eric Christensen, director of the Catalina Sky Survey. (Its sister, a wide-angle Schmidt scope, works in parallel.) Once they’ve discovered an object, Christensen and his team typically need a few days’ worth of data to figure out its orbit. Then comes the impact risk calculation: Will this object hit Earth soon? That means—despite whatever nail-biting fear the headlines may have instilled—Christensen and his team figured out this thing wasn’t an immediate hazard about three years ago.
Nor was 2014 JO25 a rare object: In the three days before discovering it, astronomers found more than 640 other near Earth objects.
But common doesn’t mean boring. Once the Catalina Sky Survey determined that this big orbital rock wasn’t an immediate danger, they passed the data along to NASA’s Near Earth Object program. It’s up to that latter group to figure out the asteroid’s return period: Is it going to come back and smack Earth sometime in the next 15, 20, 100 years? In 2014 JO25’s case, they’ll have a better idea when they process all the data from this latest fly-by. “The other thing is to get information about where and how close the object is going to pass by so other astronomers can prepare to aim their telescopes and radars to collect data,” says Kelly Fast, the Near Earth Object program manager.
This one passed by close enough to collect radar imagery. Specifically, from the massive 70 meter antenna at Jet Propulsion Laboratory’s Goldstone Deep Space Network installation. “When an asteroid is getting close, we ping it with radio waves,” says Shantanu Naidu, radar astronomer at JPL. “For this object, it took about 20 seconds for those echoes to come back, at which point we stopped transmitting, and started recording the echoes.” The results were pretty awesome:
These images accompanied other data that let Naidu make precise measurements of the object’s size, shape, and spin period. Those attributes allow astronomers to measure how asteroids get pushed around by solar photons. “Photons carry momentum with them, and when an object absorbs and reradiates the photon it gets a slight kick,” says Naidu. This triggers the Yarkofsky effect—the “afternoon” side of the asteroid gets hotter than the “morning” side, reradiates more photons, and the asteroid starts to spin.
And as the asteroid spins, it morphs. Loose material on its surface starts to float outward, overcoming gravity. Sometimes this material accretes into another asteroid, forming a binary pair orbiting each other. In some cases, these two orbiting bodies reform into one. That explains the double-headed lump that is 2014 JO25, a shape astronomers call a contact binary. “This happens in about 15 percent of asteroids bigger than 200 meters,” says Naidu. He says the data he collected from 2014 JO25 will help astronomers learn more about the mechanics of these small space rocks.
And let’s not forget: The photon-induced spinning creates perturbations in the asteroid’s orbit, not just its shape. Which is how astronomers will figure out whether Earth will be so lucky the next time 2014 JO25 swings by this part of the solar system.