Remember our not-so-little friend asteroid 2014 JO25? On April 19, it approached within 4.6 lunar distances (1.1 million miles) of Earth, the closest an asteroid at least 600 meters (1,968 feet) in size has come since 2004. The flyby provided the perfect opportunity for astronomers to study the physical properties of the asteroid and make images of its surface via radar that rivaled those a spacecraft flyby might return.
Shortly after its discovery in 2014, Jet Propulsion Laboratory astronomer Joe Masiero used observations made by the NEOWISE spacecraft to estimate 2014 JO25’s size as roughly 2000 feet (650 meters) with a surface twice as reflective as the moon’s.
Masiero also measured the asteroid’s position in space relative to stars in the background sky to determine a more precise orbit. This is crucial for radar observations since astronomers operating these big dishes have to know exactly where to point them. Two radio telescopes focused on 2014 JO25, the National Science Foundation’s Arecibo Observatory in Puerto Rico from April 15-21 and NASA’s Goldstone Solar System Radar in California from April 16-21.
Radio waves were beamed at the asteroid and their reflections collected, analyzed and assembled into hundreds of radar images with resolutions of 25 feet per pixel from both observatories and a smaller number of images at 12 feet per pixel resolution at Goldstone. Those numbers mean that the smallest details discernible in the images were either 25 or 12 feet across.
Thanks to all this beaming and bouncing, we know that 2014 JO25 has an irregular, double-lobed shape with a narrow neck. A Rubber Ducky or lopsided peanut comes to mind. The asteroid’s bigger lobe is about 0.6 miles (1 km) long, and the short about 0.4 miles (600 meters) for a total length of about a mile.
The most detailed radar images of 2014 JO25 reveal smaller scale features such as flat regions up to ~650 feet (200 m) long, ridges, concavities, possible impact craters dozens of feet in diameter, hills, and collections of bright spots that may indicate large boulders. By tracking specific features, the radar teams were able to pin down the asteroid’s rotation period: 4.5 hours. With a day that short, I’d never get anything done.
Radar measurements probed the asteroid’s surface roughness on the order of a couple feet and discovered it resembled other asteroids we’ve examined up close with spacecraft. All are rocky with boulders, pebbles and fine dust.
The NASA Infrared Telescope Facility (IRTF) also got into the act. The 3-meter (10-foot) telescope is located atop Mauna Kea, Hawaii and can parse the reflected light of an asteroid using its SpeX instrument to better determine its composition. On the evening of April 21, SpeX measurements showed that the 2014 JO25 had a stony composition and was moderately bright.
Among the hundreds of near-Earth asteroids studied with radar to date, about 50 have double-lobed or “contact binary” shapes. For near-Earth asteroids larger than about 490 feet (150 meters) across, about 1/6 of them have this type of shape, so it is clearly quite common. The 4.5-hour rotation period is fast for a contact binary shape, and given the dimensions of the asteroid, 2014 JO25 is rotating almost fast enough for it to split in two.
So how did 2014 JO25 get its shape? While we don’t know for sure, it may be that the asteroid formed during a slow collision between separate objects. Or from slowly spinning up and now starting to come apart. There’s strong evidence that many near-Earth asteroids are weakly bound collections of rocks and dust that are held together primarily by their very feeble gravity. If so, then the asteroid’s shape could distort if its rotation accelerates, which can happen when irregularly-shaped asteroids absorb visible sunlight and then re-radiate it as heat. The difference in the direction of the heat emitted relative to the absorbed sunlight produces a gentle torque that can gradually change the spin. If the rotation spins up enough for a rubble pile object, then the shape can change and double-lobed objects may form.
Another possibility is that the asteroid could have more closely approached either Mercury or Earth and that planetary tides (like tides caused by the moon on Earth) could have started to pull it apart. Still another possibility is that an older, larger asteroid was shattered by a collision with another object, and some of the remaining debris re-accumulated by its weak mutual gravitational attraction into a few individual rubble-piles which settled onto each other.
Once again, gravity, sunlight and spin work their subtle magic.