NASA Discovers Potentially Hazardous Asteroid Has A Moonlet During Close Encounter

On June 27, asteroid 2011 UL21 made a relatively close encounter with Earth, flying by our planet at a distance of 6.6 million kilometers (4.1 million miles), or roughly 17 times the average distance from the Earth to the Moon.

While not close enough to worry about, the encounter gave astronomers an opportunity to get a closer look at the object. Doing so can help us learn more about such asteroids, as well as narrow down their orbit, allowing us to know whether they will pose risks to the planet further in the future.

“The term ‘Potentially Hazardous Asteroid’ (PHA) is a precise formal definition, referring to minor planets larger than approximately 140 meters [459 feet] that can come within 7.5 million km [4.6 million miles] from the Earth,” Gianluca Masi, astrophysicist and scientific director of the Virtual Telescope Project, said in a statement ahead of the flyby. “In other words, only the largest asteroids capable of approaching close enough to our planet are flagged as PHAs, which does not mean they are going to hit the Earth, but they nonetheless warrant a better monitoring.”

During this year's flyby, NASA's Deep Space Network’s Goldstone planetary radar kept a close watch on 2011 UL21, imaging it seven times as it passed at 25 kilometers (16 miles) per second. This was the first chance that NASA had to image the asteroid using radar, and when they did so they discovered the asteroid is actually a binary system. The asteroid has its own moonlet, orbiting at a distance of about 1.9 miles (3 kilometers).

“It is thought that about two-thirds of asteroids of this size are binary systems, and their discovery is particularly important because we can use measurements of their relative positions to estimate their mutual orbits, masses, and densities, which provide key information about how they may have formed,” Lance Benner, principal scientist at JPL who helped lead the observations, said in a statement .

During the approach, NASA discovered that the asteroid is roughly spherical. Prior to radar imaging, there was uncertainty about the object's size, with estimates suggesting it could be as small as 1.7 kilometers and as large as 3.9 kilometers (1.05 to 2.4 miles). After radar imaging, NASA puts its size at nearly 1 mile wide (1.5 kilometers) wide, so a little smaller than expected.

It was actually a pretty busy week for the radar system, which observes space objects by transmitting radio waves and then receiving the reflected signal back to the same antenna. On June 29, a second object – only discovered on June 16 – made a much closer approach, passing within 184,000 miles (295,000 kilometers) of Earth. That's a little over three-quarters of the average distance between the Earth and the Moon, a pretty close approach by the asteroid provisionally named 2024 MK.

"For these observations, the scientists also used DSS-14 to transmit radio waves to the object, but they used Goldstone’s 114-foot (34-meter) DSS-13 antenna to receive the signal that bounced off the asteroid and came back to Earth," NASA explained. "The result of this 'bistatic' radar observation is a detailed image of the asteroid’s surface, revealing concavities, ridges, and boulders about 30 feet (10 meters) wide."

The asteroid's path was altered slightly by Earth's gravity, shortening its 3.3-year orbit around the Sun by about 24 days. The asteroid, which was discovered by NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) just 13 days before its closest approach, is classed as potentially hazardous. However, calculations of its orbit show that it poses no threat to Earth for the foreseeable future.