NASA discovers potentially hazardous asteroid has a moon during close encounter

On June 27, asteroid 2011 UL21 passed relatively close to Earth, flying past our planet at a distance of 6.6 million kilometers (4.1 million miles), which is about 17 times the average distance from Earth to the Moon.

While it wasn’t close enough to worry about, the encounter gave astronomers a chance to get a closer look at the object. That could help us learn more about such asteroids, and refine their orbits so we can know if they pose a risk to the planet in the future.

“The term ‘Potentially Hazardous Asteroid’ (PHA) is a precise formal definition, referring to small planets larger than about 140 meters [459 feet] which can reach within 7.5 million km [4.6 million miles] “The Earth is the largest asteroid in the Universe,” said Gianluca Masi, astrophysicist and scientific director of the Virtual Telescope Project, in a statement ahead of the flyby. “In other words, only the largest asteroids that can get close enough to our planet are marked as PHAs, which does not mean they will hit Earth, but they deserve closer monitoring nonetheless.”

During this year’s flyby, NASA’s Deep Space Network 2011 Goldstone planetary radar kept a close eye on UL21, snapping seven images of it as it passed by at 25 kilometers (16 miles) per second. This was NASA’s first chance to image the asteroid using radar, and when they did, they discovered that the asteroid is in fact a binary star system. The asteroid has its own moon, which orbits the asteroid about 1.9 miles (3 kilometers) away.

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

Radar image of asteroid 2011 UL21 and its moon.

The moon can be seen at the bottom of these radar images.


During the approach, NASA found that the asteroid is roughly spherical. Before the radar images, there was uncertainty about the object’s size, with estimates suggesting it could be as small as 1.7 kilometers (1.1 miles) and as large as 3.9 kilometers (1.05 to 2.4 miles). After the radar images, NASA estimates the size to be nearly 1 mile (1.5 kilometers), slightly smaller than expected.

It’s actually been a pretty busy week for the radar system, which tracks space objects by sending out radio waves and then receiving the reflected signal back to the same antenna. On June 29, a second object—only discovered on June 16—came much closer, passing 184,000 miles (295,000 kilometers) from Earth. That’s a little over three-quarters of the average distance between Earth and the moon, a pretty good approach by the asteroid provisionally named 2024 MK.

Radar image of asteroid 2024 MK tumbling through space.

Asteroid 2024 MK, tumbling through space.

Image Credit: NASA/JPL-Caltech

“For these observations, scientists also used DSS-14 to beam radio waves toward the object, but they used Goldstone’s 114-foot (34-meter) DSS-13 antenna to receive the signal bouncing off the asteroid and back to Earth,” NASA explained. “The result of this ‘bistatic’ radar observation is a detailed view of the asteroid’s surface, showing depressions, ridges and boulders about 30 feet (10 meters) wide.”

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

Leave a Comment