New images show a small moon around an asteroid as it flies past Earth


NASA’s Goldstone Solar System Radar recently observed asteroid 2024 MK, which made its closest approach to Earth on June 29.

Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news about fascinating discoveries, scientific developments and more.


When NASA scientists recently tracked the orbits of two space rocks as they made close approaches to Earth, they discovered a surprise: One of the asteroids has a tiny moon.

Astronomers regularly monitor the orbits of asteroids to make sure they are not on a collision course with our planet.

While neither of the recent asteroids passed by at a disturbing distance, the space rocks could provide valuable information that NASA is using to prepare for potential future collisions.

Asteroids, remnants from the formation of the solar system, are also of interest because recording details about their size, orbit and composition can provide insights into our part of the cosmos.

Astronomers at NASA’s Jet Propulsion Laboratory in Pasadena, California, used a system called planetary radar via the Deep Space Network to track and image the asteroids.

The Deep Space Network is a system of radio antennas on Earth that allows the agency to communicate with spacecraft exploring our solar system. The network also transmits radio waves that act as radar in space.

The first space rock, asteroid 2011 UL21, passed Earth on June 27 at a distance of 4.1 million miles (6.6 million kilometers), or 17 times the distance between Earth and the moon. Researchers first discovered the asteroid in 2011 using the Catalina Sky Survey in Tucson, Arizona. But since the space rock was first spotted, its flyby of Earth in June is the closest it has ever swung past our planet that radar has recorded.

Astronomers beamed radio waves from the 230-foot-wide (70-meter-wide) satellite dish of the Goldstone Solar System Radar, near Barstow, California, toward the space rock. The waves bounced off the asteroid and traveled back to the network’s satellite dish antenna.

Researchers have classified the nearly 1.5-kilometer-wide asteroid as potentially hazardous, meaning there’s a chance it could hit Earth in the future. But astronomers don’t think it will pose a threat to our planet in the near future, after calculating its future orbits and determining it won’t come too close to Earth.

The radar images showed that the asteroid is roughly spherical and is one of a pair, called a binary system. The space rock has a small moon orbiting it at a distance of 1.9 miles (3 kilometers).


Seven radar observations show the mile-wide asteroid 2011 UL21 as it approaches Earth on June 27 from about 4 million miles away. The asteroid and its tiny moon are circled in white.

“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 lead scientist at JPL who led the observations, said in a statement.

NASA missions, including the Lucy spacecraft scheduled to explore a mysterious population of space rocks called the Trojans later this decade, have helped reveal how many moons exist around asteroids in our solar system.

And the DART mission deliberately collided with a moonlet called Dimorphos, which orbits a larger asteroid called Didymos, to alter the motion of a celestial body in space for the first time. This was a way to test asteroid-deflecting technology in 2022.

Sometimes astronomers don’t know that an asteroid is in an orbit that brings it close to Earth until just before it approaches. That uncertainty is one of the reasons NASA is stepping up its efforts to better understand the population of asteroids that come closest to our world.

The researchers discovered asteroid 2024 MK just 13 days before it flew past Earth, at a distance of just 184,000 miles (295,000 kilometers) from our planet — just over three-quarters of the distance between Earth and the moon — on June 29.


A mosaic shows 2024 MK as the asteroid rotates in one-minute increments, about 16 hours after its closest approach to Earth.

The Asteroid Terrestrial-impact Last Alert System, or ATLAS, at the Sutherland Observing Station in South Africa first spotted the space rock on June 16. While the asteroid is also considered potentially hazardous, it doesn’t appear to be on a concerning orbit relative to Earth anytime soon.

Astronomers beamed radio waves to the space rock and captured a detailed image of asteroid 2024 MK. Its surface is littered with 10-meter-wide boulders, as well as hollows and ridges. The asteroid is 150 meters wide and appears angular and elongated, but also has some noticeable flat and rounded areas.

When the space rock passed by our planet and encountered Earth’s gravity, its orbit changed. Now the asteroid’s 3.3-year journey around the sun has been shortened by about 24 days.

Objects the size of asteroid 2024 MK only pass near Earth once every few decades, so astronomers gathered as much data as possible.

“This was an extraordinary opportunity to investigate the physical properties and obtain detailed images of a close-to-Earth asteroid,” Benner said.

Leave a Comment