Planetary Defense: NASA Radar Captures Two Large Asteroids Zooming Past Earth

Deep Space Network’s Goldstone planetary radar had a busy few days observing asteroids 2024 MK and 2011 UL21 as they safely passed by Earth.

Researchers at NASA‘s Jet Propulsion Laboratory (JPL) in Southern California recently tracked two asteroids as they flew past our planet. One turned out to have a small moon orbiting it, while the other was discovered just 13 days before its closest approach to Earth.

Although there was no risk that either of the objects that passed close to Earth would hit our planet, the radar observations made during these two approaches will provide valuable knowledge for planetary defense. They will also provide information about their sizes, orbits, rotation, surface details, and clues about their composition and formation.

The Goldstone Solar System Radar, part of NASA’s Deep Space Network, made these observations of the recently discovered 150-meter-wide asteroid 2024 MK, which made its closest approach on June 29 — coming within about 295,000 kilometers of Earth. Credit: NASA/JPL-Caltech

Insights from the 2011 UL21 asteroid encounter

The asteroid 2011 UL21 passed Earth on June 27 at a distance of 4.1 million miles (6.6 million kilometers), or about 17 times the distance between the moon and Earth, and was discovered in 2011 by the NASA-funded Catalina Sky Survey in Tucson, Arizona. But this is the first time it has come close enough to Earth to be detected by radar. Although the nearly 0.9-mile (1.5-kilometer) wide object is classified as potentially hazardous, calculations of its future orbit indicate that it poses no threat to our planet in the near future.

The Goldstone Solar System Radar (GSSR) is a large radar system used to study objects in the solar system. It is located in the desert near Barstow, California, and consists of a 500-kW X-band (8500 MHz) transmitter and a low-noise receiver on the 70-m DSS 14 antenna at the Goldstone Deep Space Communications Complex. Credit: NASA

Using the Deep Space Network’s 70-meter-wide Goldstone Solar System Radar, called Deep Space Station 14 (DSS-14) near Barstow, California, JPL scientists beamed radio waves toward the asteroid and received the reflected signals through the same antenna. In addition to determining that the asteroid is roughly spherical, they discovered that it is a binary system: a smaller asteroid, or moon, orbits it about 3 kilometers away.

“About two-thirds of asteroids of this size are thought to be binary star 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,” said Lance Benner, a principal scientist at JPL who helped lead the observations.

These seven radar observations by the Deep Space Network’s Goldstone Solar System Radar 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 (a bright dot at the bottom of the image) are circled in white. Credit: NASA/JPL-Caltech

Second close approach

Two days later, on June 29, the same team spotted asteroid 2024 MK passing our planet at a distance of just 184,000 miles (295,000 kilometers), or a little more than three-quarters of the distance between the moon and Earth. This asteroid is about 500 feet (150 meters) wide and appears oblong and angular, with noticeable flat and rounded areas.

For these observations, the 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. 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.

Because asteroids the size of 2024 MK pass relatively rarely, JPL’s planetary radar team gathered as much information as possible about the near-Earth object. This mosaic shows the spinning asteroid at one-minute intervals, about 16 hours after its closest approach to Earth. Credit: NASA/JPL-Caltech

Close-Earth objects the size of 2024 MK are relatively rare, occurring on average every few decades. That’s why the JPL team wanted to gather as much data about the object as possible. “This was an extraordinary opportunity to investigate the physical properties and obtain detailed images of a close-Earth asteroid,” Benner said.

Combined in this animation, observations from NASA’s Goldstone Solar System Radar show the tumbling asteroid 2024 MK shortly after it made its closest approach to our planet on June 29. The 150-meter-wide asteroid’s orbit was altered slightly by Earth’s gravity as it passed. Credit: NASA/JPL-Caltech

Contributing to planetary defense

Asteroid 2024 MK was first reported on June 16 by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) at the Sutherland Observing Station in South Africa. Its orbit was altered by Earth’s gravity as the asteroid passed by, shortening its 3.3-year orbital period around the Sun by about 24 days. Although the asteroid is classified as a potentially hazardous asteroid, calculations of its future motion indicate that it poses no threat to our planet in the near future.

The Goldstone Solar System Radar Group is supported by NASA’s Near-Earth Object Observations Program within the Planetary Defense Coordination Office at the agency’s headquarters in Washington. The Deep Space Network, managed by JPL, receives programmatic oversight from the Space Communications and Navigation program office within the Space Operations Mission Directorate, also at NASA Headquarters.