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The Deep Space Network’s Goldstone Planetary Radar had a busy few days watching asteroids 2024 MK and 2011 UL21 as they safely passed Earth.

Scientists at NASA’s Jet Propulsion Laboratory 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. There was no risk of any near-Earth objects impacting our planet, but the radar observations made during these two close approaches will provide valuable practice for protecting the planet, as well as information about their sizes, orbits, rotation, surface details and clues as to their composition and formation.

Passing by 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, asteroid 2011 UL21 was discovered in 2011 by the Catalina Sky Survey in Tucson, Arizona. But this is the first time it has come close enough to Earth to be captured by radar. Although the nearly mile (1.5 kilometer) wide object is classified as potentially hazardous, calculations of its future orbits indicate that it will not pose a threat to our planet in the foreseeable future.

Using the Deep Space Network’s 230-foot (70-meter) Goldstone Solar System Radar, called Deep Space Station 14 (DSS-14), near Barstow, California, JPL scientists transmitted radio waves to the asteroid and received the reflected signals from the same antenna . In addition to determining the asteroid to be roughly spherical, they discovered that it is a binary system: a smaller asteroid or moon orbits it from a distance of about 1.9 miles (3 kilometers).

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

Second close approach

Two days later, on June 29, the same team watched asteroid 2024 MK fly past our planet from a distance of just 184,000 miles (295,000 kilometers), or just over three-quarters of the distance between the Moon and Earth. About 500 feet (150 meters) wide, this asteroid appears elongated and angular, with prominent flat and rounded areas.

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 bounces off the asteroid and returns to Earth . The result of this “bistatic” radar observation is a detailed image of the asteroid’s surface, revealing depressions, ridges and boulders about 30 feet (10 meters) wide.

Close approaches of near-Earth objects the size of 2024 MK are relatively rare, occurring on average every few decades, so the JPL team tried to collect as much data on the object as possible.

“This was an outstanding opportunity to study the physical properties and obtain detailed images of a near-Earth asteroid,” Benner said.

Asteroid 2024 MK was first reported on June 16 by the Asteroid Earth Impact Alert System (ATLAS) at the Sutherland Observatory in South Africa. Its orbit was altered by Earth’s gravity as it passed by, reducing its 3.3-year orbital period around the sun by about 24 days. Although it is classified as a potentially dangerous asteroid, calculations of its future motion show that it does not pose a threat to our planet in the foreseeable future.