Scientists at NASA's Jet Propulsion Laboratory (JPL) in Southern California monitored these asteroids during their close approaches. One asteroid was found to have a small moon, while the other was discovered just 13 days before its closest pass. Neither posed any risk to Earth, but the radar data collected will enhance planetary defense techniques and provide insights into their characteristics and formation.

Asteroid 2011 UL21, which 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, was initially discovered by the NASA-funded Catalina Sky Survey in 2011. This flyby marked the first occasion it was close enough to be imaged by radar. Although classified as potentially hazardous, future orbit calculations indicate no threat to Earth. The radar observations revealed that the nearly mile-wide (1.5-kilometer-wide) asteroid is roughly spherical and part of a binary system, with a smaller moonlet orbiting it at about 1.9 miles (3 kilometers) distance.

"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," said Lance Benner, principal scientist at JPL who helped lead the observations.

On June 29, the team observed asteroid 2024 MK as it passed within 184,000 miles (295,000 kilometers) of Earth, slightly more than three-quarters of the distance to the Moon. This 500-foot (150-meter) wide asteroid appears elongated and angular, with both flat and rounded regions. The observations, using the 230-foot-wide (70-meter) DSS-14 to transmit radio waves and the 114-foot (34-meter) DSS-13 antenna to receive the reflected signals, resulted in detailed surface images showing concavities, ridges, and boulders about 30 feet (10 meters) wide.

Close approaches of near-Earth objects the size of 2024 MK occur about every few decades. The JPL team aimed to collect extensive data on this asteroid. "This was an extraordinary opportunity to investigate the physical properties and obtain detailed images of a near-Earth asteroid," said Benner.

Asteroid 2024 MK was first detected on June 16 by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) at Sutherland Observing Station in South Africa. Earth's gravity altered its orbit during the flyby, shortening its 3.3-year orbital period around the Sun by about 24 days. Despite being classified as potentially hazardous, future orbit calculations show no threat to Earth.

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. Managed by JPL, the Deep Space Network receives oversight from the Space Communications and Navigation program office within the Space Operations Mission Directorate, also at NASA Headquarters.

More information about planetary radar and near-Earth objects can be found here.

Related Links
Asteroid Radar Watch
Deep Space Network
Planetary Defense Coordination Office
Asteroid and Comet Mission News, Science and Technology