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

The Dual Asteroid Observations

NASA's Deep Space Network's Goldstone planetary radar successfully observed two distinct near-Earth asteroids within a two-day period in June 2024. The first, asteroid 2011 UL21, passed Earth on June 27 at a distance of approximately 4.1 million miles (6.6 million kilometers) – about 17 times the distance between the Moon and Earth. Though discovered in 2011 by the NASA-funded Catalina Sky Survey, this marked the first time the asteroid came close enough to be imaged by radar.

Just two days later, on June 29, the same radar system tracked asteroid 2024 MK, which approached much closer at approximately 184,000 miles (295,000 kilometers) from Earth – slightly more than three-quarters of the distance to the Moon. This asteroid had only been discovered 13 days prior to its closest approach by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) at the Sutherland Observing Station in South Africa.

Revealing Characteristics Through Radar Technology

The Goldstone Solar System Radar (GSSR), located in the desert near Barstow, California, utilizes a 500-kW X-band transmitter and a sensitive receiver on the 70-meter DSS 14 antenna. For the observations of 2011 UL21, scientists transmitted radio waves to the asteroid and received the reflected signals using the same antenna – a monostatic radar technique.

Analysis revealed that the nearly mile-wide (1.5-kilometer-wide) 2011 UL21 is roughly spherical and, significantly, a binary system. A smaller asteroid, or moonlet, orbits it at a distance of about 1.9 miles (3 kilometers). As noted by Lance Benner, principal scientist at JPL who helped lead the observations, '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.'

For the closer approach of 2024 MK, researchers employed a bistatic radar configuration, using the DSS-14 antenna to transmit signals and the 34-meter DSS-13 antenna to receive the reflections. This technique produced detailed images showing the 500-foot-wide (150-meter-wide) asteroid to be elongated and angular, with prominent flat and rounded regions, as well as surface features including concavities, ridges, and boulders approximately 30 feet (10 meters) wide.

Orbital Changes and Future Trajectories

Both asteroids experienced gravitational interactions with Earth during their passages, altering their orbits around the Sun. The asteroid 2024 MK saw its 3.3-year orbital period reduced by approximately 24 days due to Earth's gravitational influence as it passed by.

Despite being classified as potentially hazardous asteroids, detailed calculations of their future motions indicate that neither 2011 UL21 nor 2024 MK poses a threat to Earth for the foreseeable future. This classification stems from their size and proximity to Earth's orbit, but ongoing monitoring allows scientists to refine trajectory predictions with increasing confidence.

The rarity of such close approaches adds significance to these observations. Close encounters with asteroids the size of 2024 MK occur, on average, only about every couple of decades, making each opportunity to gather detailed data particularly valuable for scientific understanding and defensive preparedness.

Advancing Planetary Defense Capabilities

These radar observations contribute directly to NASA's planetary defense efforts, which are coordinated through the Planetary Defense Coordination Office at NASA Headquarters in Washington. The Goldstone Solar System Radar Group receives support from NASA's Near-Earth Object Observations Program within this office.

The Deep Space Network, which manages the Goldstone facilities, operates under programmatic oversight from the Space Communications and Navigation program office within the Space Operations Mission Directorate, also at NASA Headquarters. This organizational structure ensures that asteroid tracking and characterization efforts are integrated into broader space situational awareness initiatives.

By practicing radar observations during safe passages, scientists refine techniques that would be essential if a hazardous asteroid were detected on a collision course with Earth. The data gathered – including precise measurements of size, shape, rotation, surface composition, and internal structure – informs potential deflection strategies and impact prediction models.

Technological Implications for Space Monitoring

The successful application of both monostatic and bistatic radar techniques during these observations demonstrates the versatility of NASA's Deep Space Network for planetary science and defense applications. The ability to switch between transmission and reception configurations allows scientists to gather different types of data depending on the asteroid's distance, size, and characteristics.

These observations highlight the importance of sustained investment in space monitoring infrastructure. Systems like ATLAS, which discovered 2024 MK just weeks before its approach, and the Catalina Sky Survey, which found 2011 UL21 over a decade ago, form a critical detection network that provides early warning of near-Earth objects.

The combination of discovery telescopes and follow-up radar characterization creates a comprehensive approach to near-Earth object studies. While optical telescopes excel at initial detection and tracking, radar provides unique capabilities for determining physical properties that cannot be measured through optical means alone.

What this means

The June 2024 radar observations of asteroids 2011 UL21 and 2024 MK exemplify how NASA's planetary defense system operates during routine celestial encounters. While these particular asteroids posed no danger to Earth, the opportunity to characterize them in detail strengthens our collective ability to respond effectively to genuine threats. The combination of early detection systems like ATLAS and Catalina Sky Survey with powerful characterization tools such as the Goldstone Solar System Radar creates a layered defense approach. As near-Earth object monitoring continues to advance, each successful observation adds to our knowledge base and technical readiness, contributing to the long-term safety of our planet in the dynamic environment of near-Earth space.