On June 27, asteroid 2011 UL21 made a relatively close encounter with Earth, flying past our planet at a distance of 6.6 million kilometers (4.1 million miles), or roughly 17 times the average distance from the Earth to the Moon.
While it wasn’t close enough to worry about, the encounter gave astronomers an opportunity to get a closer look at the object. Doing so can help us learn more about such asteroids, as well as narrow down their orbits, allowing us to know whether they will pose risks to the planet further into the future.
“The term ‘Potentially Hazardous Asteroid’ (PHA) is an accurate official definition, referring to minor planets larger than approximately 140 meters. [459 feet] which can reach within 7.5 million km [4.6 million miles] from Earth,” Gianluca Masi, astrophysicist and scientific director of the Virtual Telescope Project, said in a statement before the flight. “In other words, only the largest asteroids capable of coming close enough to our planet are marked as PHAs, which does not mean they will hit Earth, but they still warrant better monitoring.”
During this year’s flyby, NASA’s Deep Space Network’s Goldstone Planetary Radar closely observed 2011 UL21, imaging it seven times as it passed by at 25 kilometers (16 miles) per second. This was the first chance NASA had to image the asteroid using radar, and when they did, they discovered that the asteroid is actually a binary system. The asteroid has its own moon, which orbits at 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 mutual orbits, masses and densities, which provide key information about how they may have formed,” Lance Benner, the principal scientist at JPL who helped lead the observations, said in a statement.
The moon can be seen at the bottom of these radar images.
NASA/JPL-Caltech
During the approach, NASA discovered that the asteroid is roughly spherical. Before the radar image, there was uncertainty about the size of the object, with estimates suggesting it could be as small as 1.7 kilometers and as large as 3.9 kilometers (1.05 to 2.4 miles). After the radar image, NASA puts its size at nearly 1 mile wide (1.5 kilometers), slightly smaller than expected.
It was actually quite a busy week for the radar system, which observes space objects by transmitting radio waves and then receiving the signal reflected back to the same antenna. On June 29, a second object – discovered only on June 16 – made a much closer approach, passing 184,000 miles (295,000 kilometers) from Earth. That’s just over three-quarters of the average distance between Earth and the Moon, a close enough approach from the asteroid tentatively named 2024 MK.
Asteroid 2024 MK, hurtling through space.
Image credit: NASA/JPL-Caltech
“For these observations, 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 bounced off the asteroid and back to Earth. “, NASA explained. “The result of this ‘bistatic’ radar observation is a detailed image of the asteroid’s surface, revealing concavities, ridges and boulders about 30 feet (10 meters) wide.”
The asteroid’s path was slightly altered by Earth’s gravity, shortening its 3.3-year orbit around the Sun by about 24 days. The asteroid, which was detected by the NASA-funded Earth Impact Early Warning System (ATLAS) just 13 days before its closest approach, is classified as potentially hazardous. However, calculations of its orbit indicate that it poses no threat to Earth for the foreseeable future.