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Astronomers recently got a closer look at a “potentially dangerous” asteroid as it whizzed safely past Earth, and what they saw surprised them: The Rombergart is unusually elongated for an asteroid and spinning much slower than expected.
The asteroid anomaly, known as 2011 AG5, was discovered in January 2011 by the Mount Lemmon Survey using a telescope based near Tucson, Arizona. Rombergart made headlines at the time because scientists predicted that the asteroid’s orbit around the Sun, which takes about 621 days, could put it on a devastating collision course with Earth in 2040. But follow-up observations in 2012 revealed that its orbit had been significantly miscalculated and that it poses no real threat to our planet.
On February 3, 2023, the asteroid passed within about 1.1 million miles (1.8 million kilometers) of Earth, or nearly five times the distance between Earth and the Moon. The close flyby gave astronomers the chance to scan it properly for the first time.
Using the powerful 230-foot-wide (70-meter) Goldstone Solar System Radar antenna dish at NASA’s Deep Space Network facility in southern California, scientists took several images of the asteroid. The blurry images revealed that 2011 AG5 is 1,600 feet (500 m) long and about 500 feet (150 m) wide—about the size of the Empire State Building, according to a statement (opens in a new tab) from NASA’s Jet Propulsion Laboratory (JPL).
“Of the 1,040 near-Earth objects observed by planetary radar to date, this is one of the most elongated we have seen,” Lance Benner (opens in a new tab), a principal investigator at JPL who helped lead the observations, said in the statement. Most of the scanned space rocks that have whizzed off Earth are much more rounded, he added.
The researchers did not provide an explanation for why 2011 AG5 is so elongated.
Related: Why are asteroids and comets such strange shapes?
The radar scans also allowed scientists to calculate the asteroid’s spin, revealing that it takes around 9 hours for the oblong object to complete a single rotation. This is a much longer rotation period than most asteroids, researchers wrote in the statement, and may be influenced by the rock’s unusual shape. But it is unclear exactly why the asteroid spins so slowly.
The new images also showed subtle dark and lighter patches several feet across the asteroid’s surface, which could indicate that there are several small-scale surface features sprinkled across the asteroid. But what they might be remains a mystery.
The researchers hope that the additional data about the asteroid’s orbit gathered by the new radar scans can constrain where it will be in the future, which could help explain its unusual properties.
“These new planetary radar team distance measurements will further delineate exactly where it will be far into the future,” Paul Chodas (opens in a new tab), the director of NASA’s Center for Near Earth Object Studies (CNEOS) at JPL said in the statement. This will increase our chances of learning more about this bizarre space rock, he added.
Although 2011 AG5 will not collide with Earth, it is expected to pass much closer — within 1.1 million km of our planet — when it returns for its next flyby in 2040. Any asteroid that passes within 7.5 million miles (7, 5 million miles) km) of Earth is classified as a “potentially hazardous asteroid,” according to NASA, so it’s important to keep an eye on them.
