Scientists have spotted two space rocks that may be Earth's freshest asteroid neighbors.
The strange pair of near-Earth asteroids is separated by about 600,000 miles (1 million kilometers), and researchers calculated that they likely broke off the same asteroid just a few centuries ago.
"It's very exciting to find such a young asteroid pair that was formed only about 300 years ago, which was like this morning — not even yesterday — in astronomical timescales," Petr Fatka, lead author on the new research and an astronomer at the Astronomical Institute of the Czech Academy of Sciences, said in a statement published by Lowell Observatory in Arizona, where some of the observations used in the work were gathered.
The two asteroids were discovered separately in August 2019, but within a month, the similarity of their orbits quickly stood out to researchers, who decided to take a closer look.
Strangely, both asteroids happened to be a variety that scientists refer to as D-type. Scientists believe this category of asteroid is rich in compounds that quickly disappear into space if the rocks get too warm, and D-type asteroids are quite rare to find close to the sun. The fresh duo of rocks never gets closer to the sun than Earth's orbit and wander out as far as Saturn's orbit. Earth is about 93 million miles — 150 million kilometers — from the sun, on average, while Saturn is about 886 million miles, or 1.4 billion km.)
But here they were, dubbed 2019 PR2 and 2019 QR6, one just over a half-mile (1 km) wide and the other about half that size. And the story became more complicated when the researchers turned to the archives, where they spotted the two asteroids in data gathered in 2005 by the Catalina Sky Survey in Arizona, which hadn't been able to detect the rocks at the time.
Those additional observations sharpened scientists' orbital calculations enough to "rewind" the asteroids' locations over the years, determining that in the not-so-distant past, the two objects had in fact been just one. Depending on the model the scientists used, the pair separated between 230 and 420 or between 265 and 280 years ago.
However, the sibling rocks still have some secrets. In particular, scientists can't quite make their histories line up with the usual factors that influence orbits. Instead, it seems like the objects may have been off-gassing some material, like a comet does, even though there's no sign of any such activity today.
"In the present day, the bodies don't display any signs of cometary activity," Nicholas Moskovitz, an astronomer at Lowell Observatory and co-author on the new research, said in the same statement. "So it remains a mystery how these objects could have gone from a single parent body, to individually active objects, to the inactive pair we see today in just 300 years."
The two asteroids made their closest approach to Earth in October 2019; the next such visit will occur in November 2047. However, scientists hope that they won't need to wait quite that long to learn more about the strange space rocks, which will again be observable from Earth in 2033, according to Fatka.
The research is described in a paper published Wednesday (Feb. 2) in the journal Monthly Notices of the Royal Astronomical Society.
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Meghan is a senior writer at Space.com and has more than five years' experience as a science journalist based in New York City. She joined Space.com in July 2018, with previous writing published in outlets including Newsweek and Audubon. Meghan earned an MA in science journalism from New York University and a BA in classics from Georgetown University, and in her free time she enjoys reading and visiting museums. Follow her on Twitter at @meghanbartels.