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Images from the Arecibo Observatory in Puerto Rico show a massive space rock's close approach to Earth on Dec. 16. 

The asteroid 3200 Phaethon zipped to within about 6.4 million miles (10.3 million kilometers) of Earth, or about 27 times the distance from Earth to the moon. This was the space rock's closest approach to our planet since 1974 (when it also flew by on Dec. 16). Phaethon will not come that close to Earth again until Dec. 14, 2093. [In Images: Potentially Dangerous Asteroids]

Radar images of near-Earth asteroid 3200 Phaethon were generated by astronomers at the National Science Foundation's Arecibo Observatory on Dec. 17, 2017.
Radar images of near-Earth asteroid 3200 Phaethon were generated by astronomers at the National Science Foundation's Arecibo Observatory on Dec. 17, 2017.
Credit: Arecibo Observatory/NASA/NSF

The images, though grainy, have helped researchers clarify the size of 3200 Phaethon and some of its physical features, according to a statement from NASA. The rock appears to be roughly spherical, with a diameter of about 3.6 miles (6 km), or about 0.6 miles (1 km) larger than previous estimates, according to the statement. 

The images of 3200 Phaethon show that it also has "a large concavity, or depression, at least several hundred meters in extent near its equator, and a conspicuous dark, circular feature near one of the poles," according to the statement. The dark feature could be a crater or a depression that does not reflect the light emitted by Arecibo's radar instrument, Patrick Taylor, a scientist at the Universities Space Research Association (USRA) and group leader for Planetary Radar at Arecibo Observatory, said in the statement. Arecibo has the most powerful radar system on Earth and is used by NASA's Planetary Defense Coordination Office to monitor "potentially hazardous asteroids," or PHAs. The observatory suffered "minor structural damage" when Hurricane Maria made landfall on Sept. 20. 

Near-Earth asteroid 3200 Phaethon, imaged by the radar instrument at the Arecibo Observatory.
Near-Earth asteroid 3200 Phaethon, imaged by the radar instrument at the Arecibo Observatory.
Credit: Arecibo Observatory/NASA/NSF

"Arecibo is an important global asset, crucial for planetary defense work because of its unique capabilities," Joan Schmelz, of the Universities Space Research Association (USRA) and deputy director of Arecibo Observatory, said in the statement. "We have been working diligently to get it back up and running since Hurricane Maria devastated Puerto Rico."

Phaethon may have once been a comet, meaning it contained water ice near its surface that would sublimate as it flew by the sun. It's also likely that a trail of debris left behind by Phaethon is responsible for the annual Geminid meteor shower

Follow Calla Cofield @callacofield. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.