'As of December 24, there are 17,495 known Near-Earth Objects (NEOs) around our planet; 17,389 are asteroids'. Arecibo's radar images of Phaethon have resolutions as fine as about 250 feet (75 meters) per pixel.
"These new observations of Phaethon show it may be similar in shape to asteroid Bennu, the target of NASA's OSIRIS-REx spacecraft, but more than 1,000 Bennus could fit inside Phaethon", he said.
Fresh measurements were calculated from the Arecibo Observatory Planetary Radar in Puerto Rico, putting the diameter of the asteroid at 5.8 kilometres or 3.6 miles - that's about 1 kilometre or 0.6 miles bigger than experts thought. The researchers also used Arecibo's radar to spot a depression near 3200 Phaethon's equator that stretches a few hundred meters at minimum, as well as a dark, spherical spot close to one of the asteroid's poles.
It will not make another approach as close until after 2093. High frequency radio signals from Earth did not return from the mysterious dark zone, suggesting it could be a crater. Phaethon is the second largest near-Earth asteroid classified as "Potentially Hazardous".
To qualify as potentially hazardous objects, these kind of space rocks have to achieve a certain level of size - and their orbits have to bring them close enough to Earth, even if there's very little chance of a direct strike in the near future.
Tracking and characterizing PHAs is a primary mission of NASA's Planetary Defense Coordination Office.
Asteroids and comets are differentiated based on what they are mad of.
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The Arecibo Observatory has the most powerful astronomical radar system on Earth. The observatory suffered "minor structural damage" when Hurricane Maria made landfall on September 20.
Though Arecibo's structure was only minimally damaged when the hurricane hit, a long-term lack of power and diesel fuel for its generators has drastically hampered its radar operations.
Asteroid 3200 Phaethon was discovered on October 11, 1983, by NASA's Infrared Astronomical Satellite (IRAS), and the planetary dust that produces the annual Geminid meteor shower originates from this asteroid.
The new observations conducted from December 15-19 show Phaethon to be about 3.6 miles (six kilometres) wide.
Earlier this month a massive asteroid called 3200 Phaethon made its closest approach to Earth at around 6.4 million miles (10.3 million kilometers) away. The Arecibo Observatory is a facility of the National Science Foundation operated under cooperative agreement by SRI International, USRA, and Universidad Metropolitana.
In a statement about the asteroid, which was first discovered in 1983, a spokesperson for Nasa said: "With a diameter of about 5km, Phaethon is the third largest near-Earth asteroid classified as 'Potentially Hazardous"'.