As the “Halloween” asteroid approached its flyby of Earth on Saturday, the Arecibo Observatory in Puerto Rico captured images of the space rock using its large radio telescope. The rock, likely a dead comet, was due to come within 486,000 kilometers of Earth, or about 1.3 times the distance to the Moon at 1pm ET. In radar images it resembled a human skull.

The 1,000-foot Arecibo instrument, managed by the National Science Foundation, is the largest single-aperture telescope in the world and is used primarily for radio astronomy. By looking in the radio portion of the spectrum, astronomers were able to better characterize the size and roughness of the asteroid, formally known as 2015 TB145.

Astronomers found that the Halloween asteroid reflects only about six percent of the light from the Sun. "That is similar to fresh asphalt, and while here on Earth we think that is pretty dark, it is brighter than a typical comet which reflects only 3 to 5 percent of the light,” said Vishnu Reddy, a research scientist at the Planetary Science Institute, Tucson, Ariz. “That suggests it could be cometary in origin—but as there is no coma evident, the conclusion is it is a dead comet."

The Halloween asteroid has raised some concerns among skywatchers because it was only found about three weeks before making its closest approach. The radar data indicates the asteroid is about 600 meters across and spherical.

If an asteroid of that size hit Earth, it would do considerable damage. Scientists estimate the Tunguska event in Siberia, which flattened 2,000 square kilometers of forest in 1908, was less than 200 meters in diameter.