A large space rock that will zip past the Earth this Halloween on Saturday is most likely a dead comet that bears an eerie resemblance to a skull, said US space agency NASA.

Discovered three weeks ago, the asteroid, 2015 TB145, will fly by our planet at just 1.3 lunar distances, or about 490,000 km, at 1.00 p.m. on Saturday, that means it will pose no threat to the Earth, Xinhua quoted the space agency as saying.

Scientists observing asteroid 2015 TB145 with NASA' s Infrared Telescope Facility (IRTF) on Maunakea, Hawaii, determined that the celestial object may have shed its volatility after numerous passes around the sun.

Radar images generated by the US National Science Foundation's 305-metre Arecibo Observatory in Puerto Rico showed the object is spherical in shape and about 600 metres in diameter.

"The IRTF data may indicate that the object might be a dead comet, but in the Arecibo images it appears to have donned a skull costume for its Halloween flyby," Kelly Fast, IRTF programme scientist at NASA Headquarters said.

Vishnu Reddy, a research scientist at the US Planetary Science Institute, found that the asteroid, nicknamed 'Spooky' is similar to dark carbonaceous meteorites.

"We found that the object reflects about six percent of the light it receives 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," Reddy said.

"That suggests it could be cometary in origin but as there is no coma evident, the conclusion is it is a dead comet."

Asteroid 2015 TB145 was discovered on October 10, by the University of Hawaii' s Pan-STARRS-1 (Panoramic Survey Telescope and Rapid Response System) on Haleakala, Maui, part of the NASA-funded Near-Earth Object Observations Programme.

The next time the asteroid will be in Earth's neighbourhood will be in September 2018, when it will make a distant pass at about 38 million kilometres, or about a quarter the distance between Earth and the Sun.