An asteroid's close approach to Earth this Christmas has revealed it to be similar in shape to a "hippopotamus wading in a river", according to NASA.
The object, called 2003 SD220, is now the closest to Earth it has been in 400 years, NASA said.
It safely passed by the planet last Saturday at 2.9 million kilometres away — about seven and a half times the distance from the Earth to the Moon.
It will next come back to Earth in 2070, at a slightly closer range.
Three deep-space communications facilities gathered new data on the asteroid over three days from December 15, revealing its shape and surface and giving astronomers a better understanding of its orbit, NASA said.
The images are 20 times more detailed than previous readings of the asteroid.
The 'hippo' is 1.6 kilometres in length and has an "extremely slow rotation" of about 12 days, the space agency said.
"It also has what seems to be a complex rotation somewhat analogous to a poorly thrown football," NASA said in a statement.
The radar images of the asteroid, taken using one antenna to transmit and another to receive, are comparable in detail to a spacecraft flyby, such as when NASA's OSIRIS-REx took photos earlier this month of asteroid Bennu, the agency said.
