This collage of radar images of near-Earth asteroid 1999 JD6 was collected by Nasa on July 25, 2015. The images show the rotation of the asteroid, which made its closest approach on July 24 at 9:55 p.m. PDT (12:55 a.m. EDT on July 25) at a distance of 4.5 million miles - about 19 times the distance from Earth to the moon
Nasa has captured images of a peanut-shaped asteroid as it made its closest approach to Earth last weekend.

The bizarrely-shaped asteroid appears to be what is known as a contact binary, which is an asteroid with two lobes that are stuck together.

The images show the rotation of the space rock, named 1999 JD6, which made its closest approach on July 24 at 9:55 p.m. PDT (12:55 a.m. EDT on July 25).

The asteroid remained at a distance of about 4.5 million miles (7.2 million km), or about 19 times the distance from Earth to the moon.

'Radar imaging has shown that about 15 per cent of near-Earth asteroids larger than 600 feet (180 metres), including 1999 JD6, have this sort of lobed, peanut shape,' said Lance Benner, who leads Nasa's asteroid radar research program.

To obtain the views, researchers paired Nasa's 230ft-wide (70-metre) Deep Space Network antenna at Goldstone, California, with the 330-foot (100-metre) National Science Foundation Green Bank Telescope in West Virginia.

Using this approach, the Goldstone antenna beams a radar signal at an asteroid and Green Bank receives the reflections.

The technique, referred to as a bistatic observation, dramatically improves the amount of detail that can be seen in radar images.

The new views obtained with the technique show features as small as about 25 feet (7.5 metres) wide.

The individual images used in the footage were generated from data collected on July 25.

They show the asteroid is highly elongated, with a length of approximately 1.2 miles (2km) on its long axis. The film spans a period of about seven hours, 40 minutes.

This week's flyby was the closest approach the asteroid will make to Earth for about the next 40 years.

The next time it will approach Earth this closely is in 2054, at approximately the same distance of this week's flyby.

Data from the new observations will be particularly useful to Sean Marshall, a graduate student at Cornell University in Ithaca, New York.

'I'm interested in this particular asteroid because estimates of its size from previous observations, at infrared wavelengths, have not agreed,' he said.

'The radar data will allow us to conclusively resolve the mystery of its size to better understand this interesting little world,' he said.

Despite the uncertainty about its size, asteroid 1999 JD6 has been studied extensively and many of its physical properties, as well as its trajectory, are well known.

It rotates in just over seven-and-a-half hours and is thought to be a relatively dark object.

Asteroid 1999 JD6 was discovered on May 12, 1999, by the Lowell Observatory Near-Earth-Object Search, located in Flagstaff, Arizona.