Astronomers using the US space agency's Spitzer Space Telescope measured the size of the asteroid, called 2011 MD, for the Asteroid Redirect Mission (ARM), which seeks to drag a space rock into orbit around the moon for future visitation by astronauts, Xinhua quoted NASA as saying.
"The near-Earth asteroid ... was found to be roughly 20 feet (6 metres) in size, and its structure appears to contain a lot of empty space, perhaps resembling a pile of rubble," it said in a statement.
"The Spitzer results confirm that asteroid 2011 MD has characteristics suitable for the ARM proposal, elevating it to the 'valid candidate' level."
According to NASA, valid candidates are those asteroids with the right size, mass and rotation rate to be feasibly captured by the robotic spacecraft.
Prior to the Spitzer study, the size of 2011 MD was only very roughly calculated with visible light observations. Spitzer, however, can use its heat-sensitive infrared vision to spy asteroids and get better estimates of their sizes.
Previously, two other valid candidates, 2009 BD and 2013 EC20, have been identified. NASA said it continues to search for and find new potential candidates.
The Asteroid Redirect Mission aims to identify, capture and redirect a near-Earth asteroid to a stable orbit around the moon with a robotic spacecraft. US astronauts will then visit and explore the asteroid using NASA's Orion spacecraft and Space Launch System rocket in the 2020s, returning to Earth with samples.