Hayabusa 2 dipped as low as 55 meters (180 feet) for the release, then retreated back from the asteroid. The robots move by hopping because extremely weak gravity makes rolling hard.
Japan's Hayabusa2 spacecraft arrived at diamond-shaped asteroid Ryugu on June 27 after three and a half year journey.
As the craft approached to within 100 meters (328 feet) of the space rock, it sent back a photo showing Hayabusa's shadow projected onto its barren, cratered surface. "This is probably due to the rotation to Ryugu, and MINERVA-II1 is now on the far side of the asteroid". Once on the surface, the robots will hop around, jumping as high as 49 ft (15 m), and staying in the air as long as 15 minutes.
Beneath their desolate surface, asteroids are believed to contain a rich treasure-trove of information about the formation of the solar system billions of years ago.
Hayabusa2 carries four payloads - three rovers and the 10kg lander, the Mobile Asteroid Surface Scout (MASCOT) built by the German Aerospace Centre (DLR) and the French space agency (CNES). "Additional instruments on the rovers include optical sensors, an accelerometer and a gyroscope". "The mother ship itself will also make several forays to the surface in 2019, to grab asteroid material that will be returned to Earth in December 2020".
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The rovers are stored in drum-shaped container at the base of the Hayabusa-2 "mothership". "Gravity on the surface of Ryugu is very weak, so a rover propelled by normal wheels or crawlers would float upwards as soon as it started to move", mission team members wrote in a MINERVA-II1 description.
Japanese scientists are now racing NASA for that historic achievement, with the U.S. agency's own sample retrieval mission due to arrive back on Earth in 2023.
If all goes well, Hayabusa-2 will be the first spacecraft to successfully place robot rovers on the surface of an asteroid. It is a "C-type" asteroid which is the most common class of asteroids.
The cautious announcement came after a similar JAXA probe in 2005 released a rover that failed to reach its target asteroid.