As asteroid 2005 YU55 sped away from its close encounter with Earth on Tuesday, astronomers in Hawaii snapped its photo using one of the world’s largest optical-infrared telescopes.

The image from the Keck II Telescope, made possible with adaptive optics that correct for atmospheric distortion, shows a 788-foot-wide spherical rock with no smaller orbiting companions.

Earlier estimates set the diameter at 1,200 feet.

On the team behind the camera work were asteroid investigators William Merline and Peter Tamblyn of the Southwest Research Institute in Boulder, Colo., and Chris Neyman of the Keck Observatory on Hawaii island.

Their observing run also was webcast live on UStream from the Keck remote operations room in Waimea.

The asteroid made its closest approach to Earth — about 201,000 miles, inside the orbit of the moon — at 1:26 p.m. Hawaii time.

But the departing boulder was still visible to the Keck’s sharp eyes as night fell over the islands.

Although smaller than expected, YU55 still packs a punch.

Had it hit Earth at its relative speed of 8.5 miles per second, it would have generated an explosion likely greater than the one that leveled trees across a wide expanse of Siberia in 1908. The Tunguska explosion was generated by a rock only 120 feet across.

Meteor Crater, a 3,900-foot-wide hole in the desert east of Flagstaff, Ariz., was made 50,000 years ago by an impact from a rock 160 feet in diameter, scientists estimate.

There was no crater at Tunguska, scientists say, because the rock exploded in the atmosphere.

Don Yeomans, a senior research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., has estimated that a Tunguska-sized asteroid enters Earth’s atmosphere on average once every 300 years.

Detecting and tracking such asteroids is the mission of Yeomans’ Near-Earth Object Program Office and a Hawaii program called Pan-STARRS, which operates an observatory on Haleakala.

The program now is finding about one such object a day, according to Ken Chambers, a University of Hawaii astronomer and director of the Pan-STARRS Telescope 1.

Another asteroid, 2000 YA, will approach Earth within three lunar distances on Dec. 26, according to JPL.