The asteroid named 2005 YU55 is being watched by ground antennas as it approaches from the direction of the sun. The last time it came within so-called shouting distance was 200 years ago.

Closest approach will occur at 6:28 pm EST on Tuesday when the asteroid passes within 325,000 km of Earth. That's closer than the roughly 386,000 km between the Earth and the moon.

The moon will be just under 241,000 km from the asteroid at the time of closest approach.

Both the Earth and moon are safe - "this time", said Jay Melosh, professor of Earth and atmospheric sciences at Purdue University.

If 2005 YU55 were to plow into the home planet, it would blast out a crater 6.4 km across and 510 meters deep, according to Melosh's calculations. Think a magnitude-7 earthquake and 21-meter-high tsunami waves.

Scientists have been tracking the slowly spinning, spherical, dark-colored object since its discovery in 2005, and are positive it won't do any damage.

Smaller objects come close all the time, Yeomans noted, but nothing this big will have ventured so close since 1976. And nothing this large will again until 2028.

Radar observations from California and Puerto Rico will help scientists ascertain whether the asteroid is pockmarked with craters and holds any water-bearing minerals or even frozen water.

Amateur astronomers would need a 15-centimeter-or-bigger telescope and know exactly where to look to spot it.

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