A massive asteroid will make a rare fly-by early on Wednesday and although it poses no danger of crashing to Earth, US scientists said this week they are eager for a closer look.
"This is not a potentially hazardous asteroid, just a good opportunity to study one,'' said National Science Foundation astronomer Thomas Statler.
The circular asteroid, named 2005 YU55, is about 400 metres wide and will come closer than the Moon, zipping by at a distance of 325,000 kilometres, the US space agency said.
The time of the nearest flyby is expected to be at 1.28am Wednesday NZ time.
The encounter will be the closest by an asteroid of that size in more than 30 years, and a similar event will not happen again until 2028.
However, those who want to see will need a telescope.
The asteroid is "going to be pretty faint when it flies by,'' said Scott Fisher, program director of the National Science Foundation's Division of Astronomical Sciences.
"It will not be visible to the naked eye. You will need a telescope that has a mirror at least six inches in size to see it. To make it even more difficult to observe, it will be moving very quickly across the sky as it passes.''
Several radar telescopes are set up in North America to catch glimpses of the space rock, Fisher added.
"The best time (and place) to observe it would be in the early evening on November 8th from the east coast of the US.''
Astronomers who have studied the object, part of the C-class of asteroids, say it is very dark, like the color of charcoal, and quite porous.
It was first discovered in 2005 by Robert McMillan of the Spacewatch Project, a solar-system-scanning group of scientists near Tucson, Arizona.
While 2005 YU55 will stay a safe distance away, it is part of a crew of 1,262 big asteroids circling the Sun and measuring more than 150 metres across that NASA classifies as "potentially hazardous.''
"We want to study these asteroids so if one does look like it may hit us someday, we'll know what to do about it,'' Statler said.
The asteroid's closest pass is set to take place in 2094, at a distance of 269,000 kilometres, according to forecasts.
"The observations will give us a piece of the puzzle, one we don't get many chances to see,'' said Don Yeomans of NASA's Jet Propulsion Laboratory.
"At one time, we thought these were the asteroids that delivered carbon and other elements to the early Earth, so they are pretty important.''
NASA said the last time a space rock this big approached Earth was in 1976, "although astronomers did not know about the fly-by at the time.''