An asteroid a third of a mile across is due to pass close by the Earth at the end of the month.
The space rock, code-named 2004 BL86, is expected to reach a point about 745,000 miles from our planet, or three times the distance to the Moon, on January 26.
Although easily far enough away to be safe, the flyby counts as a narrow encounter in astronomical terms.
It will be the closest any asteroid comes to the Earth until the predicted fly-past of another rock,1999 AN10, on August 7, 2027.
Dr Don Yeomans, retiring head of the US space agency Nasa's Near-Earth Object Programme office at the jet propulsion laboratory in Pasadena, California, said: "Monday January 26 will be the closest asteroid 2004 BL86 will get to Earth for at least the next 200 years.
"And while it poses no threat to Earth for the foreseeable future, it's a relatively close approach by a relatively large asteroid, so it provides us a unique opportunity to observe and learn more."
The asteroid is likely to be bright enough for amateur astronomers to spot with small telescopes and strong binoculars.