An asteroid half-a-mile across is set to come close enough to Earth tonight to be seen through small telescopes or binoculars.
The 800-metre wide space rock, named 2002 NY40, will pass at a distance of 329,000 miles.
Although slightly further away than the Moon, it counts as a "near miss" by astronomical standards.
The asteroid will be 100 times fainter than the naked eye can see. But a small four inch telescope will be powerful enough to pick it up.
Sharp-eyed observers might be able to spot it with a pair of binoculars, said Robin Scagell from the Society for Popular Astronomy.
Despite its size the asteroid will look like a fast-moving faint star. It will cross the sky at about the same speed as the minute hand of Big Ben, seen from the foot of the clock tower.
Mr Scagell said: "This will be a fascinating event. It may not be spectacular but it is very unusual to see a space rock up close like this - usually you have to wait for hours or days to detect any movement in the sky, apart from such things as meteors and satellites.
People looking for the rock will need a detailed map of its track, showing stars at least as faint as the asteroid. Details can be obtained from the Society For Popular Astronomy website.
The asteroid passes Vega, the brightest star in the summer sky, at about 4.30am BST.
PA