Rock's fleeting visit

Asteroid is visible in a small telescope

A half-kilometre-wide (546 yard) space rock hurtled past Earth on Friday, just 12 times further from our planet than the Moon. In cosmic terms, that's a near miss.

Scientists said there was no danger of a collision. Instead, the close encounter gives astronomers an opportunity to study a bright near-Earth asteroid from close range.

The asteroid, which has the catchy name 2000 QW7, was discovered on 26 August using the Near Earth Asteroid Tracking system (Neat), which is run by Nasa' s Jet Propulsion Laboratory.

QW7 caught the attention of Neat project scientists because it was fast moving and unusually bright.

2000 QW7 is racing through Aquarius toward Pisces

At 13th magnitude, amateur astronomers can easily spot the minor planet through a 20cm (8-inch) or larger telescope.

According to Neat principal investigator Eleanor Helin, QW7 offers an exceptional opportunity for earthbound observers to study a near-Earth asteroid: "This is a very important object," she said. "It's so bright that amateur astronomers can track it now and through to the end of this year."

Future danger

A group of astronomers led by Jean-Luc Margot of the Arecibo Observatory made the first radar detection of the space rock using Nasa's Goldstone antenna in the Mojave Desert.

"An improved orbit from the radar data will help us run the orbit backwards and search for pre-discovery images of the asteroid," said Helin. "It's a bit of a mystery why we haven't seen this one before."

Asteroid 2000 QW7 falls into a category of Near-Earth Objects (NEOs) called Potentially Hazardous Asteroids, or PHAs.

"Technically an asteroid is a PHA if it can get within about 7,450,000 kilometres (4.65 million miles) of Earth's orbit and if it's larger than a few hundred meters," said Donald Yeomans, manager of Nasa's Near Earth Object Program office at the Jet Propulsion Laboratory.

"There are currently 266 known Potentially Hazardous Asteroids - none of them pose an immediate threat to the Earth."

Although PHAs in their current orbits won't collide with Earth, astronomers monitor them because one day they might become dangerous.

Gravitational nudges by Earth, Mars or Jupiter can potentially set such asteroids on a collision course with our planet, says Yeomans.