By Eugenie Samuel Reich of Nature magazine

Marshalling everything from major radar facilities to backyard telescopes, astronomers are gearing up for a fantastic view of an asteroid called 2005 YU55. The 400-meter-diameter rock is predicted to narrowly miss Earth on November 8, scraping past us at just 0.85 of the distance between our planet and the Moon.

Large enough to cause regional devastation if it were to hit the Earth, 2005 YU55 is the closest pass by an asteroid this big since 1976, and there won't be another until 2028. The near miss provides an unparallelled opportunity for radar, optical and infrared observations of a mysterious charcoal-black world similar to the type of asteroid that astronauts may one day set foot on.'

Radar bonanza

"It's a bit like a spacecraft fly-by with the Earth being the spacecraft," says astronomer Don Yeomans at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "It's going to be an extraordinary target for radar."

Of all the methods for observing asteroids from Earth, radar is the most powerful. Sending a chirp of radio waves and receiving the bounce-back, or echo, enables astronomers to pinpoint the location, shape, size and rotation of close-passing asteroids. But the technique only works when a target is close enough to Earth that radio waves can be bounced off it with detectable strength.

2005 YU55 will come within that distance on 4 November, when two radio antennas at the Goldstone Deep Space Communications Complex in California's Mojave Desert will be trained on it. Astronomers at Goldstone will use the fly-by to try out an upgrade to the facility that, with such a close target, promises images with a resolution of just 3.75 meters, comparable to or better than those obtained in spacecraft fly-bys of asteroids.

"We regularly observe at 10 to 20 lunar distances but it is very unusual to have an object of several hundred metres in diameter between us and the Moon," says planetary radar scientist Marina Brozovic, speaking on behalf of the JPL team that will run the Goldstone campaign.

Asteroid upgrade?

The hope is that by 8 November, the team will be able to produce an animation of the asteroid showing its shape and rotation, says Jon Giorgini, a senior analyst in Solar-System dynamics at JPL. On that date, the asteroid will be within the gaze of the fixed dish at the Arecibo Observatory in Puerto Rico, which will join Goldstone in detecting radar echoes.

Astronomers already know from an earlier pass of 2005 YU55 in 2010 that it rotates about once every 18 hours and has some large structures on its surface that could be craters. Visible light spectra taken then also showed that the asteroid is carbon-like (C-type). That makes it unlike Eros and Itakawa, two asteroids that have been visited and closely mapped by spacecraft, which are stony (S-type).

Instead, Yeomans is reminded of Mathilde, a carbonaceous asteroid visited by NASA's Near Earth Asteroid Rendezvous mission in 1997. It proved to be extremely porous with massive craters from impact damage. "2005 YU55 is likely the same," he says. "I'm very much looking forward to radar images."

The radar observations should pinpoint the orbit well enough for 2005 YU55 to qualify for a numerical designation from the Minor Planet Center in Cambridge, Massachusetts. That may be a precursor to the asteroid getting a name from the International Astronomical Union.

Aside from radar, further measurements of visible light spectra are planned in an effort organized by Nick Moskovitz of the Carnegie Institution in Washington DC. There is also an amateur campaign coordinated by Brian Warner, an independent astronomer at the Palmer Divide Observatory north of Colorado Springs, Colorado.

The fly-by comes as NASA prepares for a November 14 workshop to discuss a roadmap, developed by a number of international space agencies in September, for sending humans to an asteroid by the 2020s. US president Barack Obama has expressed support for such a mission as a first step towards sending humans to Mars.

There isn't yet a shortlist of potential targets, says engineer Kathy Laurini, who is organizing the workshop for NASA, but as a large, relatively slowly rotating carbonaceous asteroid, 2005 YU55 is "illustrative of the kind of target we'd be aiming for", she says.

This article is reproduced with permission from the magazine Nature. The article was first published on November 2, 2011.