NAIC / USRA

A radar image from the Arecibo Observatory shows asteroid 2012 LZ1 from a distance of 6 million miles (10 million kilometers), at a resolution of 25 feet (7.5 meters) per pixel.

The bad news about the asteroid 2012 LZ1, which zipped past Earth last week, is that it's actually twice as wide and a lot deadlier than we thought — a kilometer (0.6 miles) wide in its largest dimension, rather than 500 meters. The good news is that we have at least seven centuries to figure out how to fight that particular space rock.

That's the verdict from astronomers using the 1,000-foot-wide (300-meter-wide) Arecibo Observatory in Puerto Rico, the world's biggest single radio dish.

"The sensitivity of our radar has permitted us to measure this asteroid's properties and determine that it will not impact the earth at least in the next 750 years," Mike Nolan, the observatory's director of planetary radar sciences, said in a news release issued today.

Another Arecibo researcher, Ellen Howell, was quoted as saying "this object turned out to be quite a bit bigger than we expected, which shows how important radar observations can be, because we're still learning a lot about the population of asteroids."

As anyone who's seen the movie "Deep Impact" already knows, a kilometer-wide space rock is considered big enough to set off an extinction-level event if it were to hit Earth. Until this month, 2012 LZ1 was among the estimated 10 percent of potentially threatening asteroids of that size that have yet to be detected. (A collision with 500-meter-wide asteroid would rank as a horrible catastrophe, but experts don't think it would kill off civilization.)

2012 LZ1 was discovered on June 10 at Siding Spring Observatory in Australia, and came within 14 Earth-moon distances (3.3 million miles, or 5.3 million kilometers) during its closest approach a week ago. There was zero risk of collision this time around, but the fact that astronomers had so little advance warning of LZ1's approach was just a bit, um, worrisome.

The big challenge for observing this asteroid appears to have been that it was unusually dark. That's why previous estimates of its size were so far off: Without precise observations of the object's shape, astronomers base their size estimates on the relative brightness of an asteroid at a given distance.

The Arecibo Observatory is well-suited for making radar observations of passing asteroids by reflecting radio signals off their surfaces — and the radar image of 2012 LZ1, captured on Tuesday, was good enough to show the object's shape and size. From that, scientists determined that the rock reflected only 2 to 4 percent of the light striking the surface. That suggests that the asteroid was as black or even blacker than charcoal.

The case of the big black asteroid serves as another reason why it's a good thing that the B612 Foundation is planning to put up a privately funded space telescope to look for such rocks. More details about the Sentinel Space Telescope are due to come out in a week. In the meantime, check out today's Weekly Space Hangout, in which yours truly and other space scribes discuss the asteroid threat and what humanity is doing about it:

Scientists who worked on the 2012 LZ1 investigation include Howell and Nolan as well as Israel Cabrera, Jon Giorgini and Marina Brozovic.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.