Ordinarily, the weekly public-viewing night at Clay Center Observatory, located at the Dexter and Southfield Schools in Brookline, Massachusetts, draws a few dozen people.

But last night, which coincided with the close flyby of asteroid 2005 YU₅₅, was downright nuts! A conga line of television trucks lined the access road, their crews jockeyed for the best camera angles, and hundreds of eager citizens climbed to the rooftop observing deck — all hoping for a glimpse of the cosmic interloper.

Why so much interest in a big space rock coming as close as 200,000 miles to Earth? It's not the "as close as" part. Scores of small asteroids have buzzed nearer to Earth in recent years. Heck, tiny little 2011 CQ₁ skimmed just 3,400 miles (5,500 km) from our planet last February. No big deal.

So it's got to be the "big" part — as in "a big catastrophe if something this big ever hit Earth." A quick calculation suggests that 2005 YU₅₅ would deliver a kinetic-energy wallop equivalent to 5,000 megatons of TNT, enough to make for a very bad day on whatever continent it struck.

Doomsday prophets, please take note: Astronomers calculate that there's no chance of this asteroid hitting us any time soon.

It's this seeming closeness that has drawn the attention of astronomers worldwide. Amateurs had a chance to see the asteroid whiz by at just over 11th magnitude, aided enormously by the great finder charts cooked up by S&T associate editor Tony Flanders.

It was a challenging observation, even if you had a beefy telescope. But S&T's Alan MacRobert says he picked it up rather easily in a 12½-inch reflector at 85×. Office-mate Dennis di Cicco captured the sequence of images shown above, and you'll find other nice ones at spaceweather.com.

If you saw the asteroid — or if you looked for it without success — please post a comment about your experience below. If you took images, please share the link to the website where they're posted.

On the professional front, this was one case where the Hubble Space Telescope was not the right scope for the job. The asteroid's motion with respect to the stars was just too rapid (up to 7 arcseconds per second) to keep pace with it. Nor was it in the crosshairs of the Spitzer Space telescope (too far away).

However, in Hawaii, astronomers used the Keck Telescope's exquisite adaptive-optics system to resolve the asteroid's gibbous shape (watch a webcast of the observing run here).

At the Green Bank Telescope in West Virginia, radio astronomers were able to probe the asteroid's temperature several inches (10 cm) below its surface and recorded day-night changes. (The surface material emit radio energy simply by being warm.) "Long-wavelength radiometry gives us the thermal inertia and heat capacity of the material as a function of depth," explains astronomer Michael Busch (University of California, Los Angeles), and the signal is also influence by the mix of grain sizes in the surface coating and its porosity.

The observations that I'm most eager to see are the surface maps derived from radar soundings at NASA's Goldstone track station in California, the Arecibo radio telescope in Puerto Rico, and elsewhere. The Goldstone team is still in the midst of an exhaustive (and no doubt exhausting) sequence of radar runs that are still ongoing. When the asteroid was its closest, the round-trip light travel time was just 2.2 seconds —— too close for a single Goldstone dish to both send the radar pulse and record the echoes. So astronomers planned for some observations to be bistatic, with one dish sending and another receiving.

"The Goldstone observations of 2005 YU₅₅ are going very, very well," reports Lance Benner (Jet Propulsion Laboratory). By resolving features down to just 12 feet (3.75 m) across, he says, "The images have revealed more detail on the asteroid's surface than I expected" — evidence for boulders, two candidate craters, and the vague hint of a ridge near the equator. And the rotation period, as previously estimated, is a puzzlingly slow 18 hours.

Of greater long-term interest is how the radar data will refine the asteroid's orbit. Before this week, explains JPL dynamicist Jon Giorgini, "We could predict its motion only over the interval 1827-2011 before statistical uncertainties blurred things out. It looked like there could be Earth encounters in the 2040s, but it wasn't certain."

But it didn't take much radar data to get a much more solid lock on where this body's been and where it's going. "We were able to eliminate a possible Earth encounter in 2041, confirm one in 2045, and gain predictability through the year 2075." Benner adds, "The improved orbit significantly reduced the uncertainties during a close flyby of Venus in 2029, and then enabled us to discover a subsequent close flyby of Earth in 2075 that we hadn't previously recognized."

But the pass in 2075 will likely be at a nice, safe distance of 360,000 miles and can't be any closer than 117,000 miles — about half the Moon's distance.

I expect to find out a lot more about this asteroid next March at the annual Lunar & Planetary Science Conference. So stay tuned for further developments.