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.

Path of asteroid 2005 YU<sub>55</sub>

This animation shows the trajectory of asteroid 2005 YU55 as it cruises past Earth on the night of November 8-9, 2011.


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.

Asteroid 2005 YU<sub>55</sub> among stars

In Sudbury, Massachusetts, Dennis di Cicco captured asteroid 2005 YU55 passing through Pegasus on the evening of November 8, 2011. Taken with a Meade 16-inch telescope, the 45-second-exposures were made as fast as his SBIG ST-8300 camera would download the images.

Dennis di Cicco

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).

Resolved image of 2005 YU55

Astronomers used the 10-m Keck Telescope to record asteroid 2005 YU55, which reveals a gibbous phase. The image was acquired at 7:20 Universal Time on November 9, 2011.

W. Merline & others / Keck Obs.

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.

Radar movie of asteroid 2005 YU<sub>55</sub>

This six-frame movie of asteroid 2005 YU55 was generated from data obtained by NASA's Goldstone tracking station on November 7, 2011. At the time the asteroid was 860,000 miles (1,380,000 km) from Earth. Radar illumination is effectively from the top, though this is not a true image because each pixel combines a reflection from two or more points on the body. Note the hint of a large crater to the right of center in the later frames.

Update Nov. 11: NASA has released an updated 28-frame Goldstone radar movie.

Kelly Beatty

"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.


Image of Rick Baldridge

Rick Baldridge

November 9, 2011 at 1:27 pm

Great view of 2005 YU55 from Foothill College Observatory in Los Altos, CA (near San Jose) last night. Took 2 hours of video with StellaCam III through a Meade 16" LX200 operated by the Peninsula Astronomical Society. Video posted at: http://youtu.be/u25iCJE8XBU

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Tim Hutton

November 9, 2011 at 1:46 pm

As a subscriber of S&T I had anticipated the hunt for YU55 since recieving the Nov.issue. The chart helped in my success in witnessing the yellow-orange marble wiz by. I do however feel a little luck was involved, due to my lack of experience. I have a 10in. reflecter I've had for about 6 mo.(my 1st scope) and I'm still learning how to use it. Using the RA & dec #'s on the chart I was able to ambush the rock. Thinking I had set up to see it at 7:50 MST, I acually saw it at 7:42 MST, that's where the luck came in. I assume I was in the wrong place at the right time. Nonetheless, without the chart I wouldn't have been there to begin with. Thanks S&T for all the great articles that inspire me to seek out the wonders of our Universe.

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Brad Timerson

November 9, 2011 at 3:32 pm

I was able to videotape the asteroid from my home observatory in Newark, NY (near Rochester) using a 10" Meade LX200GPS and PC164C low light video camera. This is the same equipment I use for asteroidal occultations. I've posted a couple videos on YouTube.

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Tom Stone

November 9, 2011 at 5:07 pm

I was in my front yard in upstate New York last night after dinner around 6:25 to see if I could catch a glimpse of the asteroid. After a couple of minutes I was looking almost directly overhead and saw what I believe was 2005 YU55. It was quite bright when it was directly overhead and was moving from North to South. Its speed reminded me of the speed of the ISS as I've seen it passing over the house, perhaps a little quicker. As it proceeded South it grew dimmer and finally became completely non-reflective from my vantage point at 50-55 degrees above the horizon (just guessing). I have been trying to confirm my observations of the trajectory of this object with the trajectory of the asteroid, but have been unable to find any mention of a N to S trajectory. I am a complete beginner here, but am a lifelong interested observer. If anyone here could help me confirm that I may or may not have seen the asteroid, I would greatly appreciate it. Thank you.

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November 9, 2011 at 5:14 pm

I set up my 10-inch reflector and saw it at 6:30 EST. It was definitely moving! I then accidentally lost it and looked for it for two hours without finding it until the sky got too cloudy. I'm not sure why I couldn't find it again, but I'm glad that I at least saw it for a little bit!

"The heavens declare the glory of God, and the skies proclaim the work of His hands." -Psalm 19:1

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Brad Timerson

November 9, 2011 at 6:01 pm

Tom, you most likely did not see the asteroid. It required a telescope and precise knowledge of its location since it was about 100x dimmer than what can be seen with the eye alone. Even in images and videos, it shows up as quite dim.
Also, the asteroid was moving almost directly west to east from near Altair shortly after skies darkened here in NY to central Pegasus later in the evening. You might want to check with a website called Heavens-Above for information about earth satellites, since that's what it sounds like you saw.

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Angela Johnson

November 9, 2011 at 6:03 pm

I was able to view the asteroid with my scope last night. It was definitely a spectacular event to which I had the company of my girlfriends aunt, uncle and cousin. I set up my scope around 6pm and got my 10mm lens ready. I zoned in on the moon as I thought this would be the best place to see the flyby due to the moons illumination and using that light as a backdrop. My scope is too large and heavy to maintain a constant moving of the scope so one set place and observation began. At exactly 6:30pm, the asteroid flew by with such speed and force that if I had blinked I would have missed it! It was an amazing evening for me! I look forward to the next event.

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Tom Stone

November 9, 2011 at 7:46 pm

Thank you Brad, I suspected it could very well be a satellite as it was as bright as other stars, but confirmation of the trajectory clinches it. Judging from other comments, the speed of the object I saw seems to be on the slow side for sure. Thanks for your response and the web site suggestion.

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Tom Stone

November 9, 2011 at 8:33 pm

Brad, very nice videos. After seeing your videos I can tell what I saw was definitely a satellite. To correct my previous statement about its speed, the satellite I saw appeared to be moving faster than the asteroid because of its proximity. Though YU55 was close, I understand its still much further than even geostationary satellites (which I know if you could see would appear "stationary") and therefore would appear to be moving slower than these other objects. From most of the media coverage, I was mistakenly expecting first of all to be able to see it just by stepping outside, and secondly expecting to see something that was moving faster than it was. Thanks again.

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Dieter Kreuer

November 10, 2011 at 4:25 am

I couldn't view the asteroid on the night of closest approach, as seen from Germany, it was too low on the horizon already shortly after dark when I returned from work, but according to some information by Daniel Fischer, I had another chance on the evening of the 9th, when the asteroid was to culminate around 10:45 pm local time (21:45 UT). I set up my Skywatcher 120ED refractor and pointed it to the coordinates which I had generated with the JPL ephemeris calculator (http://ssd.jpl.nasa.gov/?horizons) in 10 minute intervals. There wasn't much to see visibly, though, just 2 weak stars on a bright blue background so near to the almost full moon. None of them was moving, the asteroid was too faint to be seen.
So I attached my DSLR to the scope and took some 27 photos (15s at 1600 ISO, 900mm FL @ f/7.5, motorized w/o guiding, 30 s interval + 4 darks, 21:26...21:45 UT), which I combined with Deepsky Stacker. This is the result: http://tinyurl.com/c57csgg.

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Lee Scarborough

November 10, 2011 at 7:06 am

Thanks to your finder chart I was very successful in locating and tracking 2005 YU55 with my Celestron CPC11 from my Observatory 7 miles southwest of McDonald Observatory in the Davis Mountains. I plotted the path for my location, then set the scope on the 2 brighter stars located on the 8:30pm CST mark on the chart at 8:25 and waited. Right on time she came right by the 2 stars at 8:30. Faint but easily seen and tracked, I followed her for about an hour through mostly sparse skies. Thanks for the excellent chart that allowed me to observe such a rare celestial event.

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Gordy Thomas

November 10, 2011 at 8:13 am

The animation with this story implies that 2005 YU55 passed near enough to both the earth and the moon to be influenced by their respective gravitational fields. Has anyone heard of any calculations that have been made regarding how this might effect the path/trajectory of the asteroid when it draws near to us again?

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Conrad Jung

November 11, 2011 at 9:58 am

We observed the asteroid using Chabot Space & Science Center's 36-inch reflector. We had mounted cameras on the two 5-inch guidescopes and the main scope. One of the guidescopes had a Mallincam astro-video camera feeding a widescreen monitor and was uploading an image to the Internet as well so visitors could watch the asteroid live. The other guidescope had a DSLR taking stills.
The main 36-inch telescope had a CCD camera which was doing science imaging collecting positional data to be later submitted to the Minor Planeet Center. Later on, after the sciencee imaging was completed, the CCD camera was removed and visitors could then step up and look through the telescope and watch the asteroid drift by the stars.

We animated some of the still images from the DSLR and you can find them here:


and a composite of the same frames can be found here:


All in all a fun event for visitors and us alike.

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Bill Watson

November 11, 2011 at 10:20 am

I found with a nearly full moon the more than abundant cirrus clouds are a big problem in the City of Tonawanda, NY, with an 30 year old 8 inch celestron. I watched nothing go threw my star field asteroid trap and as 2005 YU55 was approching the moon and its glar I decided to draw one more star trap field to try and catch this elusive asteroid. Success! I finally caught a few looks of this cyptic visitor between 5:32 and 5:38 UT. After plotting my three scetched positions on my Millennium Star Atlas and measuring the Right Ascentions and Declinations, I found it had residuals of Dec. +.4" and RA -4 seconds. So it was a little north of its JPL predicted position and maybe a very little east. (My standard error was larger on my RA measurements.) ...but then again on only plotted three positions before those nasty cirrus clouds ruined my fun.
Bill Watson

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Scott Turnquist

November 11, 2011 at 2:21 pm

I used the S&T finder charts and had my 12.5 Newtonian set and looking at the base of Delphinius, Deneb Dulfim, with my 35mm panaoptic eyepiece for a magnification of 75x. I was set up and looking from 6:45 to about 7:30 EST and caught it right on time.

I watched its track across my FOV even though the moon was very bright. I set my telescope next to the West side of my house in the shadow from the moon.

Needless to say, the S&T charts were dead on.

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Robert Sheaffer

November 11, 2011 at 2:56 pm

I set up my 11" Celestron GPS in my driveway in suburban San Diego. This was about 20:00 PST. The sky would have been semi-dark without the nearly-full moon, but moonlight stole away a lot of faint stars. I was using RTGUI, the real-time astronomy program, and downloaded 24 hours' topocentric positions for the asteroid at one-minute intervals. RTGUI selects the appropriate entry, then I press Goto.

The asteroid was in the field using a 40mm wide-angle eyepiece, but it took a fair amount of effort to identify it. Its motion was slow but noticeable, and it's not easy to identify the one slow-moving "star" among a field of many. I went in the house briefly, and when I came back outside I was not able to identify it again. Finally after several more Gotos, I identified it in a field having few stars, and watched it again cross the field. I had the impression that the asteroid was not at constant brightness.

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November 11, 2011 at 3:04 pm

For one of the few nights this summer, the sky was decently clear in Cuyahoga Falls, OH. I had added the asteroid's orbital elements into TheSky and when I used it to point my AT01RCF and G-11, there it was on the CCD chip of my ST-8XME. I tracked it for a half hour or so and took a series of 25s images. I made up a 3-frame animated gif for a local television station and it was seen by watchers of the WEWS Channel 5 news (our local weather people are all strong supporters of astronomical science).

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Don R.

November 11, 2011 at 4:14 pm

I tried to see the asteroid with my 8 in Newt. I used the S&T star chart to set up on a distinctive (to me) asterism and sat and watched at 31X from 7:00 'til nearly 8:00. No luck, but then the sky was so bright that I could barely distinguish the mag 9 - 10 stars, much less a mag 11 object. I did get some nice views of Jupiter though. The seeing was bouncy enough that 100X was about the best view.


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November 11, 2011 at 4:41 pm

From my home in Virginia Beach, Virginia, USA I Observed asteroid YU55 11/9/2011 between 0210 and 0240 UTC at 80X with 6" Newtonian reflector that I made in Istanbul Turkey at the ATY2009 workshop sponsored by Istanbul Kultur University (IKY).

It was difficult and would not likely be seen in a smaller scope under the sky conditions of that night.

I could not have done it without the excelent Sky & Tel charts.

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November 11, 2011 at 5:48 pm

I tried from South Florida with my Dobsonian 8 inches.
Very early I had everything ready with the chart, etc.
Sorry I missed it. Tried and tried but with no findings or any luck to see that piece of rock. Finally spent time at the full Moon and Jupiter. Jupiter and its moons are now spectacular!

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Jon S

November 11, 2011 at 11:33 pm

The caption to the delay-Doppler image animation is misleading and does not represent the correct interpretation. The image is not a 2-D spatial image. As indicated by the pixel resolution in the "radar imaging runs" link, the pseudo-image has units of distance (vertical, meters) by Doppler frequency shift (horizontal, Herz). The "image is not illuminated from above, as it looks to be. Instead the brighter pixels are the higher reflectivity regions (nearest earth, "zero" Doppler shift) the fainter ones are further away from us, with the left side being the red-shifted regions (I believe), and the right side the blue-shifted regions. Please note that 1/2 of a hemisphere is only visible because the other (bottom) half cannot be distinguished-the radar collects signal from the entire hemisphere. Most dramatically, if the asteroid was not rotating, the image would collapse to a single line of pixels at the center (zero Doppler shift). Although features and shapes can be distinct (and periodic) in this many-snapshot rendering, it is very deceptive (as your picture caption is). This image is, in fact, not a crescent phase view of the asteroid. Now, with many resources collecting radar imaging and other data, a 3D rendering will likely be obtained, but this frequency/spatial rendering is not a true image. Much more data (e.g. from other view angles and other passes) is required.
For a nice introduction discussion of these delay-Doppler radar images, please read Emily Lakdawalla's blog at http://www.planetary.org/blog/article/00003248/
Thank you,

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Jon S

November 13, 2011 at 10:53 pm

Kelly - Thank you for modifying the radar image caption to include that it is not a true image. Although more complicated to describe briefly, at least one's curiosity to understand these images may lead to more fruitful discussion or research to understand them better 🙂


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Warren Odom

November 15, 2011 at 9:40 pm

I took my Celeston 11" to a friend's house outside the city, and shivered in the cold for close to an hour, but didn't manage to see it -- despite having the detailed finder chart. I'm not sure what happened, unless somehow I didn't get the scope set up accurately enough to go to exactly the right place when I entered the coordinates. There was some indication that might be the case, when afterward I told it to go to Jupiter and it was off by maybe a degree or so (which has never happened before with this scope). But it's not clear because at least once I saw a star pattern that exactly matched the chart (near 2:50 UT). I kept wishing for a wider field of view; I think that would have helped. I'm now considering getting one of those focal reducers that turn the f/10 primary optical system into an f/6.3.

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Warren Odom

November 15, 2011 at 9:52 pm

Angela, it sounds like you saw something other than the asteroid. You looked near the Moon, but the rock didn't get anywhere near there until several few hours later -- not 6:30 PM in any U.S. time zone. Plus it's doubtful ymou could have seen such a faint object so close to the Moon's glare. Finally, you said it went by so fast that if you had blinked you would have missed it, but in reality it would have taken several minutes to travel the distance equivalent to the Moon's diameter.

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