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Kelly Beatty
HOMEPAGE OBSERVING by Kelly Beatty

Mini-Asteroid Makes a House Call

Update: The flyby of asteroid 2005 YU55 generated a lot of interest among the news media and general public. Click here for details.

Roll out the red carpet! Earth is about to be visited by the largest close-approaching asteroid on record. Known as 2005 YU55, it comes closest to us on November 8th at 23:28 Universal Time (6:28 p.m. EST), when it passes 198,000 miles (319,000 km) from Earth's surface — closer than the Moon's orbit. It will be visible from the Americas and Europe through much of the night.

Radar image of asteroid 2005 YU55
This radar image of asteroid 2005 YU55 was generated from radar data taken on November 7, 2011, using NASA's giant radio dishes in California. At the time the asteroid was 860,000 miles (1,380,000 km) from Earth. Radar illumination is from the top, so only half of the asteroid is apparent.
NASA / JPL
Discovered nearly six years ago by Robert McMillan at Steward Observatory's Spacewatch Telescope in Arizona, 2005 YU55 has been this way before. In April 2010 it passed close enough for detailed radar probing by the giant radio dish at Arecibo, Puerto Rico.

The Arecibo observations showed this asteroidal emissary to be a quarter mile (400 meters) across and remarkably round. Given its size and dimness, its surface must be quite dark and thus likely carbon-rich. Its rotation period is relatively long, 18 to 20 hours.

In the grand scheme of things it's more micro-planet than minor planet, but we've never knowingly had something this big come this close before. Were it to strike Earth, 2005 YU55 would deliver a kinetic-energy punch equivalent to several thousand megatons of TNT. It's the kind of potential threat that outer-space sentries lose sleep over.

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 from the top, so only half of the asteroid is apparent. Note the hint of a large crater to the right of center in the later frames.
Kelly Beatty
But fear not: the Arecibo observations allowed dynamicists to recompute the big rock's orbit with enough accuracy to ensure that it won't strike Earth within the next 100 years. (That said, it will pass just 175,000 miles from Venus in 2029, close enough to alter its orbit slightly. This adds uncertainty to predictions for its next close encounter with Earth in 2041, when the minimum distance could be anywhere from 200,000 to 30 million miles.)

So we might as well just enjoy this month's show. The asteroid will approach Earth from the sunward direction, so it will be a daylight object until just before the time of closest approach. A few hours later 2005 YU55 should reach a visual magnitude of 11.1, within reach of backyard telescopes with apertures of at least 6 inches under fairly dark skies — though you'll be fighting light from the nearly full Moon. (By the way, that bright thing near the Moon tonight is Jupiter.)

Path of asteroid 2005 YU<sub>55</sub>
Best seen from North America, the little asteroid 2005 YU55 will race far across the constellations in just 11 hours. Click here for a more detailed chart and instructions.
Sky & Telescope illustration
The pass's track is especially favorable for western Europe and North America. But you'll need to know exactly where to look at exactly what time: the object will traverse the 70° of sky eastward from Aquila to central Pegasus in just 10 hours, clipping along at 7 arcseconds per second. Use the chart here to get a sense of what part of the sky it's in, then download our detailed finder chart for use between 9 and 10 p.m. November 8th Eastern Standard Time (2:00 and 3:00 November 9th Universal Time).

If you don't have a suitable scope, or if it's cloudy tonight, check out the live video webcast of asteroid 2005 YU55 from the 25-inch telescope at Clay Center Observatory in Massachuetts (continuously from about 6 p.m. to 3 a.m. EST). Another live webcast is available from Bellatrix Astronomical Observatory in Italy.

Amateur asteroid sleuths Brian Warner and Robert Stephens have mounted a campaign to obtain detailed photometry (brightness measurements) of the asteroid. You'll need at least a 10- or 12-inch scope, a CCD camera, and ideally one or more of the standard photometric filters commonly used by professional astronomers. Details.

Part of the Goldstone Deep Space Communication Network, the Mars 70-meter antenna is often used for radio and radar astronomy. It serves a dual purpose as a communication receiver for interplanetary spacecraft.
Courtesy GDSCC/JPL/NASA.
Meanwhile, this visit by 2005 YU55 is providing an unprecedented opportunity for high-reolution radar study. Astronomers have lined up extensive radar campaigns with Arecibo and with NASA's Goldstone facility in California's Mojave Desert, using big radio dishes in West Virginia and elsewhere as receivers. "The signal-to-noise ratios will be more than 1 million for Goldstone observations on November 8–9," explains Lance Benner (Jet Propulsion Laboratory). This SNR is heady territory for radar work, high enough to yield thousands of pixels across the object and to achieve surface resolution "comparable to what can be obtained by a spacecraft flyby mission."

So I hope you all get a chance to spot 2005 YU55 as it zips past Earth.

Posted by Kelly Beatty, November 1, 2011
related content: Celestial events
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First comments (from 25)

Odd that ~20K miles makes that much difference

Posted by J Craig November 3, 2011 At 05:56 PM PDT
Interesting that the 175K mile pass by Venus in 2029 is going to "alter the orbit slightly" while the 23K mile less distant pass of both the earth and moon this month apparently isn't going to add much uncertainty to the predicted orbit. Is this more a question of relative orbital speed during the pass or what? (If anyone knows, I'd be curious to hear.) It's not entirely clear from the animation, but given the orbital elements (finally chased down about 1/3 of the way down this page: http://echo.jpl.nasa.gov/asteroids/2005YU55/2005YU55_planning.html) it would seem that 2005 YUsub55 is crossing earth's orbit at pretty nearly 90 degrees, so it is hard to imagine it paralleling Venus so that there's more time to change its velocity in 2029. And, it'll certainly be closer to aphelion at that point so the asteroid itself will be moving faster in its own orbital path.... So why the difference?


re: ~20K miles

Posted by Kelly Beatty November 4, 2011 At 05:44 AM PDT
J. Craig: a very insightful question! the difference is that radar will be tracking the asteroid throughout its pass by the Earth, so astronomers will know the orbital parameters (distances, velocity vectors) exactly as it leaves the Earth-Moon system. that won't be the case at Venus.


Orbital Uncertainty

Posted by Tony Flanders November 4, 2011 At 05:53 AM PDT
I'm no expert on orbital dynamics, but it seems clear that the uncertainty about the Venus encounter's effects on this asteroid's orbit has to do with time, not space. We're tracking the asteroid daily, so we know how close it's going to come to Earth on Tuesday with exquisite precision. But if you extrapolate that orbit out 18 years, the uncertainties add up. We can't tell exactly how close it will come to Venus in 2029, and a very small variation in that distance would have a relatively large effect on how much Venus alters the orbit.


detailed finder chart

Posted by Luca Vanzella November 4, 2011 At 09:29 AM PDT
FYI, the detailed finder chart linked in the online article is missing a phrase in the instructions on how to offset from Kansas City. The print article in the Nov 2011 issue says at the end of the instructions: "Then copy the 10-minute tick marks, noting your offset from Kansas City on the U.S. map." By offset, I assume you mean in both the x and y axes of the chart.


Sign correction

Posted by J Craig November 4, 2011 At 10:34 AM PDT
I mistakely said "less" when I should have said "more": while ...the 23K mile MORE distant pass... Thanks for the reply, Kelly B (and thanks for overlooking this error!). Maybe by 2029 there'll be a craft orbiting Venus that can do radar measurements (a Magellan successor, as it were).


Asteroid 2005 YU55

Posted by Jason November 4, 2011 At 09:55 PM PDT
Can somebody please help me! I live in Windsor, Ontario, Canada, basically a mile from Detroit, MI for reference. I want to observe the asteroid Nov 8th but I really find that the information is either vague or maybe I just cant see it because of my geographic location, I dont know. I would really appreciate if somebody could help me so that I could view it when the Sun goes down. If somebody could give me a reference point and time. For example, I always can find Cassiopeia and Orion this month, so if someone could tell me where to look relative to one of these at say midnight Eastern Standard Time. I believe we are at the 42 parallel in the northern hemisphere. Thank you for your time and help!


What about the moon?

Posted by chicknlady November 4, 2011 At 11:04 PM PDT
Everyone talks about what a disaster it would be if an asteroid hit the earth. Well, we need to worry about the moon too. How big would an object have to be to alter the moon's orbit? The moon keeps us steady around the sun. Now there's a doomsday scenario, only slower. Exciting stuff none the less.


(NEAs) do not have enough mass to perturb the orbit of the moon

Posted by Kevin Heider November 5, 2011 At 02:19 AM PDT
chicknlady, near-Earth asteroids (NEAs) do not have enough mass to perturb the orbit of the moon even if they impact. From an orbital dynamics standpoint NEAs are basically massless. While on the subject, NEAs also do not have their orbits perturbed by Coronal mass ejections (CMEs).


2010 XC15 = 1976

Posted by Kevin Heider November 5, 2011 At 02:21 AM PDT
It was NEA 2010 XC15 that passed close to the Earth in 1976. http://en.wikipedia.org/wiki/2010_XC15


Uncertainty region is magnified with every significant close approach to a major body

Posted by Kevin Heider November 5, 2011 At 02:41 AM PDT
The uncertainty region is magnified with every significant close approach to a major body. After we get radar measurements during the 2011 Earth approach we will know the exact distance and speed of the asteroid post-Earth close approach, and as a result we will have a much more precise solution for the 2029 Venus passage and the 2041 Earth passage.


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