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.
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.
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.

27 thoughts on “Mini-Asteroid Makes a House Call

  1. J Craig

    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: 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?

  2. Kelly BeattyKelly Beatty

    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.

  3. Tony Flanders

    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.

  4. Luca Vanzella

    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.

  5. J Craig

    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).

  6. Jason

    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!

  7. chicknlady

    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.

  8. Kevin Heider

    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).

  9. Kevin Heider

    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.

  10. Ross

    2005 YU55 has been described as ‘very nearly spherical’, and ‘remarkably round’. Everything i’ve read on the subject says an object is space must be, at least, somewhere between 100 and 200 miles in diameter for its gravity to be strong enough to pull it into a sphere. The asteroid is said to be 400 meters in diameter. How is it that so closely resembles a sphere?

  11. Wayne-MansfieldWayne Mansfield

    Chicknlady’s comment about an asteroid impact causing a Lunar Orbital change raises another question.What if an asteroid or other large object struck the Moon? What the ejecta from the impact send a volley of Lunar material hurtling toward the Earth?

  12. Robin

    Is it possible for any part of this asteroid to break apart from the main body? Is there any possibility for a smaller piece of YU55 to hurl toward our moon or our planet?

  13. Pete Jackson

    I suspect that the image obtained from the 2010 flyby is not a true image of how the asteroid would look in the sky, but a plot of returned signal strength plotted against time (increasing as we go down) and frequency (left to right).

    Increasing time will represent returns initially from the part of the asteroid nearest us and then extending towards the limb in all directions. Increasing frequency will represent returns from the receding limb extending across the face to the approaching limb. Bright reflective features on the asteroid will show up in the radar image, but you can’t unambiguously locate them on the asteroid.

    I recall back in the 1960s, before they did radar interferometry, radar images like this were made of Venus.
    This time, of course, with the added antennas at Goldstone and elsewhere acting as an interferometer, a true image of the asteroid will be obtained.

  14. Fernando

    Hi to everyone!
    Im from Portugal and im new here and i would like that someone help me because i would like to see the 2005 yu55 here in Portugal. Could someone tell me what time of the 8 of november will pass here and where i should looking?
    Thank you very much!!

  15. Robert Sheaffer

    The easiest way to observe this close approach is to use RTGUI, the free Windows Real-Time astronomy program (

    First, make sure your location, date, system time etc are all set correctly in RTGUI. To prepare for observing in advance, you need to set the program time in RTGUI to a few hours before you will begin observing the asteroid (say, 18:00 hours local on Nov. 8). Press "Get Comet/Asteroid". Make sure you check the box "Near Earth Object," that way you can use a non-real time starting time. For the name enter 2005 YU55, and press OK. You’ll download a file from the Minor Planet Center, with both an object name and a starting time in the filename. The positions will be topocentric for your exact Lat/Long (an asteroid this close can have considerable parallax). This file can only be used during the 24-hour period you specify, but you will have minute-by-minute positions for the asteroid that will update automatically, and you can use your Goto function at any time if you connect your telescope to your computer. (You have to tell RTGUI which virtual COM port is connected to the telescope). Probably you will be able to see the asteroid moving against the background of stars.

  16. Sergio 120857

    Me encuentro en Lima Peru , quisiera saber hacia donde tengo que dirigir el telescopio para poder apreciar el asteroide 2005yu55. Gracias por la ayuda

  17. Michael C. Emmert

    If 2005 YU55 passes so close to both Earth and Venus, it might be on or evolving towards the Hoohmann minimum energy trajectory. Dr. Hohmann invented this trajectory to get from one planet to another. Interesting if 2005 YU55 is on this trajectory but there is a psrecedent; it used to be called 2008 TC 3, but it crashed into the Earth and is now displayed somewhere in Sudan as the Almahata Sitta meteorite, an historically significant object being the fisrt meteorite observed and tracked in space prior to impact.

    It was on the Earth-Mars Hohmann minimum energy trajectory.

    With two examples of close encounters from asteroids on the Hohmann minimum energy trajectory, one has to wonder if maybe it is a chaos attractor? If so, possibly such orbits might be monitored.

  18. Sarah Bourt

    As chicknlady’s post got me thinking, it’s rather a shame in a way that this object doesn’t collide with the moon. The possibilities of what the impact could turn up would no doubt help us learn a little bit more about our closest neighbor. It’d also be one hell of a show in the sky!

  19. rairden

    8-inch f/10 Meade LX200 was not enough to eyeball YU55 under clear but bright sky in Palo Alto. 35mm eyepiece with about 1 deg fov. Stars down to 11.2 or 11.3 were my limit. Attempt 2:30-3:15 UT. Haven’t seen actual magnitude vs time data. Maybe phase hadn’t reached brightest before I gave up and looked at the moon and Jupiter. It was fun to get outside anyway.

  20. Terry Blanchard

    My suggestion is that Earth should send a rocket to this thing, at the next good opportunity and alter its orbit enough that it plows into Venus, providing some wonderful science and ridding ourselves of that little nuisance.

  21. JoSeL

    Hello, I tried observing with my 150 mm Maksutov at the right place and time (I recognized in my scope the little asterism from the detailed finder chart above) but did not see it; I believe the faintest stars in that chart were mag. 11, which I was able to recognize. The sky was fairly bright from the nearby moon (naked eye limit was about 3rd magnitude). So I think 2005 YU55 must have been fainter than magnitude 11. I am in the Bay Area, CA.

  22. andysky

    So, a little over 18-mo ago it came by Earth?
    I felt a bit confused when I read…
    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.

    Then further down in the article it says this…
    …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.)

    Is this to mean that the orbit is some how going to take some unknown drastic, out of the ordinary maneuver?

    Is that math mixed up?
    It has been this way before and now it comes by again on Nov 8th 2011
    Why is this sentence in this article?
    Am I to expect that it will come by this way again in May of 2013?

    If I am full of unthinking, then please disregard me in the highest order.

  23. Bill Duane

    I was able to see 2005 YU55 in my 14" LX200R, but it was a difficult target from Westford, MA. I located it by starting at Epsilon Delphini and sweeping in RA, because I started observing before the time of the detailed finder chart. It was faint, even in the 14", and seemed to fade in and out. I am not sure how fast the rotation is, but perhaps one side is slightly darker than the other. The other possibility is that there were some high thin cloud stripes it was passing through. In any case, I lost it after 20 minutes and was not able to recover it.

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.


This site uses Akismet to reduce spam. Learn how your comment data is processed.