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2005 YU55 Closest Approach Today

2005 YU55, an asteroid roughly the size of a city block, makes its closest pass today, approaching within 325,000 kilometers, closer than the distance between the Earth and the Moon. It will be another seventeen years before we get an asteroid as substantial as this in such proximity. That one is 2001 WN5, which will pass halfway between the Moon and the Earth in 2028. Today’s object of interest, 2005 YU55, isn’t in danger of hitting the Earth on this pass, but astronomers track these objects closely because over time their trajectories are known to change.

Image: This radar image of asteroid 2005 YU55 was obtained on Nov. 7, 2011, at 11:45 a.m. PST (2:45 p.m. EST/1945 UTC), when the space rock was at 3.6 lunar distances, which is about 860,000 miles, or 1.38 million kilometers, from Earth. Image credit: NASA/JPL-Caltech.

The asteroid’s discoverer, Robert McMillan (University of Arizona) calls it “…one of the potentially hazardous asteroids that make close approaches from time to time because their orbits either approach or intersect the orbit of the Earth,” all of which reminds us of the need to keep an eye on it and other Earth-crossing objects. McMillan is co-founder and principal investigator for SPACEWATCH, whose job it is to find and track objects that might pose a threat to the Earth. Both it and the Catalina Sky Survey are based at the University of Arizona’s Lunar and Planetary Laboratory, where the CSS has NASA support to discover potentially dangerous asteroids.

SPACEWATCH uses charge-coupled devices (CCDs) and specialized software to study the images it generates, passing suspect objects on to the Minor Planet Center at the Smithsonian Astrophysical Laboratory. It was on one of these searches that 2005 YU55 turned up six years ago, an apparent Near Earth Object. Subsequent investigation refined its trajectory:

“The MPC posted it on their confirmation page, which is monitored by everybody who follows up newly discovered Near Earth Objects,” McMillan said. “So we followed it up on subsequent nights and over the following month. Over time, we refined its orbit to the point that NASA’s Jet Propulsion Laboratory listed a large number of potential close encounters with the Earth. Now, after 767 observations by ground-based observers, we have the orbit of that asteroid really nailed down, so we know it’s not going to hit the Earth on Nov. 8.”

What we know about this object is that it is roughly spherical and measures about 400 meters in diameter, with a complete rotation every 18 hours. The asteroid is a carbonaceous chondrite, but in the absence of more detailed information about its physical properties, its long-range trajectory is hard to predict, especially given variables like the Yarkovsky Effect, which results from uneven heat distribution on the surface as the object radiates sunlight back into space. All the more reason, then to look forward to the OSIRIS-REx mission, a sample return of the carbonaceous chondrite 1999 RQ36 that will bring material from the asteroid back to Earth in 2023.

The Near-Earth Object Observations Program at JPL known as Spaceguard is also part of the detection program for nearby asteroids, discovering and tracking them to analyze their trajectories for possible danger to Earth. What we know of 2005 YU55 is that its orbit regularly brings it close not only to the Earth but also to Venus and Mars, but the 2011 encounter with our planet is the closest it has come to us for the last 200 years. We should have new Arecibo radar images of the asteroid after closest approach at 1828 EST (1128 UTC). More in this NASA news release, and see this University of Arizona page for further background on the 2005 YU55 encounter. Finally, amateur astronomers equipped to do accurate photometry are being sought to help observe the object. If you’re interested, this Sky & Telescope article has the details.


Comments on this entry are closed.

  • Paul Titze November 8, 2011, 20:29

    I’m surprised no one is taking the opportunity to land a probe on it being that close to Earth, maybe next time.

    Cheers, Paul.

  • Raffaele Antonio Tavani November 9, 2011, 6:54

    Ho un paio di domande da porre, all’attenzione dei lettori e del curatore di questo “blog” scientifico.
    Come è possibile che un corpo così piccolo, abbia una forma quasi rotonda?Mi sembra, una stranezza…
    E poi, vorrei sapere se c’è qualche possibilità di studio di questo piccolo asteroide, da parte di qualche satellite scientifico, oltre che degli osservatori da terra.
    Un saluto a tutti voi, da Antonio.

  • Paul Gilster November 9, 2011, 8:06

    Antonio’s comment via Google Translate:

    I have a couple of questions to ask the attention of the readers and the editor of this “blog” scientific.
    How can such a small body, has a nearly round shape? It seems to me, a strange …
    And then, I would like to know if there is any possibility to study this small asteroid, by some scientific satellite, as well as observers from the ground.
    Greetings to all of you, Antonio.

  • A. A. Jackson November 9, 2011, 10:54

    It is kind of mystery for a body so small seemingly not in a self gravitating Newtonian equilibrium regime to seem to be ‘spherical’ or spheroidal.
    But it can be the case by way erosion.


    The shape distribution of asteroid families: Evidence for evolution driven by small impacts
    Gyula M. Szabóa, and László L. Kiss
    Volume 196, Issue 1, July 2008, Pages 135-14

    From the abstract:
    A statistical analysis of brightness variability of asteroids reveals how their shapes evolve from elongated to rough spheroidal forms, presumably driven by impact-related phenomena. Based on the Sloan Digital Sky Survey Moving Object Catalog, we determined the shape distribution of 11,735 asteroids, with special emphasis on eight prominent asteroid families. In young families, asteroids have a wide range of shape elongations, implying fragmentation–formation. In older families we see an increasing number of rough spheroids, in agreement with the predictions of an impact-driven evolution. Old families also contain a group of moderately elongated members, which we suggest correspond to higher-density, more impact-resistant cores of former fragmented asteroids that have undergone slow shape erosion. A few percent of asteroids have very elongated shapes