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Brian D. Warner, Robert D. Stephens
On November 8-9, 2011, the Earth-crossing asteroid 2005 YU55 will speed past us at a distance closer than the Moon. With a diameter of about 400 meters, it will be the largest object that's ever been seen passing so close (not counting those that then hit Earth, of course).
Lance Benner (NASA/JPL) has put together an extensive web site detailing what's known about this object and the plans for radar observations.
For additional coverage, in particular about a campaign by the pro-am community to obtain high-quality photometry on the asteroid, see the articles in the 2011 November Sky and Telescope and the Minor Planet Bulletin (38-4).
To be of most use, photometry of the asteroid should be adjusted to standard magnitudes as observed through one of the filters commonly used by professional astronomers. These are usually the Johnson-Cousins V or R filters, though the Sloan Digital Sky Survey standard filters are gaining popularity. The RBG filters used by many astroimagers will only have limited use. Clear (unfiltered) observations can be used as well if they can be converted to a standard magnitude system.
Converting your observations to standard magnitudes might seem daunting, but an overview of the process and a spreadsheet can be found by clicking here.
If you would like to participate in the photmetry campaign, please contact Brian Warner.
Everyone who contributes useful data will be credited in any research paper(s) that result. This web page will be updated daily as new data become available. Observers can, of course, also do their own analysis and report results independently.
Posted: 2011 Oct. 23, 17:00 UT
The team headed by Nicholas Moskivitz (Carnegie Institute) will be taking spectra of 2005 YU55 during its fly-by using the Gemini telescope in Hawaii. Pointing accuracy will need to be on the order of 1 arc minute. To test the abilty to track the asteroid, team member Mikael Granvik found an earth-orbiting satellite whose motion would be close to that of the asteroid. He used a program he wrote to generate sky positions at ten-minute intervals as seen from the Palmer Divide Observatory (PDO) run by Brian Warner.
This picture shows one of the images that captured the satellite. Warner used MPO Canopus to measure the end points of the satellite trails. Since the ephemeris from Granvik gave positions "Of Date" (current epoch, not J2000), the positions were converted to J2011.Oct22 before being sent to the team. After some checking and corrections to the PDO longitude/latitude so that they were also referenced to the current epoch, the measured and predicted positions were within 20 arcseconds of one another, or about 1/3 the maximum error for the Gemini observations.
Posted: 2011 Oct. 26, 00:00 UT
About a half-dozen observers have expressed interest in taking part in the YU55 campaign. All are from the United States. It would be great to have some observers from other parts of the world join in. Since there is a possibility the period of YU55 is near 18 hours, it will help tremdously to have those additional observers. In the meantime, interest in the flyby is growing. Queries have come in from Nature, Space.com, and some professionals.
Posted: 2011 Oct. 27, 16:00 UT
Leonard David wrote to get some quotes on the flyby. The result is an article on the
Huge Asteroid to Creep Near Earth on Nov. 8
Posted: 2011 Oct. 28, 15:00 UT
A couple more observers have expressed interest in the photometry campaign, bringing the total to 8. The better news is that at least one observer in Europe (Denmark) will be trying to get observations. He, like all of us, are hoping that the weather gods will be kind.
Many have expressed concern about trying to get data the day of or after closest approach because of the large sky motion. While observations on those days would be helpful, doing so requires some experiece with working fast targets to balance minimizing trailing while keeping exposures sufficiently long to avoid scintillation noise dominating the data (when using exposures of about 10 sec or less). There is a expression in American football: "When in doubt, punt," meaning in this case that it's better to wait a couple of days (weather permitting) to let the asteroid slow down a little.
There's also that darned full moon hanging near-by. Do the best you can; that's all you can do and we'll try to make the most of the situation.
Not just for this flyby but other news as well, check out the Minor Planet Center blog, The Daily Minor Planet, run by J.L. Galache. You'll find a list of upcoming asteroid/comet close approaches. The page for 2005 YU55 is here.
Posted: 2011 Nov 7, 18:00 UT
More observers have lined up to be part of the project. So far, there are about a dozen in locations around the world, including the United States, Denmark, Australia, and Brazil. This is good news. The suspected period is about 20 hours, so getting enough data from one station in the few days the asteroid will be easily worked would be difficult. Having observations coming in from around the world will, if weather allows, provide almost continuous coverage for extended periods.
I've done a couple of radio interviews for stations in the Denver, CO, area. Timothy Brothers reports that a crew from NBC Nightly News visited the George R. Wallace Observatory at MIT today. A producer for "NBC Nightly News with Brian Williams" told me there will be a report on the asteroid flyby on Tuesday night (November 8). Be sure to watch for it.
The weather for the Palmer Divide Observatory looks iffy for Tuesday night/Wednesday morning. However, the next couple of days look promising. Radar observations at Goldstone have reduced the position uncertainties considerably, down to 14 arcsec as of Nov 6. Be sure to get the latest elements from the MPC before you start observing. With this close of a pass, the elements are bound to change. Also, many programs may not be able to do the numerical integrations for such a close pass. This is why I'm planning my more serious efforts starting a day or two after closest approach.
Posted: 2011 Nov 8, 00:15 UT
The Goldstone radar team has generated a new image for the asteroid. The image was taken on Nov. 7 at 11:45 a.m. PST, when the asteroid was approximately 860,000 miles (1.38 million kilometers) away from Earth.
Goldstone started tracking the asteroid on November 4. Arecibo in Puerto Rico, will start observations later today (November 8).
Posted: 2011 Nov 9, 21:15 UT
The Goldstone radar team has generated another new image for the asteroid. This has twice the resolution of the earlier one.
Lance Benner reports: "So far the images look consistent with a rotation period of roughly 18 hours. We haven't seen enough of our own data yet (which are still processing from yesterday) to estimate a precise period but there are enough features that we may be able to estimate the period if things go well tonight."
It's a couple of hours before sunset here in Colorado. I've had reports observers in Europe getting data. We'll see how things go. Keep checking back.
Posted: 2011 Nov 18, 17:15 UT
Three observers managed to submit data to the campaign that were used for lightcurve analysis: Robert D. Stephens, James W. Brinsfield, and my self. We obtained data on 2011 November 9, 10, 14-17. We found a period of 16.3 ± 0.2 h. The plot below shows our data phased to that period.
We were also able to determine the absolute magnitude (H) and phase slope parameter (G) for the asteroid.
Using H = 21.40 and the diameter of 0.400 km based on the latest radar observations, this gives an albedo of p_V = 0.0304, meaning that only 3% of the light that hits YU55 is reflected. This is a very dark asteroid. The value of G = -0.045 is consistent with such a dark asteroid.
There were some doubts about the period solution, one possibility being 27 h. However this was ruled out by the radar images as discussed by radar team member, Marina Brozovic:
The radar animation from Nov 07 has time tags: 19:06-23:36 UT, so 4.5 hours rotation coverage. When I look at some bright surface features, it does look like they rotate about ~100 deg as Brian's period of ~16 h would suggest....
Some more lightcurve data may be coming. If so they will help confirm the period. Additional radar observations are also planned that, combined with the lightcurve data, may produce an accurate shape and spin axis model.