The November Asteroid Flyby:
Photometry Needed!

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. It should reach about magnitude 11.2 at its brightest, as told in the November Sky & Telescope, page 53.

Several professional groups plan to study the asteroid during the crucial days and hours of closest approach. They are seeking help from amateurs who are set up to do accurate photometry.

A team led by Nicholas Moskovitz (Carnegie Institution) is planning spectroscopic observations. Their data may give new information on the mineralogy of primitive solar-system bodies; 2005 YU55 has a dark, carbonaceous surface. But they will need to link their observations to an accurate rotation model for the asteroid.

Several teams of radar observers will be bouncing signals off the asteroid hoping to develop a high-resolution shape model and confirm the rotation period found by lightcurve analysis. Lance Benner (NASA/JPL) will be heading radar efforts at the Goldstone facility in California; Mike Nolan (NAIC) will be leading efforts at the Arecibo radio observatory in Puerto Rico; and Michael Busch (CalTech) will lead Arecibo/VLBA observations and reception of Arecibo transmissions at the Green Bank Telescope.

Previous radar observations at Arecibo, and some limited visible-light photometry, indicate that the asteroid's rotation period is close to 18 hours. That will make it very hard for a single observatory to obtain a complete enough light curve covering a full rotation during the brief flyby. This is where amateurs come in. We are widely positioned around the Earth and so can obtain continuous coverage for hours, even days.

To be of most use, the measured brightness of all observations 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 at

Observations will be complicated by the asteroid’s rapid motion across the sky when it’s close to the Earth. Depending on your equipment, it may cross your field of view in minutes or less. A day or two after closest approach, when the asteroid will be much dimmer at 15th magnitude, it will be moving much more slowly across the sky.

At closest approach, a 10-inch or 12-inch telescope will be required to get sufficient signal even though shorter than normal exposures will be needed to avoid excessive trailing. You will have to experiment, but 10- to 30-second exposures will be the maximum near the closest approach. After November 11, a 14-inch telescope with 120- or 180- second exposures will probably be needed.

Although accurate photometry has been done with DSLR cameras on telescopes, it is not recommended that you use those for this project. It is best to use a CCD camera and process with darks and flats for the highest accuracy. If you are using a CCD with small pixels such as those favored for astroimaging, you should consider binning your images, since high resolution is not required.

There are several packages for producing the raw instrumental magnitudes: Astrometrica, Maxim, CCDSoft, MPO Canopus, etc. Astrometrica and Canopus have built-in means of converting the magnitudes to a standard system.

Photometric filters such as Johnson-Cousins or Sloan can be purchased from AstroDon, Custom Scientific, Optec, or one of the online astronomy stores.

You’ll need accurate positions for the asteroid. Ephemerides are available from JPL Horizons or the Minor Planet Center (MPC) ephemeris generator. You can also load the latest orbital elements from the MPC into your planetarium program to get accurate positions. Be sure to specify positions corrected for your location on Earth (topocentric correction). The asteroid will be close enough that this matters.

Brian Warner will coordinate the photometry campaign in support of the two science teams. Interested observers should write to Brian well beforehand so that data-reduction techniques can be discussed and tested in advance. His address is brian-AT-MinorPlanetObserver-DOT-com.

In addition to sufficient equipment, you will need basic telescope and image processing skills to participate in this project. Beyond that, you will need one of the software packages capable of measuring the images and producing magnitude readings. Finally, you will need to convert those internal magnitudes to a standard magnitude system, using one of the software packages or a spreadsheet.

There will also be some coverage in the upcoming issue (38-4) of the Minor Planet Bulletin, available as a free PDF download.

Everyone who contributes useful data will be credited in any research paper(s) that result. A web page showing the results of the analysis will be posted and updated daily: Observers can, of course, also make their own analysis.

This will be a valuable and exciting project for photometrists old and new. And the skills you’ll develop will surely be useful on other asteroid campaigns in the future!

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