Space and astronomy news and information for the American Southwest. Coverage includes Vandenberg AFB rocket and missile launches.

Asteroid's Moon to Occult Star

by David Dunham

2008 March 6

With just binoculars, you might see a relatively bright star wink out briefly as a moon of the asteroid (45) Eugenia covers it; this rare event will be visible across the southern USA Saturday evening, March 8. You could help determine the location of the moon relative to the asteroid more accurately than can be done directly with the largest telescopes on or near the Earth, if you can observe this rare event. In 1977, astronomers laughed when Paul Maley and I suggested that asteroids might have small moons based on visual observations of eclipses of stars. In 1994, they stopped laughing when they saw Dactyl, the small moon of the asteroid (243) Ida in images returned from the Galileo spacecraft. Now, many dozens of moons of asteroids have been discovered, but only three times have confirmed eclipse timings been made when these moons covered stars, providing the most accurate information about them (all of those observations were made in Japan). Saturday evening, March 8, you could join the very small number of people who have seen a star eclipsed by a moon of an asteroid.

Eugenia will eclipse the 5.7-mag. star ZC 741 in Mexico and its moons will eclipse it in the southern USA.

We could use your help to map the asteroid and its small moons. The eclipse (called "occultation" by astronomers) by either Eugenia or by its moons can be seen with binoculars, if you can find the star not far from Aldebaran in Taurus; we have sky charts that should make locating the star quite easy.

The occultation by Eugenia, about 215 km in diameter, will last up to 12 seconds in its path crossing northern Mexico over Loreto, Baja California sur; Torreon; Saltillo; and Monterrey.

The occultation by Eugenia's larger (about 13 km) moon, Petit-Prince, will last about 0.7 second in a path passing over southern California (nominally over King City and Tipton, but observers all the way from San Francisco to Los Angeles all have almost an equal chance to see it), southern Nevada, northern Arizona (Flagstaff) and New Mexico, Texas (Lubbock and Dallas/Ft. Worth, but Waco to southern Okla. have a chance), northern Louisiana, and southern Mississippi. This is the latest prediction, just updated earlier tonight; an earlier prediction posted a couple of nights ago was significantly farther north.

The occultation by Eugenia's smaller (about 6 km) moon, S/2004(45)1, also called Petite-Princesse, will be more difficult to observe, lasting about a third of a second in a narrow path passing over northern Mexico and southern Texas. The location of its path is especially uncertain, it might be almost anywhere between San Antonio and Brownsville, Texas (or farther south in Mexico; there's some chance that those in the northern part of the occultation path for Eugenia could have an event by Petite-Princesse).

The star, number 741 in the Zodiacal Catalog and also known as SAO 94227 or HIP 23043 (other catalogs), is in Taurus about 5 degrees above and to the left of Aldebaran. It can be easily located using all-sky charts and a blowup chart of the Hyades region of Taurus showing Aldebaran, the V-shaped Hyades cluster, and other stars visible with binoculars to find the unique group of 5 stars that includes ZC 741 as its brightest member; these charts are posted on the Web site of the International Occultation Timing Association (IOTA) devoted to occultations of stars by asteroids at (the box at the bottom has the charts, other useful links to path maps, and power point files that describe how to make observations). Also for making observations, with whatever you have available, see; just knowing if an eclipse of the star occurred or not at your location can be important, especially if it did occur. For those interested in the details, the star's coordinates are J2000 RA 4h 57m 22.3s, Dec +17 deg. 09' 13".

The International Occultation Timing Association (IOTA) needs as many observers as possible to try to time the occultations by Eugenia and its moons, to better determine their sizes, shapes, and relative orbits. If you are anywhere near the paths described above, you are encouraged to watch or record ZC 741 from 5:42 to 5:45 UT (Universal or Greenwich Mean Time) of March 9, or, in local time, Saturday evening, March 8, at 9:42 to 9:45 pm PST, 10:42 to 10:45 pm MST, and 11:42 to 11:45 pm CST. The Petit-Prince occultation might even be seen at low altitude in the Florida panhandle early Sunday morning, March 9, from 12:42 to 12:45 am EST. In Mexico, Kerry Coughlin, e-mail, is coordinating observations in Baja California while Pedro Valdez Sada, e-mail, is coordinating them in Nuevo Leon and nearby areas of northeastern Mexico. David Dunham, e-mail, is coordinating plans for the observations of the occultations by the Eugenian satellites.

Especially for the satellites, we need many observers set up at pre-determined distances across the path in a way that will cover the uncertainty zone of the path, which is considerably wider than the path itself. We need the locations of observers who will try it from home locations or from fixed observatories so that mobile observers can fill the gaps in the coverage that the fixed-site observers can provide. The interactive Google maps on Derek Breit's Web site at (click on "Spectacular Triple Asteroid Occultation - 45 Eugenia" near the top) also includes static maps showing the paths, with green lines showing the predicted central line, blue lines showing the northern and southern limits, red lines showing the "1-sigma" error limits, and gray lines showing the less likely "2-sigma" error limits. Read the information in the boxes for the interactive maps if you use them; they can be used to view the paths on detailed maps, satellite and in some cases aerial photography, and other map sources to almost any desired level of detail. Derek Breit's site also includes a list of stations and cities in and near the predicted path, listed in order of distance in kilometers from the predicted central line (distances north of the line are considered as negative), and for each gives the predicted time of the center of the occultation, the probability that an occultation will occur, and the local circumstances (mainly in this case, the altitude above the western horizon) of the event. Notice that the event will occur at about 12 deg. altitude above the horizon along I-35 between Dallas and Oklahoma City, so observers in that area and farther east, where the altitude will be even lower, need to take care to find large open fields, the east sides of lakes, or otherwise places with an unobstructed view of the western horizon.

To help publicize this event, and provide basic information and observing tips for timing it, Pedro Valdez Sada has prepared a 3.2-megabyte Power Point file in Spanish and English on the main Web site mentioned above. It concentrates on observing the occultation by Eugenia in Mexico but has useful general information for the satellite events in the U.S.A., also.

The Accuweather forecast for this event calls for clear skies in California's Central Valley and the Los Angeles area; it should also be clear in eastern Texas and eastward from there to Florida. There's a 50% chance of significant high clouds from southern Nevada to central Texas; later forecasts will tell better which areas will have better prospects for seeing the event. Marine clouds are expected in the coastal areas of central California; observers there are encouraged to travel to the Central Valley.

Much information about observing occultations of all types is in "Chasing the Shadow: The IOTA Occultation Observer's Manual" available for free at

This story was originally titled "See an asteroid's moon occult a 5.7-mag. star Sat. evening".

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