Profile of asteroid 8 Flora

This partial profile of 8 Flora was obtained when several observers timed the occultation of a 9th-magnitude star by the asteroid. See page 3 for an explanation of how this was done.

Sky & Telescope illustration

To see a star suddenly snap out of sight, then reappear several seconds later, is so startling that you never forget it. Such an occultation happens when a tiny minor planet passes directly between you and the star. Events like this are much rarer than lunar occultations (see "Lunar Occultation Highlights for 2005"), because asteroids have much smaller angular sizes than the Moon and because they move much more slowly across the sky. On the other hand, there are lots of them!

It doesn’t matter if the asteroid is too faint to see — you just need to be able to see the star. The value of these events is in recording exactly when the star disappears and reappears. Amateurs using simple timing methods allow us to map the size and shape of the asteroid’s silhouette — if enough observers are spaced at different locations. New close double stars (including spectroscopic binaries) can also be resolved this way, and the angular diameters of some stars can be measured. That’s why the International Occultation Timing Association (IOTA) works so hard to seek and collect these observations.

Map of world occultation paths

The author’s map shows the best asteroid occultation paths predicted to cross well-populated parts of the world from March through July 2005 (click on the image to see the world map). Paths are plotted only where the asteroid and star can be seen in a dark sky at a favorable altitude. An asteroid’s shadow generally takes 5 to 15 minutes to sweep across Earth, so to get a more useful local time prediction (as well as likely path changes), observers should subscribe to the Occultation AstroAlerts, or check IOTA’s Web site a day or two beforehand, as described in the text.

Sky & Telescope illustration.

The maps here describe many asteroid occultations for March through July 2005. Only the best ones are shown on the world map above; many more are within reach, as shown by the map of North America with paths of 22 occultations visible with 8-inch telescopes in March alone. Each year, a given location will experience about five predicted asteroid occultations (some of which will be clouded out). About 30 will occur within a 200-mile radius for mobile observers who can load up the car and hit the road.

Predictions have become more reliable in recent years, since most occulted stars’ positions were measured very accurately by the European Space Agency’s Hipparcos satellite in the 1990s. Those results were incorporated in the highly refined Tycho-2 Catalogue, published in 2000, which has in turn been improved and greatly expanded with the US Naval Observatory CCD Astrometric Catalog-2, known as UCAC-2, released in 2003. The asteroids’ orbits are continually being improved as well.

Anyone with an interest in observing these dynamic events should get on the list for Sky & Telescope’s e-mail AstroAlerts for Occultations. This is the easiest way to get predictions of the best events, late changes to predictions, finder charts, and anything else you need.

Help Wanted

Occultation paths across North America

This map shows all asteroid occultation paths predicted as of early December 2004 to cross well-populated parts of North America in March 2005. It is like the world map but includes paths involving stars down to 12th magnitude that might be seen with 8-inch telescopes. Details about each event are available here.

Sky & Telescope illustration.

Each occultation can be thought of as the asteroid casting a “star shadow” that sweeps across Earth’s surface. The shadow’s actual path can differ by a couple hundred miles from the predictions on the maps here. Fortunately, IOTA’s Steve Preston in Washington state and Jan Manek in the Czech Republic can refine a path’s location to an accuracy of about 30 miles (50 kilometers) a few weeks before an occultation. With UCAC-2 providing a dense net of faint reference stars, many other observatories — including some run by advanced amateurs — are also now providing the accurate observations needed for refined predictions.

Such good updated predictions allow us to deploy mobile observers effectively. For many events, we try to have mobile observers fill gaps in the asteroid’s expected silhouette — by directing them a day or two in advance to positions between fixed observers who are predicted to have clear weather and who tell us they’ll be watching.

Timings: Video is Best

Timings accurate to a fraction of a second are easy to make visually using a tape recorder or camcorder and an audible time signal (see Sky & Telescope: January 2005, page 81 and S&T: November 2000, page 110). But comparing visual observations to video data of the same event shows that human reaction times are invariably larger and less certain than people report. Visual timings are useful primarily for defining the edges of an asteroid’s occultation path. Video recordings, which can provide more objective times that are accurate to just a few hundredths of a second, are much preferred.

Many observers are now using the inexpensive PC 23C and PC 164C cameras, available from SuperCircuits. The PC 164C (now also sold by Adirondack Video) can actually record fainter stars than you can see visually in the same telescope. See S&T: February 2003, page 59 for more about this camera. I have used it to record asteroid occultations of stars to magnitude 11.9 with an 8-inch telescope. (CCTV Camera’s model 2006X is an equivalent for those who live in Europe and Australia and use PAL video.) I also recommend using an f/6.3 or f/3.3 focal reducer on f/10 Schmidt-Cassegrain telescopes. These give a wider view for easier star-finding and also increase sensitivity by at least a half magnitude.

Finder charts for these occultations are on IOTA's Web site and Steve Preston's site.). I can’t stress enough that you need to allow generous time to set up, including at least a half hour for locating the target star if a bright star is not nearby — especially when doing video.

For More Information

For preliminary local data about all events possibly visible from your location, send your longitude and latitude, $1, and a large, self-addressed envelope to Jim Hart at 2616 Monte Cresta, Belmont, CA 94002, or obtain the information free by e-mail request to mailto:[email protected].

If you record an asteroid occultation on videotape along with audio time signals, please mail the tape (for analysis with our special equipment) to David Dunham, 7006 Megan Lane, Greenbelt, MD 20770; include $4 if you want the tape returned. Please send visual observation reports to Jan Manek at Stefanik Observatory, Petrin 205, 11846 Praha 1, Czech Republic, or [email protected]. A report form is available from IOTA’s Web site or from Manek by e-mail. Remember that if you’re near a path and watch the target star but see no event, your negative report is still important and should go to Manek.

Notes on Individual Events

Profile of asteroid 8 Flora

Several observers timed the occultation of a 9th-magnitude star by the asteroid 8 Flora on October 29, 2004. Their six “chords,” representing the times when the star vanished, define the silhouette of the asteroid’s southern portion, suggesting its size and shape. David Dunham’s “remote” observation was made by a video camera on a telescope that he set up in the New Mexico desert to run unattended; he then drove about 28 miles north to make the other recording in person. Another observer or two farther north anywhere along the path would have filled in a more complete profile of the asteroid. Volunteers are always needed — especially those willing to get in the car and go.

Sky & Telescope illustration

March 10 (Ianthe) The star is SAO 138670. The asteroid may have a satellite.

March 18 Antiope is a binary asteroid, with each component about 80 km across; they are separated by 170 km.

March 19 (1999 CO153) An occultation of a star this bright by a Kuiper Belt object (KBO) is extremely rare, so this event deserves special efforts. The star is SAO 98133, spectral type K2. The star’s angular diameter of about 1.1 milliarcseconds subtends 33 km at the distance of this KBO. So a central occultation will have a duration of 5 seconds preceded and followed by 2-second partial phases.

April 26 (2002 GW31) This is also a KBO.

May 17 and 30 (Lucina) A secondary event recorded during an occultation in 1982 shows that this asteroid may have a small moon.

June 5 Ceres, the largest main-belt asteroid, is so bright that there will be a combined drop of only 0.3 magnitude during this event, hard to notice.

June 19 Delta Librae is a spectroscopic binary, but with a period of only 2.3 days it is probably too tight to be resolved during this occultation.

July 15 The star has an 8.4-magnitude companion 42arcseconds away (in position angle 349°) that will not be occulted. This event was found by Jean Meeus.

July 29 (Mathilde) Observations of this event would help complete the 3-D shape model for this asteroid, whose sunlit side was imaged during a 1997 flyby by the NEAR Showmaker spacecraft.

July 30 A secondary extinction during a 1978 occultation indicated that Melpomene may have a large moon.

October 19 (Regulus) An occultation of 1.4-magnitude Regulus by 166 Rhodope is predicted to cross Iberia, southern Italy, and Greece. This rare 1st-magnitude event can be recorded directly by most camcorders. Details will appear later.


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