The Tale of Two Asteroids

Many asteroid moons were discovered during stellar occultation events in the late 1970s, and binary asteroids were featured in the review volume "Asteroids" published in 1979. When the phenomenon started to become an embarrassment for the standard model, participation in observing efforts by professionals was sharply curtailed by the early 1980s, and asteroid moons were barely mentioned except in one negative chapter in "Asteroids II" a decade later.

The International Occultation Timing Association (IOTA) provides the predictions and coordinates observers, both professional and amateur. Their newsletter usually reports early results even before the journals publish the final papers. Not only were professional efforts by U.S. astronomers down, but some amateurs were driven away by the stinging criticisms of professionals who have done no analysis of the reports, yet claim that flying birds and such are more likely to explain multiple occultations than objects in the asteroid's environment.

The most famous case was the first asteroid occultation where photometers were used. Visual observer James McMahon in California observed a 21-second occultation of a bright star by a much fainter asteroid, Herculina. He also reported some equally distinct but shorter secondary occultations, the longest of which lasted 5 seconds.

Lowell Observatory recorded a photoelectric record of the combined lightcurve that likewise showed a 21-second occultation of the star by the asteroid, with a magnitude drop of 4-5. David Dunham called Lowell and asked about secondary occultations. The observers reported seeing none. David then questioned McMahon closely for any possible alternate explanation of his visual sightings, telling him that Lowell saw no secondary events in the photoelectric record. Every possibility anyone could conceive was ruled out, especially when considering that the faint asteroid remained visible when the bright star disappeared.

Dunham calculated that only the secondary event that had produced the 5-second occultation for McMahon was large enough that Lowell almost certainly must have seen it too, if it was real. So he called Lowell back and again asked them to scan their record for secondary events. The observers assured him there were none.

Dunham said, "Look at the record at 91 seconds before the primary event. Is there anything unusual there?" The Lowell observers responded rather sheepishly with words to the effect, "Oh, that. That does look like a 5-second, full-light-drop occultation, but the altitude then was only 2 degrees, so we ignored it!"

A visual observer saw an occultation. A major observatory hundreds of miles away obtained a photoelectric record of the same event at the same time relative to the main occultation. The magnitude drop was nearly five magnitudes. The asteroid remained visible when the star disappeared -- both to the visual observer and to the photometer. No other events were seen in the entire photoelectric record besides the main 21-second occultation and this 5-second secondary event. What more could you ask for by way of confirmation?

To put this bluntly, observers sometimes see what they expect to see. The story of the Herculina satellite is another illustration of that, since a definite occultation event was denied to exist by the professional Lowell observers until their attention was directed to that exact place in their lightcurve. Occultations were previously an unchallenged type of reliable observation, even when done by amateurs. But when occultations of stars by asteroids began finding a phenomenon that did not fit in with current thinking, the observers and the technique were faulted. This is bias, pure and simple.

I spent the first half of my career as a part-time observational astronomer, and have observed and timed hundreds of occultations. I feel strongly that one is not fully competent to reduce data without the experience of collecting it to get a feel for the weaknesses first hand. My Melpomene occultation photoelectric lightcurve was published in Asteroids I.

The Melpomene event is another example in reverse: At USNO we had no occultation, but a very close appulse. We recorded for 40 minutes. During all that time, the only time the star's light dimmed significantly was for a few brief periods close to the time of closest approach. But the dimmings were not as deep as total occultations, and were irregular.

Any other observer was likely to have discarded that data as changes in seeing or some such. It might have been. But I saw the statistical improbability of that happening just when the star and asteroid were unresolvably close, but not at other times. And I understood the dynamics of close satellites and how the small bodies would tend to cluster into ring arcs and could produce a series of partial occultations such as those we saw. So I reported the data instead of tossing it.

I suggest these two anecdotes encapsulate the difference between myself and some mainstream astronomers: I would rather err in the direction of presenting possibly significant data and let history judge its usefulness. Others clearly would not publish data of whatever certainty that did not fit in with existing theory. I would question whether such an approach is doing "serious science", since I am convinced it retards progress.

Actually, there are more than three dozen cases of asteroid moons suggested by earlier observations prior to Galileo's discovery of Ida's Dactyl. Of these, perhaps a dozen are strong cases with no other viable explanation. A second observer gave independent confirmation in two cases. See, e.g., "Minor planets: the discovery of minor satellites", R.P. Binzel and T.C. Van Flandern, Science 203, 903-905 (1979), for the status 18 years ago. More recent discoveries are usually reported in IOTA's "Occultation Newsletter".

It is a shame that the individual discoverers of these asteroid moons, and D.W. Dunham in particular who predicted the events and organized the observer efforts, have received so little credit for their discoveries, made at a time when asteroid moons were still thought to be a theoretical near-impossibility. Dunham reported a satellite of 6 Hebe to the AAS in 1977, and the most common response was to assume that Dunham was not a careful observer or a reliable astronomer. Some reward for his role in discovering and publicizing something important and wonderful!

I'll accept a finding that Eros has no moons visible to a working, orbiting NEAR spacecraft as a good indicator that most of the thirty or so previous observations of secondary occultations near asteroids were spurious, and as evidence that my best professional judgment was extremely faulty. OTOH, if NEAR shows us that the spontaneous 1973 observations by relatively inexperienced observers of secondary bodies close to Eros were real moons of that asteroid, then we are probably safe to conclude that most of the later observations by more experienced observers better prepared for the phenomenon were likewise reliable, that phenomena repeatedly reported even by amateur astronomers should be taken seriously, and that asteroid moons are indeed numerous and commonplace.

Tom Van Flandern (December 1997)