In spite of recent indications that asteroid 302 Clarissa has a moon circling around it, evidence now suggests that the object may be alone in the cosmos after all. On June 24th, four observers in the northeastern United States watched as the asteroid occulted the star SAO 118999. Astronomers predicted that the magnitude 9.6 star would drop in brightness for about 1.8 seconds as Clarissa eclipsed it. But surprisingly, three observers, David Dunham, Frank Suits, and Michael Richmond, timed a much longer extinction almost 3 seconds indicating the asteroid was larger, and therefore covered the star longer than predicted.
Preliminary calculations by David Dunham, president of the International Occultation Timing Association, suggested that Clarissa is 64 kilometers long by 35 km wide, nearly twice its expected diameter. Yet the fourth observation, taken by Phil Dombrowski, was much shorter than predicted, only a 0.25-second-long disappearance. Dunham initially reported this short observation to be a possible companion of Clarissa, perhaps one 5 or 6 km across. Dombrowski observed the event visually and recorded it on video from outside the predicted path where he should have seen any dimming, suggesting that he observed a mini moon swinging past the star.
Dombrowski's moonlet observation hasn't held up to further scrutiny, however. Frank Abnet (University of California, Los Angeles) found Dombrowski's video recording problematic and filled with scintillation or "seeing" effects (due to the low altitude of the star), causing the star to fade out when it should have been fully visible.
Even with doubts about the video recording, Dunham still remains hopeful. Observations of doublets do happen, says Dunham. "So I am not willing to discard Dombrowski's visual observation outright."
Dunham doesn't need to look back too far for suspected double asteroids. Earlier this year two other amateur stellar occultation observations contained evidence of companions. In May, Roger Venable recorded a secondary occultation following the 98 Ianthe event, and another claim by Red Sumner was reported during an occultation by 772 Tanete in April. These pairs remain unconfirmed.
In Clarissa's case, the answer may be forthcoming. William J. Merline (Southwest Research Institute) will attempt to observe the asteroid with the European Southern Observatory’s 8.2-meter Very Large Telescope in Chile. Using adaptive optics, Merline will have the necessary resolution to obtain direct observational evidence of the moonlet. "Having followed up on several claims, the chance of it actually being a real binary aren’t high," says Merline. But that isn’t dissuading him. "Eventually one will be real."
Dunham's preliminary write-up of the Clarissa event appears at http://iota.jhuapl.edu/mp302625.htm.