Less than one week after the publication of Gaia Data Release 1 (Gaia DR1), New Zealand amateur astronomer Graeme McKay successfully observed the occultation of a star by an asteroid. Just three weeks earlier, the predicted occultation path lay more than 4 path widths — or hundreds of kilometers — from his location at Blenheim, New Zealand.

On Sep 20, the 60-kilometer minor planet (671) Carnegia was due to pass in front of a 10th-magnitude star with the romantic-sounding designation TYC 6888-01416-1. Predictive calculations by the International Occultation Timing Association (IOTA) said the event should occur along a narrow strip (known as occultation path) of the remote watery solitudes of the Pacific and South Polar Ocean. McKay would probably not have tried any observation that night.

But then two things happened and changed this situation, as reported by IOTA’s Australian activist Dave Herald in a IOTA Yahoo group:

1. A short-notice improvement of the asteroid's orbital elements by IOTA’s president Steve Preston (USA). This resulted in a lateral shift of the prediction by about 2.6 path widths.
2. The release of Gaia DR1 on September 14, giving a more precise position of the to-be-occulted star. This shifted the prediction by another 1.4 path widths.

Preston’s September 16 calculation gave a revised occultation path running right over New Zealand’s main islands. It is shown in Fig. 1 (taken from the public prediction page). Accordingly, McKay went to his home observatory in time for the event, started his “C11” telescope (28cm aperture) and “Watec-120N+” video camera and then waited to see what would happen. The star’s brightness recording derived from the resulting video is shown in Fig. 2. A very clear and pretty occultation light curve was the observer’s reward. “I feel privileged to be the first to report a positive occultation result based on the new Gaia data set.” was Graeme McKay’s comment after the observation.

The success means that both the new star position from Gaia and the revised asteroid orbit determination are correct and highly precise. In the future, IOTA’s work on the improvement of asteroid orbits and the determination of diameters and shapes of asteroids will become much more efficient: The missed opportunities — observers sitting on their telescopes in vain — due to imprecise occultation path predictions will become much less frequent.

IOTA is a world-wide amateur-professional organisation devoted to the observation and scientific usage of star occultations by planets, asteroids and the Moon.

Credits: Graeme McKay, Steve Preston, Ulrich Bastian

[Published: 21/10/2016]