On July 4th, 2015, Andy Druga and I were touring around the world using internet webcams on various observatories when we decided to make a brief visit to Spaceweather.com. The top story was an article about a large 12th magnitude asteroid (52 Europa) occulting a 10th magnitude star in Virgo (TYC 0292-00339-1) at about 11:05PM EDT (see first link below). We were intrigued by the idea of capturing the brighter star being blotted out by the fainter asteroid for the predicted duration of 17.9 seconds. This would only be visible if you were geographically within a narrow strip (see above occultation track over Chiefland). We considered ourselves very fortunate to be in the geographic track but noted that we were closer to the edge of the track and, therefore, likely to get far less than the predicted 17.9 seconds to record this event. Checking The Sky program, we could see that Virgo would be about 30 degrees elevation (adequate but not great) at the time of the occultation. We settled on 5-second exposures using small subframes in the Apogee U16M camera guiding (but turning off the “guider settle” function). Imagine if the guider was trying to settle, delaying the exposure, during the few seconds that this occultation was occurring! Thanks to Barry Riu for showing me how to avoid that potential snafu! Subframe exposures were started at 10:19PM EDT and ended at 11:27PM EDT (02:19 – 03:27 UT, July 4th). In order to catch the occultation predicted to be about 03:05:30 UT, a series of one hundred 5-second exposures were started just before the predicted start time and the occultation was captured fully on only one frame at about 03:05:56 UT! One frame shows a 12th magnitude asteroid (distance: 25 light-minutes; size: 215 mile diameter) blocking out the Virgo star which is six times brighter and exponentially bigger but much farther (81 light-years)! 52 Europa is a porous carbonaceous main belt asteroid closer to Jupiter than Mars with a low albedo. Thanks again to Barry, we were able to immediately normalize the backgrounds and view the data as an animation using Maxim 5.12 pronto! A mosaic of selected exposures is above; the center images show only the asteroid at 12th magnitude. The star is behind the asteroid disc, sort of a Virgo sun eclipse by an asteroid rather than our Sun eclipsed by the Moon leaving a shadow trail on Earth, analogous to that of a solar eclipse (watch video above to see occultation moment).
Veteran Florida photometrist and IOTA (International Occultation Timing Association) czarina Dr. Barbara Harris was also fortunate to record this event living on the east coast of Florida and having clear skies. She witnessed this occultation using a Meade 14-inch scope and non-integrating videocamera linked to GPS timing and recorded a fantastic light curve (link below with permission)! The light curve measures the light intensity of the star (Y axis) on each frame (frame # x axis). It was determined that the disappearance was on frame 1681 and reappearance was on frame 2117. She says the occultation lasted 14.524 seconds at her location. Notice on the “Results” link below that Barbara Harris was one of only three observers in the world to capture this event with quality timing as others were clouded out (“m” means missed). Not sure if Dr. Bill Cooke of NASA at Huntsville, Alabama has any data. These fine timing measurements help determine the exact size and shape of the asteroid (see inversion map below) thanks to dedicated people like Barbara. Check out the attached asteroid Fortuna map created in 2008 by Barbara’s data and others! Some “occultation chasers” drive many miles to get data! IOTA chief David Dunham began the drive from Maryland to Georgia last week to record the 52 Europa event and turned around and returned home after it was apparent that clouds would rule. I look forward to seeing another asteroid occultation before too long. According to software program “Occult Watcher”, I will be only 3 miles from the centerline of an occultation of asteroid 1995 SR1 over Sagittarius star TYC 6326-00908-1 on August 27 sporting a 6.7 magnitude drop! Unfortunately, it will only last 2.7 seconds! Guess I will have to be quick-at-the-draw!
http://www.spaceweather.com/archive.php?view=1&day=03&month=07&year=2011 – Spaceweather Alert!
http://www.nasa.gov/connect/chat/europa_feature.html#map – NASA’s mission to study asteroid size and shape
http://www.asteroidoccultation.com/observations/Results/index.html – RESULTS: IOTA site – see “52 Europa”
http://www.asteroidoccultation.com/observations/Results/Data2011/EuropaMiriadeModel1.png — Inversion Model “map” of Europa
http://www.skyandtelescope.com/news/83025892.html — Barbara Harris in S&T 2010 – U Scorpii eruption discovery