Last Sunday, 20 July 2014, a small group of amateur astronomers sat in wait around a collection of small, jury-rigged telescopes, monitoring a faint nameless star in the constellation Virgo. At about 7:18pm, with no drama or prelude, it winked out. Eleven seconds later, it reappeared, shining steadily and dimly as before. And yet only a handful of people could have seen this – observers anywhere else on Earth would have seen nothing out of the ordinary. So what happened to the star, catalogue number HIP 65106, that caused it to vanish to some witnesses yet not to others?I was one of the observers. Between us, we had set up three separate observation sites, along a roughly north-south line and spaced at roughly 10km intervals. We had been called as volunteers by one Paul Maley, who had travelled halfway around the world from Houston, Texas, to witness an occultation. Paul knew that on that day, the asteroid Patientia would occult a 7th magnitude star, and that if he could properly record the event, he would recover valuable scientific data about the asteroid. But asteroids are small, and the universe is big. Such alignments are rare, and only visible from very narrow locations. To witness such an event, one must be prepared to travel to the right spot and look in the right place at the right time.
So he came to the little town of Cullinan, South Africa, a short distance outside of Pretoria. The town is famed for its diamond mines, where the Cullinan Diamond, the largest diamond in the world, was found. It also happened to be ideally situated to witness this asteroid occultation. In astronomical terms, an occultation (from the Latin word for ‘hidden’) occurs whenever one object passes in front of another, temporarily hiding it from our view. The most interesting occultations are when an asteroid somewhere in our Solar System passes in front of a distant star. But stars are unimaginably far away, and asteroids are, cosmically speaking, tiny. These events are rare, and when they happen they cast a very small shadow on the Earth, which is why astronomers need to travel to the exact right spot.
Paul put out the call for volunteers months ago. Enough of us answered his call for him to prepare enough equipment to run three observing sites. He has accumulated an impressive collection of gear over the years, as he makes these observations on a regular basis. For the Patientia occultation, we used his home-brewed video recording system, made of an old Canon camcorder, a GPS receiver, a cheap security camera, and one objective lens from a pair of binoculars. The binoculars and the camera were assembled into a crude telescope, mounted on a lightweight camera tripod. The camera output was fed into the camcorder’s auxilliary input, so that it would record whatever the telescope saw, and the GPS receiver was wired into the system to provide a highly accurate timestamp to the video feed.
I arrived early, shortly after Paul had completed testing and preparing the equipment. He explained sadly that, although he’d originally prepared for four sites, one of the camcorders had failed and so we could only operate three. Apparently all his camcorders are old, purchased second-hand. Modern camcorders do not have the required RCA connectors to receive his external data signals, so he’s forced to buy used gear, which is usually far out of warranty. Rather than stand around waiting, we decided to take a quick drive to the first observing site, which I would be running. Most of the roads in the area are unpaved, and some of the sandiest surfaces I’ve ever driven on, making for a slow and uncomfortable drive. Because he was already feeling time pressure, Paul had not brought his notes along, and that cost us valuable minutes trying to find the site from memory. Fortunately, when we did eventually find it, he was pleased to be able to complete his inspection of the conditions – the ground needed to be firm enough to support the tripod steadily, it needed to be reasonably clear of light pollution and it needed to be out of sight of passing traffic. This task done, we raced back to the lodge, to find all the other volunteers waiting.
Traditionally, amateur astronomers have always observed asteroid occultations with a telescope, tape recorder, and some sort of accurate audio time signal (the telephone company’s speaking clock or a shortwave radio time service are popular choices). Shortly before the occultation is expected to start, they turn on the tape recorder and position the speaking clock so that it is included in the recording. They keep their eye to the telescope, and as soon as the star goes out, they say “Start”. Then when it reappears, they say “End”, and turn off the tape recorder. They then have a record of exactly when the occultation began and ended, and this information is then sent off to whichever astronomical body is interested in it. Observations like this are useful to confirm an asteroid’s position in space, and thus improve the accuracy with which we know its orbit.
But Paul had a more ambitious goal – he wanted to study the asteroid itself. We already know Patientia’s orbit well enough to predict the time of the occultation to the second, but we don’t know much about its structure beyond the fact that it has a diameter of about 235 kilometers. That makes it one of the larger asteroids, so we know it’s roughly spherical, but scientists would love to refine that information, and maybe even start building a rough map of its exact shape and dimensions. It turns out that occultation data can give them exactly that, if your measurements are precise enough, and Paul’s setup is very precise. Where the traditional method can’t be trusted to be accurate to less than one second, thanks to the time resolution of the speaking clock and the natural reactions times of any human observer, a GPS receiver can report the time accurate to the nearest microsecond. That’s one million times better than our amateur astronomer and his tape recorder.
With this level of precision, it suddenly makes sense to record at many locations across the occultation path. Stars are so far away that they are essentially point sources of light, meaning that a change in position of only a few kilometers will affect exactly which part of the asteroid blocks the starlight for a specific observer. By placing observers at specific intervals, and recording their times to a high enough precision, we can measure the length of each separate slice of asteroid (the proper term for these slices is a “Chord”, which you should remember from high school geometry). From this we can start to deduce the shape of the asteroid, and the more observations we record, the better this data gets. As we repeat the process for other occultations by the same asteroid, we build up a detailed and accurate 3d map of Patientia.
Paul explained all this to us from his room at the bush lodge he had chosen as his base of operations. He walked us through the basic science, and the timing procedure. He taught us the operating procedure, and the correct sequence of operations to perform on the equipment, and drilled us to make sure we would get it right at the critical time. And then we loaded up the gear and headed out to the sites. My site, Site One, was to be operated by myself with the assistance of Bosman Olivier, an old friend from the Telescope Making class and current chairperson of the Pretoria centre of ASSA. But even though we were to perform the actual observing, Paul made sure to set up and aim the telescopes for all three sites himself. Because the telescopes were mounted statically, he had to aim them at the point where the occulted star would be at the time of the occultation, and he had charts drawn up with the relevant locations marked for each minute of each hour during the setup time. It was clearly not an easy task, since he could not look through the telescope directly but had to view through the small low-contrast screen of the camcorder. And thanks to the timing of the event, he was trying to do it through the still-bright twilight sky. But eventually he got it right, and raced off.
So Bosman and I set our alarms to make sure we started recording when we needed to, and got back into the car where it was warm. We passed the time, swapping stories and checking email, and making a few casual observations of our own through my binoculars, when the alarms went off. Out we went, and began following the procedures. Connect power pack to the CCTV camera. Turn on the camcorder. Select input. Begin recording. Wait four minutes, turn off camera, break down equipment and load it into the car. And do all this without disturbing the tripod, or the ground beneath it in case we knocked the telescope off target. We were so focused on doing the job properly, that we paid only scant attention to the camcorder screen, and did not see the occultation. We were out there, we recorded the data, but we didn’t actually see anything. We were rather disappointed, but at least we knew we’d been a part of something.
At that point, my phone rang. It was another volunteer, the one assisting Paul at his location (Site Three). He wanted to know if we’d seen it, and seemed quite insistent. With hindsight, it’s obvious that Paul should have been so concerned: He’d just travelled halfway around the world, endured the tedium of long-haul flights at his own expense, to spend a few minutes operating a video camera and trying to capture some data. And then he had to trust the bulk of the work to a bunch of people he’d just met, and who he’d had to trust were following his earlier instructions correctly. But we assured him that we’d recorded successfully, before loading up the car and driving everything to Site Two. Bosman transferred to the vehicle which had brought him to the lodge, Paul collected his equipment, and we all went our separate ways. Paul could not invite us in to socialise as he had to calibrate time data on the camcorders not equipped with GPS before packing up for his flight home the next morning.
The story does not quite end there, however. That morning, before leaving for the airport, Paul emailed me to let me know that he’d reviewed my recording and that we had successfully captured the occultation, even though we did not manage to see it on the screen. And since then, he’s provided me with a copy of the video from Site Three, which I’ve attached below. So I finally got to see the actual occultation, even if it wasn’t live with my own eyes. What you see below is not the raw footage – I’ve cleaned it up a bit to reduce the noise and increase the contrast to make the occultation more visible. Check it out!