Late last year, word started spreading through the local astronomical community that an exciting event was coming up: An Oort cloud comet named C/2013 A1 (Siding Spring) would be passing so close by Mars that there was a chance it might hit. If it did, we on Earth would be treated to a spectacularly rare astronomical display of a comet slamming into a rocky planet. The planetary science community were simultaneously excited about what they could learn from the resulting multi-megaton blast, and frightened for the possible loss of all their various robotic missions exploring the red planet.
As time passed, though, and our measurements of the comet’s orbital path improved, it became clear that there was not going to be a collision, but that the two bodies would still pass extremely close by each other. At their closest point, they would be only 135,000 kilometers apart – about a third of the distance between the Earth and the Moon. the possibilities were endless: Gas and dust from the comet could interact with the Martian atmosphere, the comet could be ripped to pieces by tidal forces from Mars’ gravity, the robotic spacecraft orbiting Mars could even be hit by a 56,000 kilometers per hour bit of cometary gravel. It was all very exciting and newsworthy, but I found it all to be a bit abstract. Far more interesting to me was a collaborative observing program put together by Dr Padma Yanamandra-Fisher of the Space Science Institute (SSI) in Boulder, Colorado. The SSI, amongst other things, runs the spectacularly successful Cassini mission to explore Saturn and its system of moons and rings, which has been running for more than ten years. Padma’s goal was to take advantage of amateur astronomers around the world to build as complete a record as possible of observations of the comet in the weeks before and after the encounter. While the big professional observatories would capture their detailed snapshots of the two bodies, amateurs would fill in the gaps and ensure complete 24/7 coverage of the entire event.
In the months building up to the event, I and the other amateurs got to know each other in a private Facebook group set up for the purpose. We discussed observing plans, and began capturing test images: The less experienced of us wanted to master the difficult techniques need to show up a faint comet against the bright glare of Mars, and we all wanted to start producing a record of how the comet evolved in the time before, during and after the encounter. These images were tough to get from the beginning, as the comet was very small and dim – it was not visible at all through binoculars (despite some press releases to the contrary), and with its brightness never exceeding magnitude 10 it was challenging even in an 8″ telescope. My own images relied on using accurate charts to predict where it would be at the moment of the photograph and then making sure that the background stars were in my field of view. My best result is visible to the left, from about a month before the encounter.
As the big day approached, our observing windows were defined: I was asked to try to image the comet seven days before the encounter, to capture the encounter itself, and to image several days afterwards. Being in South Africa, where we were optimally positioned to see Mars and the comet at their point of nearest approach, there was special interest in my results, and considering that there were only two South Africans participating in the program, the pressure was on to do good work!
Unfortunately, I only had one single clear night of observing within the requested windows. Fortunately, it was the evening of the encounter itself, and I was in the garden preparing my observing rig just before the Sun began to set. First I mounted the telescope on its permanent pier, and fitted the camera on its T-ring (a process which converts a telescope into a gigantic camera lens – 1260mm f/6.3, for my particular configuration). I then connected the camera to a USB port on the laptop, and fired up the tethering software to allow me to operate the camera without touching it. I aimed the telescope at the most distant object I could find on the horizon, and focused. Then, with the optics all set up, I pointed up at the rapidly darkening twilight sky and snapped a series of 21 frames. These images would later be digitally combined, to produce what is called a Flat Field frame, which maps out any flaws in the optical system, including field distortions and specks of dust. Shortly afterwards, the first stars began to appear, and it was time to acquire the target.
Target acquisition would be the easiest part of the project: for the entirety of the viewing session, the comet would be no more than a few arc minutes from Mars. All I had to do was put Mars, the brightest object in the sky at that point, in the centre of the frame and I would be ready to go. Once that was done, I fired off a few test frames to find the optimal camera settings. It turned out that, for the planet itself, the best results came in with the camera at ISO100 and an exposure time of 1/100 seconds. To see the comet, however, was more difficult: I eventually settled on 30 second frames at ISO1600. And those two wildly different settings were the root of the difficulty of the shot: Mars was more than fifty thousand times brighter than the comet, and there are few cameras capable of capturing such a wide range of brightnesses. So I’m immensely proud that I managed, with my entry-level DSLR and moderate-sized telescope, managed to capture the comet at all! The image at the top of this page shows the comet and Mars together, less than ten minutes before their closest approach. They appear only 1/20th of the diameter of the full moon apart, with the comet barely visible and Mars badly overexposed to the point where it is actually not visible at all through the glare. Compare with the other image on the page, showing the comet with no planets and few stars to see how badly the glare affects the image!
A word on that second image, by the way: You’ll notice that while the comet itself is quite bright and clear, the stars are all short little streaks. To create this image, I captured a sequence of twenty images over seven minutes, which were then digitally combined to brighten the image while averaging away much of the random noise. However, the comet moved noticeably in that time, so the images also had to be shifted to match the movement. The result is exactly the same as if a more sensitive camera with better optics had tracked the comet for seven minutes as it moved slowly through the sky, leading to a nice bright comet image and motion blur on all the stars. The main image at the top of the page, by comparison, was a single frame with no discernible movement.