So these days I’m doing a bit of comet hunting. The concept is pretty easy once you know what you’re doing, but actually being successful in finding something can be a bit trickier, and takes a bit of practice.

Yesterday I came across something that looked like it was a candidate for a moving object. Here it is:

So what are we looking at here? This is compilation of three images taken at three different times – 11:48am, 12:00pm, and 12:12pm – stacked together. Those three blobs were the candidate object, showing its motion over that time of 26 minutes.

When I saw this, I saw that the brightness roughly stayed the same, it was moving in a straight line, and at a constant speed. But this was only three images: in order to confirm that a candidate is a real object, it must appear in four images. The LASCO website often has delays in updating its images, and – just my luck – there was a delay at this point. I was waiting for an image for 12:24pm for hours.

But still, when a total newb is up against well-experienced comet hunters, time is of the essence. So I submitted my finding, hoping that I could update it with a fourth image later on.

But a few things were niggling at me. In the image above, the object moves from left to right. In the region that this “object” was found, the vast majority of comets should move right to left, and at much greater angle upwards. My object was moving away from the Sun, whereas a Kreutz comet (the most common kind of comet in this region at this time of year) moves towards the Sun.

Actually, this would be easier if you saw the full image:

The thing in the middle is where the Sun is. See that oval logo in the bottom right? My object was just to the left of this, moving away from the Sun. This is where Kreutz objects can be found, but always move towards the Sun in images taken this time of year.

So. Maybe my comet was a non-group comet. It’s rare, but it happens. A random piece of rock and ice that was nudged out of its orbit beyond Pluto and made its way towards the Sun over millions – if not billions – of years.

The following morning I checked the LASCO website for new images. It had been updated with the 12:24pm image I needed. I loaded it up and added it to the other three.

There was nothing there.

Without the image appearing in four images, it cannot be considered real, and any submitted claims must be retracted. So, back to the comet-hunter website I went, and retracted my claim of a new discovery.


I knew it was highly unlikely I found something. Even after half a week of checking these images I was pretty used to spotting something, applying proper self-criticism, and realising that it was nothing. It’s no biggie: objects are found regularly enough (one lad found 40 comets in something like three or four years of occasional perusing the images), so I’m bound to find something at some stage!

These comets are so close to the Sun that they cannot be viewed from Earth. In that image above, the Sun is blocked out as it would be too bright otherwise: this allows LASCO  – Large Angle and Spectrometric Coronograph, on board the Solar Heliospheric Observatory in space – to see the amazing corona (all that flame-like stuff) of the Sun. A secondary result of this is that comets can be seen in the images. Many (most?) of these comets are in the Kreutz group, which are tiny fragments of a much older comet that disintegrated. These are called sungrazers. These comets are too faint to see from Earth, and almost skim the surface of our nearest star. Many of them are sundivers, and I think the name is self-explanatory, as they plunge into the star.

It’s thought that there are billions of comets in the Solar System, from tiny fragments to icy behemoths. The majority of them reside in the hypothesised Oort Cloud, a vast spherical shell surrounding our Solar System that measures two light-years across (the nearest star to us is 4.2 light-years away).

The comets in the Oort Cloud are in an incredibly slow orbit around the Sun. So slow, in fact, that their motion can be influenced by passing stars. A gentle gravitational nudge can cause a couple of these comets to bump into each other, and like an extremely slow game of pool, they’ll bounce and move in different directions. Some of these will begin to move towards the Inner Solar System.

Over millions of years, or billions when you consider the distances involved, they travel past Eris, Sedna, Pluto, past the ice and gas giants Uranus, Neptune, Jupiter, and Saturn, and slowly begin to warm up as they approach the Sun. The heat gradually melts the ice in the comets, which falls away from the comet, forming a tail.

By the time comets pass Earth, they are warm enough to have a significantly-sized tail, and we get to experience spectacular sights like Halley’s Comet, or the more recent Hale-Bopp.

Some of these comets swing by the Sun, in a highly elliptical orbit, making their way back into deep space again. Some will return within decades, some will return in thousands or millions of years, some will escape the Solar System and venture throughout the galaxy, and some will fall into the Sun. That final group are the sundivers, the comets that people spot in satellite images, the ones that will be seen by maybe a handful of people and never seen again.

Yeah. Being an astronomer is deadly.