Comets are small icy and rocky objects flying around our Solar System on elliptical orbits, as opposed to near-circular ones the planets are enjoying. They are strange and fantastic enough as it is: bright comets with their tails spanning across the sky have been marvelled at, recorded, and often feared as bad omens for millennia. But comet 96P/Machholz is particularly special and interesting, and it passed by the Sun again just a few weeks ago.
First of all, let me clear up any possible confusion about our comet’s name. I will be referring to it as ’96P’ or ‘Machholz’, but if you browse for ‘comet Machholz’ online, you might stumble upon an entirely different object. In fact, there are no fewer than 11 comets and one asteroid bearing the name Machholz! This is because comets are named after the people who discover them, and Donald Machholz (born 1952) is the most prolific living visual discoverer of comets in the world (‘visual’, because he is actually looking at the night sky with his own eyes and telescopes; most comets these days are discovered by automated surveys or similar). He is an amateur astronomer from the USA, and is one of the inventors of the Messier Marathon – the challenge to observe as many of the 110 Messier objects in a night as possible. The asteroid I mentioned above was given his name to acknowledge his many contributions to the field of astronomy.
The story of our comet 96P/Machholz starts in 1986 when it was discovered. As it was tracked and its orbit determined, it soon became clear that it is a rather peculiar one. All the planets, most asteroids and many comets orbit the Sun in approximately the same plane, making the Solar System look quite ‘flat’ when viewed from the outside. Comet Machholz’s orbit, however, is tilted from this plane by almost 60 degrees. And while comets generally have elliptical rather than circular orbits, this comet’s ellipse is particularly ‘squashed’ compared to many others. Moreover, while the far end of this orbital ellipse is slightly further away from the Sun than Jupiter, the near end (‘perihelion’, which simply means the shortest distance to the Sun) is almost three times closer to the Sun than Mercury, the innermost planet!
The time taken to complete one orbit around the Sun is called a period. Earth’s period is one year, and 96P’s is a little over 5 years. In fact, the ‘P’ in its name stands for ‘periodic’, which distinguishes comets whose orbit around the Sun is known and is shorter than 200 years. Among the short-period comets, 96P is the only one with such a large orbital tilt and such highly elliptical orbit at the same time, and it has a smaller perihelion distance than any others of its kind.
Comet Machholz is relatively dim when it is far away from the Sun, so it sadly cannot be seen with the naked eye. It does, like all comets, develop a tail of dust and gases the closer to the Sun it gets, but because the Sun is so very bright and the comet moves so very close to it, it has been generally quite difficult to observe its passage in that region. Kepler’s laws of motion tell us, among other things, that the closer you are to the Sun, the faster you will be moving in your orbit. This means that around the perihelion, comet Machholz is positively whizzing past the Sun at great speed.
This is where the heavy cavalry comes in: we might not have sent a probe all the way to this particular comet to observe it up close like we did with comet 67P/Churyumov-Gerasimenko and Rosetta mission (though never say never!), but there are three other equally fantastic spacecraft focused on observing the Sun in great detail, which turned out to be surprisingly useful for spying on comets, too.
The three spacecraft are SOHO, STEREO-A, and STEREO-B (the latter both part of the STEREO mission), and the way they work is quite ingenious. We know that gravity pulls objects together in proportion to their masses and relative distance. It is also easy to imagine that two massive objects will both pull on a very light object with some gravity. We can therefore expect there to be a point in space (moving along with the Earth as it orbits the Sun), where the pull from the Earth balances out the pull from the Sun. Because the Earth is about 333,000 times lighter than the Sun, this point is much closer to the former than it is to the latter. This point, 1,500,000 km away from the Earth (1/10th of the distance to the Sun) is where the SOHO spacecraft is positioned. We call it the first Lagrangian point (L1 for short) after Joseph-Louis Lagrange, the prolific 18th century mathematician and astronomer who discovered it and other points like it.
STEREO spacecraft also take advantage of physics, namely the Kepler’s laws I mentioned earlier. STEREO-A (A for ‘Ahead’) was positioned slightly closer to the Sun than the Earth, and STEREO-B (for ‘Behind’) was positioned slightly further away from the Sun than the Earth. Because you move faster when you are closer to the Sun, the Ahead spacecraft orbits the Sun a little faster than the Earth and stays ahead of it, and the Behind spacecraft moves a little slower and stays behind. Over a number of years, they move further and further apart and away from the Earth along its orbit, each in its own direction. They met on the far side of the Sun in March 2015 and have now slowly started their return to the Earth, where they will meet again in the summer of 2023. TThis means they can, especially when combined with SOHO and telescopes on Earth, together observe the Sun in 3D – or stereo – mode (hence the choice of abbreviation, which stands for ‘Solar TErrestrial RElations Observatory’. SOHO stands for ‘SOlar Heliospheric Observatory’)
Some of the telescopes on board SOHO and STEREO don’t look at the Sun directly, but are more interested in its very extended and much fainter atmosphere, which tells us more about the ‘space weather’ that causes, among other things, auroras on Earth (and other planets! Look up auroras on Jupiter or Saturn). To do that, they block out the Sun with a disc positioned in the centre of the telescope tube, much the same way the Moon blocks out the Sun in a total solar eclipse, which occasionally allows us to see the solar atmosphere and other faint objects in the sky even during the day.
While SOHO and STEREO were designed to be Sun-observing missions, they also helped us discover a vast new world of comets we never knew about before. SOHO alone has discovered more than 3000 in its 20-year history. These comets are called sungrazers, because they pass extremely close to the Sun; often even closer than our comet Machholz. Because of the intense heat so close to our star, a lot of them break apart and disappear, never to be seen again. Even the ones that survive the journey are difficult to study, because they zoom past so very quickly. A lot of these sungrazing comets are quite small and seem to come in groups, which makes scientists think that each group used to be a part of a really large comet that broke apart a very long time ago when it came close to the Sun. Now the pieces are still orbiting in a similar way and slowly disappearing or breaking apart. We don’t think Machholz is such a piece. It is instead a smaller version of those very large and very old comets, because we see small pieces breaking from it every time it goes past the Sun, which means it is actively creating its own group of sungrazing comets: some have been observed by SOHO in its 2012 passage, and more in the most recent one at the end of October . This is very exciting, because it is not often Solar System scientists get to see such a thing happening ‘live’, as it were: we usually have the rather more complicated job of detectives piecing together clues to find out what has happened in the very distant past.
The most recent passage of comet 96P/Machholz, which reached the perihelion on 27th October 2017, was the 7th one we have observed since its discovery. This time it was observed by both SOHO and STEREO-A spacecraft, which were 1,500,000 km (at L1 point) and 257,000,000 km away from the Earth, respectively – at this distance the latter needed 14.3 minutes for its signal to reach the Earth, even though the signal travels at the speed of light.
The analysis of data from its previous observations tells us even more than has already been discussed here. Detailed analysis of the light from the comet can tell us about its chemical composition, and we have discovered that 96P has much less carbon than one would normally expect from a comet. The reason for this is still a bit of a mystery. It might be caused by the comet’s small perihelion distance: the heat might be boiling off or sublimating (converting from solid straight to gas) carbon-containing molecules. It might, alternatively, mean that the comet is not from around here at all, but was perhaps captured by the Sun from a different star, similarly to the strange asteroid that was observed very recently. This theory, while perhaps more outlandish than the first, would help explain the other bizarre characteristics of the comet, like its highly elliptical and titled orbit, and its very small perihelion distance.
The data from the latest passage of comet 96P/Machholz has yet to be fully analysed, but it is bound to teach us even more about this fascinating comet, about where it came from, what it is made up of, what is happening to it now, and about its future.
Article by: Rok Nezic, PhD Student at Armagh Observatory and Planetarium