By Steve Campbell Revised March, 2016
A few years ago, there was a comet (ISON) approaching the Earth. It was discovered by two Russian astronomers, Vitali Nevski and Artyom Novichonok by way of the International Scientific Optical Network (ISON, hence the name) at the Kislovodsk Observatory. The comet was approaching from out by Jupiter and passed by the Sun in November and December of 2013.
A comet is composed of rock, dust and a lot of ice. As a comet approaches the sun, heat will vaporize ices in its body, creating a “tail”. Figure 1 is an optimistic “artist’s conception” of what it was to look like from about October 2013 through January 2014.
Figure 1: Prediction for Comet ISON (optimistic)
What actually happened was much less spectacular and could only be appreciated in a telescope. This is par for the course when it comes to comets in the popular media. I remember Comet Kouhutek, which made its appearance in late 1973. It too, was touted as the “Comet of the Century” and it failed miserably to satisfy that reputation. As usual, your humble narrator and alleged “dinosaur” was there to see it in person. I remember looking for it in the bitterly cold winter at Texas Tech University in Lubbock. The spirit was willing, but the flesh (having been raised in a more nearly tropical Houston) was weak. I never actually sighted it.
Figure 2: Comet Kouhutek (telescope view) 1973
A “big deal” was also made about the return of Halley’s Comet, which is expected every 76 years. I remember Halley’s return in 1986 and it was rather disappointing, as well. The comet was visible only in a telescope (at least from Houston) and the weather in the area was absolutely dismal for observing at that time.
There was one highlight, though. On a rare cloudless night, out at a State Park, some friends and I had the comet in my 3″ Edmund Scientific Co. reflector telescope. A very old gentleman approached and asked if he could take a look. Of course, we were proud to show off “our” comet. After taking a good look, he told us that this was the second time around for him, having seen it in 1910. So, I reckon it was we who were lucky to see “his” comet! He went on to tell us that it was a naked-eye object and quite breathtaking back then. He thanked us for the view to which we, of course, bade him quite welcome. We also pointed out some other amateur astronomers in the park, who had bigger and better ‘scopes and he wandered off in their direction.
Figure 3: Halley’s Comet 1986
Yet another comet story happened in 1997 with comet Hale-Bopp. It was much more satisfying and was a naked-eye object for months. I had taken my five year-old son out in the countryside, away from the city lights to get a good look at the comet. We were way down a dark county road with little traffic, pulled off in a driveway to somebody’s ranch. After a while, a bunch of motorcycles appeared and almost pulled in to where we were parked, but then went a bit further down the road – for a “biological break” as it turns out. After that they seemed to discuss something and then motored back to where my son and I were observing the comet.
Figure 4: Comet Hale Bopp 1997
Now, I have had nothing but good experiences with motorcycle enthusiasts, myself. My childhood neighbors were such people (“Speedy” and “Ruby” – no, really!). Speedy gave me a recommendation that got me my first real job.
But, at this late hour and so far from “civilization”, I’ll admit, I was a bit nervous at this attention. These were a club, with matching leathers and they introduced themselves – politely – and asked what I might be doing so far out in the country. I explained about the comet and pointed it out to the group. They asked a few questions about comets and such, which I was glad to answer. My son, of course, was fascinated by the motorcycles and I told him to look all he wanted, but not to touch. That is always good advice when looking at machines that are not only highly regarded by their owners, but also very hot in places.
The group started to depart, but one of them – the only woman in the group – stayed to talk briefly. She told me that she had seen me, way out here with a little boy, and was concerned that something “creepy” might be going on, so she insisted that the others come back with her to investigate. Turns out, she’s a lawyer. So, any image of a motorcycle gang intent on menace fades in the reality of a group of gentlemen – implored by a concerned lady and for the sake of a child – prepared to confront what might have been a despicable character. My good opinion of motorcycle enthusiasts was reinforced.
You will notice in Figure 4, that Hale-Bopp apparently has two tails. This is not uncommon and comes about because the gas released from the warming comet streams directly away from the Sun, while the dust, which is ionized, is affected by the Solar Magnetic field and rushes off in another direction.
Another “naked-eye” comet happened in 2007 and its name was Holmes. This was a comet that had previously neared the Earth in 1892 and at one point underwent an eruption of activity that made a blue-green glow in the sky. The same happened this time and was clearly “naked eye” visible even from my home in the glaring city light of Houston. Many folks thought they had discovered a nova (exploding star) or a new planet. I was also able to present views through my 12 inch Newtonian reflector telescope (a far cry from the early days) to family and friends. The cloud of gas and dust expanded to rival the size of the sun.
Figure 5: Comet Holmes 2007
Here’s the deal about comets. They exist out in an area called the “Oort Cloud” way beyond the outer limits of the solar system. They are composed of ice and dust and for reasons not well known, they occasionally get diverted and sent toward the inner planets (Earth being one of same). Some comets pass near planets, thus having their orbits altered and some get “captured” into an elongated orbit that brings them back around every so often. Halley’s Comet, Comet Holmes and Hale Bopp are three of these. Kouhutek and ISON are not, being one-time visitors to the Solar System. Their trajectories will never return them to our neighborhood.
There is another aspect about comets that explains the inability to predict their precise paths around the sun. As comets approach the sun –especially the one-time comets – they are warmed by solar radiation and begin to emit gas and dust. That, of course is the stuff that makes up the comet “tail”. But, that vaporization also acts like a rocket to change the orbit of the comet. How much the orbit is changed depends on the composition of the comet, of which we know little, especially for “new” Comets. At least, that is until they start spewing out tail material, which can be analyzed by spectroscope.
There was yet another comet due in 2014. Its name was Siding Spring, named after the observatory, not the discoverer. It will be passing close to the planet Mars. Very Close. In fact, a collision with Mars was within the error bars for this encounter. Chances of collision are now being quoted as 1 in 600. Mars is now being observed by two surface rovers and three orbiting spacecraft.
Update: The space probes did provide views of the comet’s close approach, but there was no collision.
This comet was estimated to be about 50 km in diameter and is traveling in a retrograde orbit – which means that it is coming in in the opposite direction of Mars’ orbit. That would have maximized the impact velocity. If had collided with Mars, the crater diameter would have been about 320 miles.
Figure 6: Artist’s impression of the comet C/2013 A1 (Sliding Spring) striking Mars. (I hasten to remind the reader that it never did this!)
Ex Scientia, Veritas