Comets

The first of our Comets of the Nineties is unusual for being the one we actually did not see - Shoemaker-Levy 9.

As we saw in the introduction, once comets enter the solar system their orbits can be modified by the gravitational pull of planets, and no planet has a greater gravitational pull than the greatest of them all - Jupiter.

Shoemaker-Levy 9's orbit had been modified to the extent that by 1970 the comet was in a highly eccentric orbit around Jupiter. This isn't unusual. At any one time there are a number of comets orbiting Jupiter instead of the Sun and, as comets sometimes just get too close to the Sun and are pulled into it, so they are also pulled into Jupiter. And so it was that in July 1992 the comet passed the planet at a distance of 40000km, or for those who prefer, about 25000 miles. This was close enough for Jupiter's gravitational pull to break the comet into twenty one pieces, and those pieces were discovered by the Shoemakers and David Levy in March 1993. Predictions of the orbit made it obvious that in July 1994 the comet would hit the planet.

In the run up to the impacts the media had one of its brief spells of sitting up and taking notice of astronomy. There were all sorts of predictions about what was and was not likely to happen. And of course the old superstitions about comets being portents of doom surfaced. What it really brought home, I think, was that although there are a lot of people who think we've got the meaning of life, the universe and everything pretty well sussed, when it comes to the universe and what's Out There, we still don't know nearly as much as we think we do.

Fortuitously the Hubble Space Telescope had just been fixed, and when the time for the 'Great Comet Crash' came it and just about every telescope anywhere that could see Jupiter was prepared to be trained on the planet. No-one knew what, if anything, we were going to see. We did know that the impacts would occur on the wrong side of Jupiter. We all hoped that by the time the impact sites rotated into view there would be something to be seen, but what size of telescope would be needed to see anything.... well, that was anyone's guess. We just hoped, and waited.

Everything started happening on the evening of July 16th, which was a Saturday. Jupiter wasn't at its best from south west England, being rather low in the sky. We had a barbeque in my garden, and as zero hour approached we sat watching Jupiter low in the west. And then its light went out. As the Sun had set the cloud had risen and that was the last we saw of Jupiter for several days. Cloud was the curse of the whole country. Of course some people who are not astronomers don't understand cloud like we do and are not prepared to accept that after all the media hype something as simple as a bit of water vapour should get in the way. I think the ignorance of a lot of people was summed up by a man who I had the misfortune to work with at the time, who was heard to comment on the Monday morning, 'Oh well, Patrick Moore says he hasn't seen anything so they've got it wrong again, haven't they?'

Astronomers don't expect great spectacles to order, but he should have been looking over my shoulder two evenings later.

I had been out, and while I was out the sky cleared. When I got home, about 9pm, although it wasn't actually dark, there was Jupiter just above my neighbour's roof in a cloudless sky. I hadn't heard any reports from other astronomers, so I got out my little six inch reflector, the only telescope I had then, and pointed it, having no idea what I was going to see. I really didn't expect to see anything at all, but what I did see is reproduced here. I think I stood there with my eye to the eyepiece for several minutes, mouth hanging open, muttering things which must have confirmed everything my neighbours had ever suspected about me. What amazed me most was not so much the sight of Jupiter with two black eyes, but the enormity of what had caused them.

So what did cause them?

Because Jupiter's gravitational pull is so strong, there are over one hundred comets known to be in short period orbit around it, and sometimes those orbits decay so that comet and planet collide. It's reckoned that an impact like that of Shoemaker-Levy 9 takes place on average about once in a thousand years.

It has proved impossible to make anything other than a very rough estimate of the size of any of the fragments of Shoemaker-Levy 9 or the original comet, but calculations from several sources suggest that the largest fragment was perhaps 1 km in diameter. A fragment of this size would hit with the equivalent of 250,000 megatons of TNT, and cause a fireball with a temperature of about 25,000 degrees C. As some sort of a comparison, the Tanguska meteorite of 1908 was about 60 m diameter and exploded 5 - 10 km high. It flattened over 2000 square kilometers of forest and broke windows 400 km, that's 250 miles, away. Similarly the Shoemaker-Levy 9 fragments burst in Jupiter's atmosphere. We think they penetrated no more than 60 km, because it is thought that Jupiter's atmosphere has water clouds at 60 km depth and no water was seen to be emitted from the impact sites. On the other hand we could just be wrong about Jupiter's atmosphere....

What you see in my drawing, of course, is the effect of the fragments on Jupiter's upper atmosphere. Professional telescopes, including the HST, filtered for various wavelengths, photographed impact flashes followed by a plume of material ejected from Jupiter's atmosphere. The plume from the first fragment to hit extended up to 1500 km above the cloud tops. The plume then slowly collapsed into a flattened pancake which was visible because of the amount of dust created. The impact scar left by fragment G was 2 or 3 times the diameter of Earth.

By the end of August the dust from the impacts was spread right around Jupiter and evidence of this could still be seen a year later. An enormous amount of data, particularly about the chemical composition of the plumes, was gathered from Shoemaker-Levy 9's death, but the fact remains that there is still a lot about comets, and about Jupiter, that we do not know.

Hyakutake and Hale Bopp