The tale of ice in the sky…

Roughly 4.5 billion years ago, something rather important happened. Maybe it was was the shock wave from an exploding giant star; maybe it was some other mechanism we don’t yet understand. But around that time, an enormous cloud of gas started to contract and form the sun, the Earth, the planets, and, possibly, a huge collection of other lumps of rock, dirt, and water. And, according to currently accepted theories, a lot of these lumps of solid matter were flung out of the inner solar system, in all directions, to form the Oort Cloud. Or so astronomers think; we’ve never actually directly seen the Oort Cloud. But, according to their current best guesses, there are billions, or even trillions, of variously-sized lumps of rock and ice present in all directions from the sun, with its highly tenuous gravitational hold resulting in orbits that last tens, or even hundreds of thousands of years.

Every so often, it seems, something throws Oort Cloud objects off course. Maybe they have a close encounter with larger Oort Cloud objects, or even collide. But, in any case, a few of them get knocked out of their slumberous existence in the eternal starlit gloom of the far, far reaches of the solar system and start making their way towards the Sun.

On the third planet from the Sun, the local inhabitants looked up at the sky, and every few decades or so would see these conically-shaped apparitions in the sky, and took them as harbingers of doom, or, in the case of Aristotle, events occurring at a relatively small distance away. But in 1577, a rather clever (if famously wrong on other issues) astronomer, Tycho Brahe, knocked that idea on the head. Brahe, and some other astronomer friends, observed a comet simultaneously from widely separated positions on Earth, and measured the angles at which they were observing as best they could with the technology of 1577. If you correct for the curvature of the earth, if the objects were close the two observers would have to look at different angles to observe the comet. But if it was a long way away, they would be looking in pretty much the same direction (again, corrected for the curvature). Brahe crunched the numbers, and it was clear that the comet was a long way away. There were other objects in the heavens sharing space with the moon and planets.

Then a rather bright English chap called Edmond Halley, who had earlier persuaded Isaac Newton that his universal theory of gravitation was pretty interesting and that he really should publish it, had another idea. Halley figured out that appearances of these stars with funny tails were occurring every 76 years or so, and guessed that comets were some kind of astronomical body with a 76-year orbit, and published his results in 1705. Halley did not live to see his predictions confirmed, as they were in 1758 (he predicted 1757, not calculating the effects of the planets interfering with the orbit properly).

Around the same time that Halley’s comet was making its confirmatory appearance, another astronomer, Charles Messier, started making a list of some rather annoying objects that looked like little comets, but were permanent rather than ephemeral. Messier never knew that many of the objects he was looking at were more important than a trillion comets. It was not until the 20th century that Edwin Hubble showed that the 31st object in Messier’s list, M31, was the Andromeda Galaxy, a spiral galaxy quite similar to our own Milky Way. Comet hunting, if indirectly, led to perhaps the greatest astronomical discovery of the 20th century; that the Milky Way, for all its immensity, was merely one of trillions of galaxies.

But the story of comets was not over yet to astronomy. It was reasonably quickly guessed that comet tails were the result of the comet throwing off matter, but why did they always seem to get blown away from the sun? Johannes Kepler, whose calculations had shown Tycho Brahe’s theories of the solar system (that everything orbited the Earth) to be wrong and whose theory of elliptical orbits around the sun paved the way for Newton’s works, suggested way back in the 17th century that the sun’s light must be putting pressure on the material being thrown off the comet. It would take until 1871, and the work of James Clerk Maxwell, to provide a theoretical explanation of this radiation pressure, and it would first be detected in the early 20th century. But it didn’t seem to be that simple. The tails of comets seemed to have occasional “shifts” in their direction; almost as if there were a changeable wind blowing parts of the tail round. And so there was; the solar wind was a stream of electrically-charged particles pushed out of the sun at enormous, but also changing speeds.

I went out to observe Comet McNaught last night, after Melbourne finally returned to the cloudless skies that have dominated this summer. Catch it while you can; McNaught is the most spectacular comet for half a lifetime; we may never see its like again. While my own photos don’t do it justice, it’s truly a spectacular sight; easily visible with the naked eye (at least for the next couple of nights), the tail extends across the sky like cosmic fireworks above the afterglow of the just-set sun. While McNaught itself is just another ball of ice and dirt,, and unlike Halley’s comet destined to return to the Oort Cloud from whence it came, to me it represents a challenge, hanging there in the sky. What we be the answers to those questions that currently puzzle astronomers – for instance, does the Oort Cloud actually resemble the theories of astronomers? What other mysteries does our universe contain? Will we continue to unravel them? And, maybe, one day, will my distant descendants go eyeball some of those mysteries in person?

Oh, and apologies to all the actual astronomers, physicists, science historians, and mathematicians who will undoubtedly be wincing over the errors and handwaves in this piece…