Comet McNaught had us talking science

Advertise

Comet McNaught had us talking science

Comet McNaught taken from Grahamstown, South Africa. Photo credit © Dr Jim Cambray

Like the solar eclipse excitement over the past few years Comet McNaught did far more than provide a visual feast in the skies over the month of January, it not only got us outdoors at sunset, it got us thinking and talking science and astronomy.

From viewing spots across the Eastern Cape you could hear non-scientists excitedly discussing Comet McNaught's tail, theorising about its trajectory. People reminisced, as most comet watchers did, at the disappointing comet fare in recent times, with Halley's Comet just about topping the list. But with comet McNaught's display, all has been forgiven. It is now billed as one of the Great Comets and the best comet display for forty years.

South Africa is fast becoming one of the hot spots for astronomy research on the globe given wide expanses of land with little to no light or radiowave pollution. With developments such as the Southern African large Telescope (SALT), existing expertise at the South African Astronomical Observatory and Hartebeesthoek Radio Astronomy Observatory (HartRAO) and the prospect of hosting the Square Kilometre Array (SKA) there is a growing interest in astronomy and science in general in the country from the general public at large. This year the South African Agency for Science and Technology Advancement (SAASTA) will for the fourth year running mark October as Astronomy month with a series of educational interactive experiences planned nationwide.

Comets and Comet McNaught

Astronomers Dave Laney and visiting scientist Albert Ziljstra at the South African Astronomy Organizations provide further insight.

Comet McNaught was discovered by R.H. McNaught at Siding Spring, Australia on August 7, 2006. At the time it was far out in the solar system, and more than 200000 times too faint to see with the unaided eye.

Most comets are found in the Oort Cloud, much farther from the sun than any of the planets, and are surviving clumps of dust and ice from the early solar system. If these are orbits are disturbed only slightly, they can plunge into the inner solar system, where we see the glowing cloud of evaporated gas and dust around the tiny (a few km across) solid lump, while the pressure of sunlight and of particles streaming outward from the sun can push some of the cloud (or 'coma') into the classic 'tail', which always points away from the sun.

Great comets are rare. Comets are occasional visitors to the inner parts of the Solar System. Since 1960, 10 comets have been visible with the naked eye. Few of these qualified as Great comets, sufficiently bright to draw worldwide attention. Comet McNaught is the brightest such comet since 1965

What is a comet?

Comets have been likened to dirty snowballs. They consist largely of frozen, fluffy water (hence snow), with dust particles embedded in them. They reside in the outer solar system, beyond the orbit of Pluto (recently downgraded to 'dwarf planet'). Small disturbances to their orbits, induced by planets, occasionally cause one to fall in towards the Sun. This is what is likely to have happened to comet McNaught.

When close to the Sun, the heat begins to evaporate the snow. The water vapour (which includes other chemicals as well) drifts away from the comet and is pushed out by solar wind - a stream of particles emitted by the Sun. This vapour forms the tail. Not surprisingly, it always points away from the Sun. The tail can reach enormous lengths, many million of kilometres. But the amount of gas in it is in fact very small. It would fit inside a soccer ball!

Comets would normally be expected to return to the outer solar system, albeit on an orbit which will eventually return to the Sun. We do not yet know the precise orbit of comet McNaught, but current estimates indicate it may be travelling on an orbit which will allow it to escape the Sun altogether. This can happen if a comet picked up some extra speed while falling towards the Sun, from coming too close to a planet. (Spacecraft travelling to the outer planets use the same trick, normally using the gravity of Jupiter.)

If this is correct, comet McNaught will never come back to us. Instead it will leave the Solar System, and may eventually (after millions of years) fall towards another star. We can expect other comets (a few each decade, typically, become visible) but it may be decades before one becomes as bright as McNaught. Or we may be lucky and get another one this year-but don't bet on it.

What is a Great Comet?

This is a phrase used for a particularly impressive comet. The brightness of a comet depends on how big it is, how close it gets to the Sun, and/or how close it gets to the Earth. Comet Halley is a large comet, but in 1987 it did not come very close to the Sun, and it remained very far from Earth. So it did not qualify as a Great Comet. On a previous appearance, in 1910, it did come close to Earth and became very impressive-and so it was a Great Comet at that time. (1910 was a good year for comets: a second comet became visible even during day light.)

Great comets of recent decades have been Hale-Bopp (1995), Hyakutake (1994), Comet West (1975) and Ikeya-Seki (1965). Of these, only Ikeya-Seki was brighter than the current comet McNaught.

The brightest comets of the past 200 years have been the Sun grazers: comets which get incredibly close to the Sun. It appears that all the brightest comets (include the one from 1965) are remnants of a much larger comet which broke in pieces during its last visit, in AD 1106. The fragments have gone off on slightly different orbits and now are coming back to the Sun one by one. Comet McNaught is not part of this family, however. It travelled well inside the orbit of Mercury, 6 times closer to the Sun than we are. Sun grazers get far closer to the Sun, however.

More information:

Information from the South African Astronomical Observatory.