Comet Hale-Bopp is a rare visitor to our solar system, writes Swapna Prabhakaran
The comet Hale-Bopp has made a spectacular entry into southern skies, 4 300 years since its last visit. The first sighting in South Africa was at Sutherland Observatory, at sunset on April 21. Cape Town spotted it two days later, and now it’s visible all over South Africa, even in smoggy Johannesburg.
The comet – really just a big snowball of ice, dust and gas hurtling through space – is easier to see in the countryside, but even through the haze, city-bound South Africans should be able to see it, if you know where to look.
John Caldwell, a senior astronomer at the SA Astronomical Observatory in Cape Town, offers these directions: “Start by looking at Orion, go down about 20 degrees towards the horizon. When you get to the star Aldebaran, look to the lower right … you can see the comet as a fairly bright fuzzy object, just after the sun sets.”
The comet is in our skies all day, but it is only visible during a “window period” – the orange hours of dusk after the sun has set. In Cape Town, you can see it just above the horizon at about 7pm, and the further north you are, the earlier you’d have to scan the skies. In Gauteng, try it out at approximately 6.20pm.
In the Northern Hemisphere, the comet has been visible since late July 1995. Astronomer Alan Hale discovered it from Massachusetts on July 22 – the same night that Tom Bopp, an amateur skywatcher, noticed the bright presence. The comet, which was then far away from Earth, near Jupiter, has since been named after them both.
The Hubble space telescope, the Nasa satellite that circles the earth taking photographs of deep space, has a year’s worth of pictures of Hale-Bopp and astronomers are using these to study the mechanics of comets. They are looking at the dynamics of the nucleus that propels it on its heady course, what kind of chemicals it’s made up of, and the rate of its evaporation.
Their observations suggest that the nucleus is huge – about 30km to 40km across, about twice as large as Halley’s comet – and gives off brief and intense bursts of activity.
Despite their reputation as bright harbingers of disaster or good fortune, comets are just space dust, born at the same time as the universe. Sometimes this dust and ice gets pulled into orbiting another object of a star – in this case, our sun’s – and begins to circle around it, like the earth orbits the sun.
Apart from having a huge nucleus, Hale-Bopp also has two tails, whose length is about 90-million kilometres. One tail is composed of ion gases and looks blue, the other is of dust and looks white or yellow-white.
Will it hit us? No, although that’s not a stupid question. Astronomers believe that either a comet or an asteroid struck Earth 65-million years ago, causing the extinction of the dinosaurs. That comet would have been about one-third of the size of Hale-Bopp.
Hale-Bopp will not be getting quite that close. It has already passed the earth in its orbit, rounded the sun and is moving away from our part of the solar system towards its outer fringes. It will be back again – in 2 300 years this time, rather than 4 300. This is because it passed too close to Jupiter and the planet’s gravity pulled it off its course into a different orbit.
Although it is on its way out, its exit will be more spectacular than its entrance. Caldwell says the comet’s going to be the brightest over South Africa next week (May 5 to 10), and then it’s going to fade from our view until it’s virtually invisible in mid-June.
Find Hale-Bopp in cyberspace at http://www.halebopp.com/