Meteor shower from comet possible

Press, Volume CXII, Issue 32880, 1 April 1972, Page 5

Meteor shower from comet possible

An English astronomer has predicted that a southern meteor shower will occur on April 23. His prediction is based on the fact that, at the time, the Earth will be only 600,000 kilometres (360,000 miles) from the orbit of Comet Grigg-Skjellerup. That is roughly half as far again as the Moon.

The nature and extent of the shower, if one does eventuate, depends on how' the meteoroids left behind by the comet have been distributed.

Before considering the possible circumstances of such a shower it is advisable to describe comets briefly. The normal appearance of a comet in a telescope is that of a nebulous haze with a star-like point in the centre of its head. This point is called the nucleus and is something of the order of tens of miles in diameter. When the comet is about three astronomical units from the Sun, a diffuse, round envelope that surrounds the nucleus expands. This is the coma. Generally, but not! always, a tail will develop when the comet is at a distance of roughly 1.5 astronomical units. A bright naked eye comet may have a coma that is several hundred thousand miles ' in diameter, whilst the tail will stretch 1 over more than 30 million miles.

Despite the enormous dimensions just quoted, comets have very little mass. They have been defined as "The nearest thing to nothing that can still be something without being nothing.” Comets have passed through the satellite system of Jupiter without causing any perturbations to the satellites.

I Far from the Sun the (nucleus of a comet is thought to resemble a 1 “dirty snowball” with all its material frozen solid. This material may be a swarm of tiny solid particles or even a single mass. The coma is , formed from the substances within the nucleus that sublimate to a gaseous state. As the comet draws nearer to the Sun and its material is heated a flow of this material is driven outwards from the head. Through a repulsion effect from the Sun this is driven radically away from the comet. Not all comets form tails. It is probable that after many returns to the Sun’s neighbourhood comets lose their ability to form tails. Being so tenuous, comets are estimated to disintegrate entirely after a hundred or so perihelion passages. Thus short-period comets are more likely to break up and disintegrate because so much of their materia! has been lost during their frequent visits to the Sun’s neighbourhood. Shooting stars The Earth moves through space, in which there are many small bodies orbiting around the Sun. These are called meteoroids. When one of them comes within the gravitational field of the Earth it is attracted. Then the small solid particle enters the Earth’s atmosphere and because of the speed at which it is travelling and the air resistance, the meteoroid heats up, dislodging small particles. These latter rapidly vapourise forming a region of glowing gases. The luminous spot of light that results is what w e see as it rapidly moves through the atmosphere and is called a meteor. More popularly it is termed a “shooting” or "falling” star although there is no connection with the stars.

Some 25 million meteors, bright enough to be visible to the naked eye enter the atmosphere every day. The atmosphere acts as a protective blanket saving us from a continual celestial bombardment since most of the meteors are completely burnt out far above our heads. Even the brightest of them rarely descends lower than 30 miles above the Earth’s surface.

Basically there are two types of meteors. The sporadic meteors, visible every fine night and these appear anywhere in the sky and move in any direction. They are simply due to random debris within the solar system. The second type are called shower meteors because they appear to radiate from a definite area in the sky.. These meteors are caused by the debris that comets have scattered in their orbits. If a comet is comparatively old then this debris is scattered more or less uniformly right round its orbit in the form of a stream of meteoroids. If the orbit of the Earth intersects that of a stream then a meteor shower results. Since the separate particles in a stream are travelling in parallel paths the resultant meteors appear to radiate from some particular spot in the sky. It is very much like looking along the parallel tracks of a railway. They appear to diverge from a point in the distance. When a comet is comparatively young the debris along its orbit tends to be bunched up at some particular spot and the meteoroids are then said to form

a swarm. Should the Earth intersect the orbit of a swarm then a most spectacular shower of meteors results. Comet Grigg-Skjellerup has a period of 5.1 years, one of the shortest known. It was first discovered on July 22, 1902, by John Grigg, of Thames, who not only found it but followed it closely enough to obtain a series of accurate positions which enabled him to compute its orbit. It was not seen at the next three returns, but, on May 17, 1922, Skjellerup, in South Africa, found what he thought was a new comet. It proved to be the second recorded apparition of Comet Grigg-Skjellerup, which has been seen since at each of its subsequent ten returns. Apart from 1902 and 1942 when it reached ninth magnitude, it has always been a rather faint and insignificant object. It has long been suspected that Comet Grigg-Skjellerup is a possible progenitor of meteors. However, no previous meteor activity attributable to it has been noted and it is only recently that it has moved into an orbit giving a very close approach to the Earth. It can be seen from the cometary aisions quoted in this e that, if the comet has scattered debris along its orbital path, the Earth will be close enough to the orbit to attract some of these particles on April 23. It appears very doubtful as to whether a spectacular display will occur because the time of closest approach will be shortly after midday. Should the meteors be in the form of a swarm the maximum display will then take place in daylight from our latitudes.

If, on the other hand, the comet has scattered its debris in the form of a meteoroid stream we could well encounter some meteors on the evenings of April 22 and 23. Bright moonlight will, however, cut down the number visible. The theoretical radiant point is in the constellation Puppis, near the bright star, Canopus. It will be interesting to watch around this point in the south-western sky for any possible meteors. John Grigg John Grigg, the original discoverer of this comet, was an English migrant to New Zealand. He settled in Auckland in 1863, moving to Thames five years later. There he built a small observatory equipped with a refractor, of 9cm aperture, and a small transit instrument. The former he used for his observations and it was with this instrument that he discovered three new comets. These were Comet GriggSkjellerup in 1902; Comet Grigg in 1903 and Comet Grigg-Mellish in 1907. Skjellerup, whose name is also commemorated by Comet Grigg-Skjellerup, made his first cometary discovery in 1862 from Copenhagen. He was not the first to see Comet 1862 111 but his was an independent discovery. Later he was stationed at the Cape, in South Africa, where he continued his search for comets. This resulted in discoveries of four new comets, one in each year from 1920 to 1923. His last discovery was .made in 1927, after he had moved to Australia. Skjellerup has connections with New Zealand and the various medals that were awarded to him are held by relatives in Christchurch.