Rudnicki Comet Has Become Brighter

Press, Volume CVI, Issue 31262, 7 January 1967, Page 10

Rudnicki Comet Has Become Brighter

A comet, discovered by Rudnicki on October 15, has become steadily brighter as it approaches the sun in the western sky. It may become just visible to the naked eye in January but is not likely to be very bright. This is partly because the comet itself is not expected to be a conspicuous object and partly because, as it nears the sun, it will be visible only during twilight. By January 21 it will reach its closest distance to the sun of about 39 million miles. Comet Rudnicki is the fifth comet to be discovered in 1966. This year is likely to prove more fruitful for comets than 1966. Some 14 periodic comets are predicted to return during 1967, but it is most unlikely that any of these will attain naked-eye brightness. Most will be very faint and visible only in large instruments as faint diffuse patches. One, Comet Encke, will become visible in small telescopes. This comet has the shortest period of any periodic comet. It is probable that Comet Encke was once a very large comet, because it is known to have contributed large streams of meteors, radiating from the constallation of Taurus, the Bull. At one time these meteors were very active, but now only a few are observable. Comet Encke has many claims to fame. One is that it has now been observed at 47 returns. It was originally discovered by Pons in 1818, and since then has been seen at each return except in 1944, when it was not observed. As a result it has assisted to disprove the theory that comets break up after a number of successive returns to the region of the sun. Though Pons was the actual discoverer of this comet it carries Encke’s name because the latter investigated its orbit and found that it was identical with comets that had been seen In 1786, 1795, and 1805. At each of its revolutions there is a very slight shortening of its period by about two hours and a halt, but in 1868 this steady shortening was decreased by half. Comet Encke passed very close to the planet Mercury in 1905, and was used to determine the mass of that planet through the perturbations on the comet. At Mount John we are often asked how many comets there are in the solar system. It appears likely that the number would be well in excess of a million. However, no definite answer can be

given because many have very long periods, possibly measured in millions of years. As a result they do not visit the sun very often and remain invisible from the earth for most of the time. WHAT IS A COMET? Comets travel around the sun in very elongated ellipses but are still gravitationally bound to the sun. They can approach the sun from any direction, and for this reason, unlike the planets, ■ they do not travel around the sun in the same direction. Just as many travel round the sun in the reverse direction as revolve in the direct direction. When a comet is nearest to the sun it can usually be divided into three parts. First, there is the nucleus, almost star-like in appearance. This is probably a swarm of solid particles, about one-third of which consists of rocky or metallic materials and the remainder of substances that vaporise more easily. As the comet comes near the sun this material sublimates to a gaseous state and forms, a coma immediately around the nucleus. The coma and nucleus together make up the head of the comet. As the materials vaporise in the nucleus this becomes a swarm of microscopic dust grains. Many comets develop tails as they approach the sun, but this is not universal, and many, especially those of short period, do not form tails. On the other hand, some comets develop tails that have a total length of more than 100 million miles. It was thought that tails were formed by the action of thermal activity caused by solar radiation heating the comet and causing the nucleus to eject material. The sun would then have two effects on the ejected material. A repulsion force would direct the material radially away from the sun whilst the gravitational force of the sun would act on each particle, causing it to move in an orbit. These two forces act in different ways, but in general the material ejected is lost to the comet for ever. HEAT MEASURED Recently scientists at the Californian Institute of Technology were able to make the first temperature measurements of a comet. These were of Comet Ikeya-Seki, which attracted so much attention in 1965 when it became a very brilliant object. The measurements were made at infrared wave lengths, which are much longer than the wavelengths of light to which the human eye is sensitive. Most infra-red light is screened out by the earth’s atmosphere, but there are a few “windows” through which such measurements are possible. Until Comet Ikeya-Seki was within 45 million miles of the sun, no measurements were possible, because the energy was below the sensitivity of the infra-red detectors. At this distance the Caltech scientists found the temperature was 700 degrees fahrenheit. By the

time the comet was only 20 million miles from the sun the temperature had risen to 1200 degrees. The overpowering effects of the sun itself prevented further measurements until the comet was receding, and when it had again reached 20 million miles from the sun a temperature of 1200 degrees was recorded. By the time the comet, on its outward journey, was 45 million miles from the sun the temperature was back to 700 degrees. It was found that for each measurement the head and the tail both had the same temperature. These observations show that the temperature of the comet was entirely dependent on the sun. Though the recorded temperatures were the same for both inward and outward journeys it was found that the total amount of energy from the comet’s nucleus had decreased sharply after the comet passed near the sun. This suggested that 65 per cent of its mass was lost as it swung around the sun. The comet was actually seen to break into two pieces, and in the process must have lost tons of iron and other metallic substances. The Caltech astronomers suggest that this comet was unusual because it had made a previous very close approach to sun, when the solar heat had "boiled away” all the lighter chemical elements in the nucleus, leaving nothing but the heavier metals. It also probably broke apart on its previous visit, and the resulting separate fragments have revolved around the sun since as separate comets. It is significant that six such fragments have been observed since 1843, all travelling in the same orbit but separated from one another by tremendous distances. LARGELY NOTHING It must not be thought that because a comet is, as Comet Ikeya-Seki has appeared, largely metallic in its nucleus that it has much mass. They certainly can cover millions of miles, but comets have been seen to pass through the satellite system of Jupiter and also close to various planets. In no case has any perturbation to a satellite or planet been caused. The comet, however, has had its period drastically altered in these cases. The tails of very bright comets may contain something like 100 or 200 million tons of microscopic iron particles. As such comets dash around the sun they could lose anything between 100 and 1000 tons of iron a second. Even this high rate would enable a comet to make 100 passages back to the sun before losing the ability to form tails. These figures may sound impressive until it is realised they are spread over vast areas. Many experts think that an average comet has a mass about a trillionth the mass of the earth. Thus a comet is largely nothing. In fact, a comet has been described as “the nearest thing to nothing that anything can be and still be something.”