February 12 February 1965 - Balloon Astronomy

In this era of rocket probes and artificial satellites, it may seem a retrograde step to turn back to the humble balloon. Yet as an astronomical vehicle, the balloon has much to recommend it, and it seemed well worth while to devote a programme to balloon astronomy in general.

To an astronomer, the Earth's atmosphere is a source of conti­nual annoyance. It is dirty and unsteady, so that images of celes­tial bodies are always to some extent blurred; to take really long- exposure photographs, of say, the surface of Mars is pointless, since the fine detail will not be registered. From this point of view the Solar System observer is in worse case than the astro­physicist, who is concerned with what are to all intents and pur­poses point sources of light; no star, apart from the Sun, shows a measurable disk even in the largest telescope yet built. However, there are more serious troubles to be faced. The atmosphere is not transparent to radiations of all wavelengths; in a large part of the electromagnetic spectrum it is depressingly opaque, so that astronomical information obtainable at ground-based observa­tories is very incomplete. From this point of view, the practice of setting up large telescopes on high mountains is of little help, though it does improve the clarity of the optical images.

The atmosphere of our world is quite extensive, and traces of it linger on up to at least 2,000 miles above sea-level, but its density falls off rapidly with height. More than 90 per cent of the total air-mass lies below thirteen miles, and more than 99 per cent of the mass below twenty miles; the upper reaches are very tenuous indeed. Unfortunately, the chief blocking-out of radia­tions is due to layers in the ionosphere, that part of the atmosphere which begins at an altitude of forty miles or so. The so-called D. E, Fi and F2 layers range up to more than 100 miles; these are regions where free electrons are found in great numbers, making ±.e levels electrically conducting (and, incidentally, making I ring-range radio communication possible, by bouncing wireless raves of certain frequencies back to earth). The layers are pro­duced by X-rays and ultra-violet rays from the Sun, which act : n the thin air; the X- and many ultra-violet rays are themselves absorbed in the process, so that they do not penetrate to the ground.

Lower down comes the ozone layer, which absorbs most of the remaining solar ultra-violet. If, therefore, we are to study these important radiations, we must send our recording instruments up to great altitudes.

Of course, rockets and artificial satellites can go far above the ionosphere, and even escape from the Earth. Yet they are com­plex and expensive, and in their present stage of development they cannot lift massive telescopes, keep them steady while the observations are being carried out, and then return them safely. Balloons are much easier to handle, and are also vastly cheaper. Their main limitation is that they are incapable of rising to the ionosphere. A height of between 80,000 and 90,000 feet is as much as can reasonably be expected, and so balloon-borne instruments can contribute little to either ultra-violet astronomy :r X-ray astronomy. All the same, the balloon has much to be said in its favour, since it can at least carry heavy equipment above most of the atmospheric mass - thus eliminating blurring and unsteadiness of the images. Moreover, water-vapour and carbon dioxide in the lower air absorb most of the infra-red radiations sent to us from the planets. Balloon ascents overcome mis hazard with ease.

Hot-air balloons date back to the year 1783, and within a few months of the first flight a French scientist, Charles, went up two miles in a free balloon. Yet there is little resemblance between less crude vehicles and a modern scientific balloon, which has : y now become an important research tool.

The main development has been carried out by M. Schwarzschild and his team at Princeton University in the United States, in collaboration with the United States Navy, the National Science Foundation, and the National Aeronautics and Space Administration. The 'Statoscope' flights of 1959, concerned mainly with studies of the Sun, were remarkably successful, and the project has now been extended. With Statoscope II, the over­all height from the telescope to the top of the launch balloon is 660 feet; the balloons together weigh over two tons, and another two tons of ballast are carried for later release if height has to be maintained during the night. The telescope, plus its controls, weighs three and a half tons. Two large parachutes are also carried; in case of emergency, the instruments and their records can be separated from the main balloon system, and brought down gently. Many of the radio and electronic devices used are similar to those of artificial satellites.

The launch of such an enormous, fragile system must be under­taken on a calm day, with a wind speed of less than fifteen miles an hour or so, and takes some time. The inflated launch balloon lifts the long, folded main balloon, the twin parachutes, and the telescopes with its associated instruments and controls. Two large lorries are used to cancel out any ground-wind; finally, when the long train of items is extended vertically, the tethering stay is dropped and the whole assembly rises into the upper air at a rate of 800 feet per minute.

A model of Stratosphere 2 shown is on the left, a view of the actual flight is shown on the right

During the ascent, helium gas from the launch balloon vents into the main balloon, so that at the chosen 'cruising' altitude (about fifteen miles) the main balloon will be fully inflated. At this point, radio commands direct the telescope towards the objects under study. The telescope locks on to each object in turn, while the observations are made. The procedure is far from simple, but is much more straightforward than with an earth satellite or space-probe; the balloon is what may be termed a 'local' vehicle, and is much more under control. The vital ad­vantage is that the actual instruments and records are recovered, to be examined at leisure and in comfort.

The new telescope for Statoscope II would be regarded as very fine even by ground-observatory standards. It is a reflector, with a 36-inch mirror made of fused quartz. The entire telescope unit will be able to point to selected objects in the sky with re­markable precision, and will remain pointing steadily to an accuracy of 1 /50 of arc second of arc. At peak altitude, where the atmosphere is so thin, the telescope will be capable of detecting fine details on the Sun, Moon, and planets only 1/10 of a second of arc across - which is comparable to using a telescope based in London to see a man who is as far away as New York.

Important discoveries have already been made by balloon- telescope techniques. For instance, detailed photographs of the Sun's surface have been taken, showing features beyond the range of ground telescopes. It has long been known that the solar surface is 'granulated', but the individual granules are compara­tively small, and are in rapid motion, so that they are not easy to study. Balloon photographs show them with amazing clarity, and show that the granules are not of uniform size. The structure of sunspots has also been shown in more detail than ever before.

Stellar astronomy has also benefited, but in view of the present emphasis upon rocket probes it may be worth saying something about the balloon results with regard to Mars and Venus.

Mars, where there is an atmosphere - even though a thin one - has been specially studied, particularly with regard to atmos­pheric water-vapour. There is so little moisture that ground-based instruments were unable to detect it at all; its traces were masked by the water-vapour in our own air. Balloon telescopes have been able to observe from heights where terrestrial water-vapour can b e neglected, and for the first time definite traces of moisture have been found in the Martian atmosphere.

The first serious balloon studies of the second of our 'neighbour' planets, Venus, were made in November 1959, by Ross and Moore in the United States. Until then it had been tacitly assumed that the atmosphere of Venus contained no appreciable water- vapour, and the clouds, whatever they might be, were not made t: H20. The balloon results showed otherwise. Instead of being bone-dry, the atmosphere contained more water-vapour than that of the Earth at an equivalent level. These early results have been substantially confirmed by more recent balloon flights, and it also appears that the clouds are composed of ice crystals. (This does not mean that the planet's surface is likely to be refreshingly cool; Venus is almost certainly very hot indeed.)

So far as balloon astronomy as a whole is concerned, we must admit that it will always be limited in scope; a balloon, by its very nature, has a definite 'ceiling'. For studies which involve going up above the shielding layers in the ionosphere the rocket must remain our only answer. Yet there can be little doubt that balloon techniques have proved their worth, and will be widely used in the future. By space-research standards they are very cheap; the main cost lies in the telescopes and other instruments which are to be lifted - but it is not essential to use giant telescopes. Reflectors of, say, forty to fifty inches aperture probably represent the practicable maximum.

Manned ascents for scientific research have often been made, but generally speaking there is no need to risk human life. The records are obtained photographically, and a visual observer on the spot could not add a great deal, interesting though the ex­perience would be. It is the unmanned, instrument-carrying balloon, operating from altitudes of fifteen miles or so, which will prove of most value in the long run.

Almost all modern work in balloon astronomy has been carried out by the Americans, but since the results are so encouraging it is presumably only a matter of time before the Russians join in ; we may hope, too, for British participation. It is too early to say just what the next developments will be, but we may be sure that the balloon, as an astronomical tool, has come to stay.