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Aviation History
1964
1964 - 0769.PDF
448 FLIGHT International, 19 March 1964 Missiles and Spaceflight BALLOONS IN SPACE OVER 30 transmissions of steady 400c/s tone, morse messages,teletype messages and facsimile photographs were made fromthe Nuffield Radio Astronomy Laboratories at Jodrell Bank to the Zimenki radio-astronomical station of Gorky University's Institute of Radio Physics via the US Echo 2 passive communi- cation satellite between February 21 and March 8. The results of this series of experiments are now being analysed in Washington, Gorky and Jodrell Bank, and further co-operative efforts in space communications between the USA, the Soviet Union and Britain are expected. It is perhaps unusual that, in these days of complex spacecraft, some of the most interesting and useful satellites should be simple balloons. The first large one, the 100ft diameter Echo 1, was designed and used successfully for the passive reflection of teletype, voice and facsimile between continents. One year after its launch, NASA announced that, although the average reflected energy had decreased to about 60 per cent of the original level, satisfactory communication was still possible. Three years after the Echo 1 launch NASA reported that the Echo experiment: the Echo 2 canister undergoing drop tests in a 33ft X 16ft vacuum chamber at Goddard Space Flight Center. The canister drops and opens and the folded balloon deploys satellite, bombarded by tiny meteoroids, had generally retained its spherical shape but that its skin was heavily wrinkled. This wrinkled condition prevented the craft from being useful in communications experiments. Balloons are also influenced by both atmospheric resistance and solar radiation pressure, so much so that small spheres are currently being launched primarily to study these two effects. The first three attempts to launch balloons into orbit failed because of faults in the boosters, the Thor Delta's first and only failure being the unfortunate Echo A-10 attempt. Three months later the back- up Echo, folded within two hemispheres, was successfully injected and inflated by sublimating powders. It carried two transmitters on 107.94Mc/s which were powered by nickel-cadmium batteries, although they became silent about a fortnight after launch. Six months later Explorer 9 (sometimes called Baby-Echo or Polka-dot) was the first satellite ever orbited by the Scout vehicle and, although its transmitters failed, it was easily followed optically. In orbit with both these balloons are several other objects (see table). A much smaller sphere was ejected from the Mercury capsule which carried Scott Carpenter. This was inflated and towed at the end of a cord which sometimes remained taut, some- times slack. Designed to measure atmospheric drag, it also pro- • vided an object for visual perception experiments. Not being an independent orbiting object, however, it burned up when the spacecraft re-entered. Another sphere, somewhat smaller than Explorer 9, was carried pick-a-back into orbit with the USAF satellite 1963-30A last July, but not until December was its presence realized by civilian tracking stations throughout the world. A natural launch to succeed Echo 1 and Explorer 9 would have involved a balloon in an orbit high enough to make atmospheric effects negligible and solar perturbations dominant. It was only after the Smithsonian Astrophysical Obser- vatory at Cambridge, Massachusetts, had requested tracking of an associated fragment 1963-30D, and the object was observed to be brighter and steadier than originally suspected, that its probable identity was discovered. Three new balloons have been in the news recently. The first, Explorer 19, was placed in orbit last December and, unlike its predecessors, was inflated by a cylinder of nitrogen. It carried a transmitter on 136.621 Mc/s. The second Echo, larger and more rigid than the first, was launched on January 25 this year, and an Explorer-Injun 4 lies waiting on the pad. The latter sphere will be carried inside a cylinder aboard the Injun, and will emerge like a sausage as it is blown up. The unusual variations that occur to the orbits of balloon satellites may be best appreciated by comparison with the behaviour of most short-lifetime Earth satellites. For initially eccentric orbits with perigee heights below, say, 300km, the apogee falls more rapidly than the perigee (the rates being proportional to the air density at the two heights). Close to the decay of the spacecraft, the orbit has changed from eccentric to near-circular, and then the height and period both diminish very quickly. In the case of balloons, however, solar radiation pressure causes the eccentricity to increase when the perigee is on the sunlit side of the Earth, and fall when the perigee is on the dark side. The shape of the orbit thus varies, the time for one cycle depending on the speed with which the perigee moves round the orbit, and the orbit rotates in space with respect to the Sun. Solar pressure on a sphere increases with the sphere's cross-sectional area and decreases with its weight. In an earlier article (Flight International, December 26, 196?) it was shown that, for Echo 1, the eccentricity cycle took almost 11 months and its maximum amplitude was 0.077. With the lowest perigee height (900km), Echo 1 is still semi-stable. For Explorer 9. however, the cycle would take several years and the perigee height would become impracticable for an Earth satellite. As the following table shows, the eccentricity reached a minimum about one yea" after launch. Since then it has been rising steadily, and the perigee will fall into the dense layers of the atmosphere before the maximum section of the cycle is reached. Air drag will then bring the orbitai
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