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Aviation History
1987
1987 - 1780.PDF
w*. A wsm S^ tin Return to Mercury Renewed efforts are under way within the planetary science community to promote an unmanned mission to Mercury before the end of the century. The solar system's innermost planet was scrutinised only briefly by the lone Mariner 10 in the mid-1970s, and an extended follow-up mission is essential to answer some fundamental questions. Neither Nasa nor Esa have current Mercury programmes, but many feel that the time is ripe for proposals to be seriously considered. The US Mariner, launched by an Atlas- Centaur in November 1973, entered a resonant solar orbit for three Mercury flybys between March 1974 and March 1975 before its nitrogen attitude-control gas was exhausted. A major surprise was the discovery of a relatively intense magnetic field, and the third flyby was brought within 327km of the surface to probe this more deeply. Unfortunately, orbital dynamics dictated that Mariner's vidicon cameras observed the same hemisphere on each encounter, and a primary goal of a new mission would be to scan the entire globe with at least 100m resolution. Mariner was constrained to about l-3km for most areas. Dr John Guest, reader in planetary science at University College, London, and a member of the Mariner 10 imaging The planet closest to our Sun, Mercury remains something of a mystery. Many scientists would like to solve that mystery by sending probes to map and sample the planet. Andrew Wilson reports. team, says "It would be dangerous to draw global conclusions about Mercury when we have seen only 50 per cent of the surface. What we have done so far is to assume that the other side has similar geology. However, that was most defi nitely not the case with Mars, where the three early flybys all looked at the southern hemisphere and gave us a totally erroneous view". It was not until Mariner 9 became Mars' first orbiter in November 1971 that the planet's rich diversity of features was revealed. Apart from resolving this problem of hemispherical dichotomy, Dr Guest and fellow specialists need to explain the origins of the different types of Mercurian Electric propulsion is one way to power a large spacecraft to Mercury 46 plains. "We need that information in order to construct a reliable model of Mercury's thermal history, to help us work towards a universal theory of planetary evolution", he says. Examples are found between craters in the highland regions and even within craters in some cases. At the time of Mariner 10 the low land smooth plains were thought to be the equivalent of the lava-filled lunar mare, but subsequent more detailed studies of the images has produced no indisputable evidence for volcanic activity anywhere on the planet. Perhaps the answer resides on the unseen hemisphere. Mariner 10 also failed to uncover any Mercurian natural satellites, although a false alarm was raised by a star, and a search would be an ideal objective for an orbiter once it had completed its primary mission. An Indian scientist has suggested that the most likely place for a moon would be in a retrograde orbit some 250,000km out. It was against this background that a team of 52 scientists, co-ordinated by DFVLR's Dr Gerhard Neukum, submitted a proposal to Esa in November 1985 for a Mercury polar orbiter (MPO). The agency's Solar System Working Group, while impressed with its content, noted that it would break the 200 MAU (million accounting units) ceiling placed on such missions. The scheme is currently being FLIGHT INTERNATIONAL, 19 September 1987
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