From Brian Wall
It would appear that one of the main reasons why Hubble has to go is that its gyroscopes are in constant need of repair, and NASA doesn’t want to continue sending up missions to repair them (31 January, p 4).
Hubble is apparently in an orbit some 150 kilometres higher than the International Space Station (ISS). So why not modify either or both their orbits to permit easy access to Hubble from the ISS on the odd occasions that this is necessary?
Their orbits only need bring them close for a short while, so Hubble could continue at its present inclination if that were a primary concern.
David L. Chandler writes:
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• This sounds like a tempting idea. There’s just one problem with it. One of the most difficult things to do with an orbiting spacecraft, in terms of energy expenditure, is to change the inclination of the orbit in relation to the Earth’s equator – in aerospace parlance, to make a “plane change”.
Hubble, launched from Cape Canaveral, Florida, is at 22.5 degrees inclination. The ISS, designed to be reachable by spacecraft launched from Baikonur, Kazakhstan, is at 51 degrees inclination – a very big difference. The energy penalty of plane changes is precisely why the ESA launch site at Kourou (latitude 7 degrees north) has such an advantage for launching geosynchronous satellites, which must end up at zero inclination.
No rocket now available is powerful enough to change the inclination of the massive ISS to 22.5 degrees – and if that were done, the ISS would become inaccessible to Soyuz, the only human-rated craft that can now reach it. Changing Hubble’s orbit would require the shuttle to attach some huge rocket to it – but, of course, the shuttle is currently grounded, which brings us back to square one.
Ferndown, Dorset, UK
