The Science of Discworld Revised Edition

that they receive in an effective manner.
    Radio astronomers use this technique all the time. The biggest technical problem is keeping the length of the path from the star to its image the same for all of the smaller telescopes, to within an accuracy of one wavelength. The technique is relatively new in optical astronomy, because the wavelength of visible light is far shorter than that of radio waves, but for visible light the real killer is that it’s not worth bothering if your telescopes are on the ground. The Earth’s atmosphere is in continual turbulent motion, bending incoming light in unpredictable ways. Even a very powerful ground-based telescope will produce a fuzzy image, which is why the Hubble Space Telescope is in orbit round the Earth. Its planned successor, the Next Generation Space Telescope, will be a million miles away, orbiting the Sun, delicately poised at a place called Lagrange point L2. This is a point on the line from the Sun to the Earth, but further out, where the Sun’s gravity, the Earth’s gravity, and the centrifugal force acting on the orbiting telescope all cancel out. Hubble’s structure includes a heavy tube which keeps out unwanted light – especially light reflected from our own planet. It’s a lot darker out near L2, and that cumbersome tube can be dispensed with, saving launch fuel. In addition, L2 is a lot colder than low Earth orbit, and that makes infra-red telescopy much more effective.
    Interferometry uses a widely separated array of small telescopes instead of one big one, and for optical astronomy the array has to be set up in space. This produces an added advantage, because space is
big
– or, in more Discworldly terms, a place to be big
in
. The biggest distance between telescopes in the array is called the baseline. Out in space you can create interferometers with gigantic baselines – radio astronomers have already made one that is bigger than the Earth by using one ground-based telescope antenna and one in orbit. Both NASA and the European Space Agency ESA have missions on the drawing-board for putting prototype optical interferometer arrays – ‘flocks’ is a more evocative term – into space.
    Some time around 2003, NASA will launch Space Technology 3 (previously named Deep Space 3), involving two spacecraft flying 0.6 miles (1 km) apart and maintaining station relative to each other to a precision of less than half an inch (1 cm). A successor, Star Light, will follow in 2005. Another NASA venture, the Space Interferometry Mission, will employ three interferometers with a 10-metre baseline and is tentatively due to launch in 2009. And NASA is thinking about a Terrestrial Planet Finder in 2012, which will look not just for planets, but for carbon dioxide, water vapour, ozone, and methane, which could be signs of life – or, at least, of a planet that might be able to support life similar to ours. Life Finder, with no specific date, would look more closely.
    The European Space Agency (ESA) has similar missions on the drawing-board. SMART-2, consisting of two satellites orbiting in formation , is planned for 2006. A more ambitious ESA project is Darwin, a flotilla of 6 telescopes that could be in space by 2014.
    The biggest dream of all, though, is NASA’s Planet Imager, pencilled in for 2020. A squadron of five spacecraft, each equipped with four optical telescopes, will deploy itself into an interferometer with a baseline of several thousand miles, and start mapping alien planets. The nearest star is just over four light years away; computer simulations show that 50 telescopes with a baseline of just 95 miles (150 km) can produce images of a planet 10 light years away that are good enough to spot continents and even moons the size of ours. With 150 telescopes and the same baseline, you could look at the Earth from 10 light years away and see hurricanes in its atmosphere. Think what could be done with a thousand-mile baseline.
    Planets outside our solar system do exist, then – and they probably exist in abundance. That’s good news if you’re hoping that somewhere out there are alien lifeforms. The evidence for those, though, is controversial.
    Mars, of course, is the traditional place where we expect to find life in the solar system – partly because of myths about Martian ‘canals’ which astronomers thought they’d seen in their telescopes but which turned out to be illusions when we sent spacecraft out there to take a close look, partly