Terry Pratchett - The Science of Discworld I

But you don't have to use just one telescope.

A technique known as 'interferometry' makes it possible, in principle, to replace a single mirror 100 yards wide by two much smaller mirrors 100 yards apart. Both produce images of the same star or planet, and the incoming light waves that form those images are aligned very accurately and combined. The two-mirror system gathers less light than a complete 100-yard mirror would, but it can resolve the same amount of tiny detail. And with modern electron­ics, very small quantities of incoming light can be amplified. In any case, what you actually do is use dozens of smaller mirrors, together with a lot of clever trickery that keeps them aligned with each other and combines the images 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 2002 NASA will launch Deep Space 3, involving two spacecraft flying 1 kilometre apart and maintaining station relative to each other to a precision of less than half an inch (1 cm). Another NASA venture, the Space Interferometry Mission, will employ seven or eight optical telescopes bolted to a rigid arm 10-15 yards (10-15 m) long. In 2009 ESA hopes to launch its Infrared Space Interferometer, not to image distant planets but to find out what their atmospheres are made of by looking for telltale absorption lines in their spectra.

The biggest dream of all, though, is NASA's Planet Imager, pen­cilled in for 2020. A squadron of 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 because conditions on Mars are in some ways similar to those on Earth, though generally nastier, and partly because dozens of science-fiction books have subliminally prepared us for the existence of Martians. Life does show up in nasty places here, finding a foothold in volcanic vents, in deserts, and deep in the Earth's rocks. Nevertheless, we've found no signs of life on Mars.

Yet.

For a while, some scientists thought we had. In 1996 NASA announced signs of life on Mars. A meteorite dug up in the Antarctic with the code number ALH84001 had been knocked off Mars 15 million years ago by a collision with an asteroid, and plunged to Earth 13,000 years ago. When it was sliced open and the interior examined at high magnification we found three possible signs of life. These were markings like tiny fossil bacteria, crystals containing iron like those made by certain bacteria, and organic molecules resembling some found in fossil bacteria on Earth. It all pointed to: Martian bacteria! Not surprisingly, this claim led to a big argument, and the upshot is that all three discoveries are almost certainly not evidence for life at all. The fossil 'bacteria' are much too small and most of them are steps on crystal surfaces that have caused funny shapes to form in the metal coatings used in electron microscopy; the iron-bearing crystals can be explained without invoking bacteria at all; and the organic molecules could have got there without the aid of Martian life.

However, in 1998 the Mars Global Surveyor did find signs of an ancient ocean on Mars. At some point in the planet's history, huge amounts of water gushed out of the highlands and flowed into the northern lowlands. It was thought that this water just seeped away or evaporated, but it now turns out that the edges of the northern lowlands are ail at much the same height, like shorelines eroded by an ocean. The ocean, if it existed, covered a quarter of Mars's sur­face. If it contained life, there ought to be Martian fossils for us to find, dating from that period.

The current favourite for life in the solar system is a surprise, at least to people who don't read science fiction: Jupiter's satellite Europa. It's a surprise because Europa is exceedingly cold, and cov­ered in thick layers of ice. However, that's not where the life is suspected to live. Europa is held in Jupiter's massive gravitational grasp, and tidal forces warm its interior. This could mean that the deeper layers of the ice have melted to form a vast underground ocean. Until recently this was pure conjecture, but the evidence for liquid water beneath Europa's surface has now become very strong indeed. It includes the surface geology, gravitational measurements, and the discovery that Europa's interior conducts electricity. This finding, made in 1998 by K.K.Khurana and others, came from observations of the worldlet's magnetic field made by the space probe Galileo, The shape of the magnetic field is unusual, and the only reasonable explanation so far is the existence of an under­ground ocean whose dissolved salts make it a weak conductor of electricity. Callisto, another of Jupiter's moons, has a similar mag­netic field, and is now also thought to have an underground ocean. In the same year, T.B.McCord and others observed huge patches of hydrated salts (salts whose molecules contain water) on Europa's surface. This might perhaps be a salty crust deposited by upwelling water from a salty ocean.