Extending the Final Frontier
Not too long ago, Gliese 581g was the name on everybody's lips (and appears on p. X) - an extrasolar rocky planet that could have the potential to support life! The discovery, if it is confirmed, profoundly changes our view on the probability of finding life outside our own solar system. Yet this is by no means the first time that astronomy has transformed our understanding of the universe. The main instrument in this endeavour has always been the humble telescope.
Gliese 581g was discovered with the help of the Keck I optical telescope in Hawaii. The Keck telescopes are the result of an inspired feat of engineering. Like most optical telescopes, they use a concave mirror to focus light onto a focal plane, where the image is recorded. However, the primary mirrors on the Keck telescopes are as long as 10 metres in diameter. Building a single mirror of this size would have been ruinously expensive. Luckily, astrophysicist Jerry Nelson came up with a brilliant idea: build a bowl-shaped reflective surface out of many thin mirror segments. Thus, each of the Keck mirrors consists of 36 hexagonal segments that can be operated together. The Keck telescopes remained the largest optical telescopes from their construction in 1995 until 2009, when the Gran Telescopio Canarias, with a diameter of 10.4 metres and a cost of around £112 million, edged in front.
Observatories like Keck and Grand Telescopio Canarias have allowed astronomers to learn a lot more about the universe, ranging from the ‘neighbouring’ asteroids in Jupiter's belt to detecting the most distant supernovae. However, ground-based telescopes are always limited by the distortions caused by the Earth's atmosphere and weather. Space-based telescopes were the obvious solution, and one cannot talk about space telescopes without mentioning the most famous example.
The Hubble Space Telescope was named after astronomer Edwin Hubble, who first discovered the existence of other galaxies besides our own Milky Way. His namesake telescope has been no less influential in astronomy. Since its launch in 1990, it has not only been producing spectacular pictures of distant stars and galaxies, such as the famous image of stars being born within the Eagle Nebula, but also enabled astrophysicists to make numerous discoveries. Among other things, data from the Hubble confirmed that the expansion of the universe is accelerating, and that black holes form the centre of most galaxies.
While Hubble is certainly the most well-known space telescope, other, more specialised space telescopes have been producing equally important scientific data. For example, the Chandra X ray Observatory has allowed us to learn about the X ray emissions of stars. This would have been impossible for a ground-based telescope, as the Earth's atmosphere absorbs most of the incoming X-rays. WISE, the Wide-field Infrared Survey Explorer, was another important mission that managed to survey the entire sky one and a half times, as well as discovering more than 150,000 objects in our solar system before running out of its on-board supply of frozen coolant.
Hubble has recently had its last servicing mission, but replacements are already being built. The most promising next-generation telescopes are the James Webb Space Telescope and the Advanced Technology Large-Aperture Space Telescope (ATLAST). Both will orbit the sun at a distance of 1.5 million kilometres hidden in the Earth's partial shadow, at the so-called L2 Lagrange point. This is an advantageous location because the pull from the Earth's gravity ensures that the telescope orbits the sun along with the Earth, keeping the same position relative to both. The Earth provides a partial shielding from the sun's radiation, but in addition, the James Webb comes with a fan-like sunshield that unfurls to protect the 6.5 metre wide mirror from interference by radiation from the sun, Earth and moon.
The James Webb’s deployment is planned for 2014 and, unlike Hubble, it will only look at the infrared spectrum. This will permit it to see through gas clouds and observe more and older stars. ATLAST has great potential as well, although it will not be deployed until sometime between 2025 and 2035. Its mirror could be up to 16 metres wide, making it the largest telescope anywhere in the solar system. The main task of ATLAST will be to look for habitable worlds like Gliese 581g, and to help settle the question of whether there is life in the universe. It seems that telescopes are destined to be the instruments that will help us advance our knowledge of the universe.