L19. Telescopes in Space

Unit 4 · Light and Telescopes

R-report

L19. Telescopes in Space

A telescope in orbit costs a hundred times more than one on the ground and usually cannot be repaired if something breaks. Astronomers build them anyway. The reason is that some things simply cannot be seen from the bottom of an atmosphere.

What the atmosphere blocks

Earth’s atmosphere is not equally transparent to all light. It has windows and walls. - Visible light gets through. That is presumably why our eyes evolved to use it. - Radio waves get through, which is why radio telescopes work on the ground. - Infrared is mostly absorbed, especially by water vapour — one reason infrared observatories sit in very dry deserts, and why the best results still come from space. - Ultraviolet is almost entirely blocked, largely by the ozone layer. - X-rays and gamma rays are completely blocked. This is excellent news for life, since ultraviolet and X-rays would be harmful. It is a serious problem for astronomy: most of the spectrum never reaches the ground at all. So an entire category of objects — the hottest gas, the most violent explosions, matter falling into black holes — was effectively invisible until instruments could be placed above the air. Ultraviolet and X-ray astronomy did not exist as fields until the space age.

Steady, dark, and cold

Getting above the atmosphere solves several problems at once. - No turbulence, so no twinkling. Images stay sharp for as long as the telescope stares. - No absorption, so every wavelength is available. - No weather, no daylight interruption, and no light pollution. - For infrared, no warm air glowing in the very band being measured. Different missions choose different orbits. Hubble orbits about 540 km up, close enough that astronauts serviced it five times — a rare luxury. The James Webb Space Telescope instead sits roughly 1.5 million kilometres from Earth, at a stable point where it can keep the Sun, Earth, and Moon all behind a single sunshield. That sunshield is the point. Webb observes infrared, so its mirror must stay near -230 °C. The five-layer shield, about the size of a tennis court, blocks solar heat and lets the instruments cool passively. The trade-off is that no repair mission can reach it.

What space telescopes have shown us

The results have reshaped astronomy repeatedly. - Hubble measured the expansion rate of the universe far more precisely than before, sharpening estimates of its age to about 13.8 billion years. - Its deep field images stared at seemingly empty patches of sky for days and found thousands of galaxies in each one. - Kepler, a space telescope dedicated to watching stars dim, discovered thousands of planets around other stars. - X-ray observatories mapped gas at millions of degrees in galaxy clusters, providing key evidence for dark matter. - Webb has observed some of the earliest galaxies known, seen as they were within a few hundred million years of the Big Bang. Ground-based astronomy has not been replaced. Enormous mirrors, long observing campaigns, and adaptive optics keep it essential, and it is far cheaper. The two approaches are complementary: space for the blocked wavelengths and the sharpest steady images, ground for size, flexibility, and cost.

Putting the investigation together

Space telescopes exist because the atmosphere blocks most of the spectrum and blurs what it does let through. Above the air, observations are sharp, uninterrupted, and open to every wavelength — at the price of enormous cost and almost no possibility of repair.

The relationship between the two approaches is worth noting. Space telescopes are not simply better; they are better at specific things. Deciding which instrument suits which question, given real limits on money and engineering, is as much a part of astronomy as the observing itself.