Why Do We Launch Space Telescopes?
Telescopes in space give us a view we literally cannot get from the ground
Astronauts Steven Smith and John Grunsfeld, appear as small figures in this wide-angle photograph from December 1999, taken during a spacewalk to service the Hubble Space Telescope.
On April 24, 1990, NASA and the European Space Agency launched an astronomical revolution. When the Space Shuttle Discovery roared into the sky on that day, it carried the Hubble Space Telescope in its payload bay, and the astronauts aboard deployed it into low-Earth orbit soon thereafter. Hubble is not the largest telescope ever built—in fact, with a 2.4-meter mirror, it’s actually considered by astronomers to be small—but it has a huge advantage over its earthbound siblings: it’s above essentially all of our planet’s atmosphere.
That lofty perch makes Hubble’s views sharper and deeper—and even broader, by allowing the telescope to gather types of light invisible to human eyes and otherwise blocked by Earth’s air. And, after 35 years in orbit, Hubble is still delivering incredible science and cosmic vistas of breathtaking beauty.
Launching telescopes into space takes much more effort and money than building them on the ground, though. Space telescopes also tend to be smaller than ground-based ones; they have to fit into the payload housing of a rocket, limiting their size. That restriction can be minimized by designing an observatory to launch in a folded-up form that then unfurls in space, as with the James Webb Space Telescope (JWST)—but this approach almost inevitably piles on more risk, complexity and cost. Given those considerable obstacles, one might ask whether space telescopes are ever really worth the hassle.
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The short answer is: Yes, of course! For astronomical observations, getting above Earth’s atmosphere brings three very basic but extremely powerful advantages.
The first is that the sky is much darker in space. We tend to think of our atmosphere as being transparent, at least when it’s cloudless. But unwanted light still suffuses Earth’s air, even on the clearest night at the planet’s darkest spot. Light pollution—unneeded illumination cast up into the sky instead of down to the ground—accounts for some of this, but the air also contains sunlight-energized molecules that slowly release this energy as a feeble trickle of visible light. This “airglow” is dim but, even at night, outshines very faint celestial objects, limiting what ground-based telescopes can see. It’s a problem of contrast, like trying to hear a whisper in a crowded restaurant. The quieter the background noise level, the better you can hear faint sounds. It’s the same with the sky: a darker sky allows fainter objects to be seen.
The second advantage to observing from space is that this escapes the inherent unsteadiness of our air. Turbulence in the atmosphere is the reason stars twinkle. That’s anathema to astronomers; the twinkling of a star smears out its light during an observation, blurring small structures together and limiting a ground-based telescope’s effective resolution (that is, how well it can distinguish between two closely spaced objects). This also makes faint objects even dimmer and harder to detect because their light isn’t concentrated into a single spot and is instead diffused. Above the atmosphere, the stars and nebulas and galaxies appear crisp and unwavering, allowing us to capture far greater detail.
The third reason to slip the surly bonds of Earth is that our air is extremely good at shielding us from many wavelengths of light our eyes cannot see. Ultraviolet light has wavelengths shorter than visible light (the kind our eyes detect), and while some of it reaches Earth’s surface from space—enough from the sun, at least, to cause sunburns—a lot of it is instead absorbed by the air. In fact, light with a wavelength shorter than about 0.3 micron is absorbed completely. (That’s a bit shorter than that of violet light, the shortest we can see, at about 0.38 micron.)
So any sufficiently shortwave light—not just ultraviolet, but also even more cell-damaging x-rays and gamma rays—is sopped up by molecules in the air. That’s good for human health but not great for observations of astronomical phenomena that emit light in these regimes.
This happens with longer wavelengths, too. Carbon dioxide and water are excellent absorbers of infrared light, preventing astronomers on the ground from seeing most of those emissions from cosmic objects, too. As we’ve learned with JWST, observations in infrared can show us much about the universe that would otherwise lie beyond our own limited visual range. As just one example, the light from extremely distant galaxies is redshifted by the cosmic expansion into infrared wavelengths, where JWST excels.
In fact, space telescopes that can see in different wavelengths have been crucial for discovering all sorts of surprising celestial objects and events. X-rays were critical in finding the first black holes, whose accretion disks generate high-energy light as the matter within them falls inward. Gamma-ray bursts, immensely powerful explosions, were initially detected via space-based observations. Brown dwarfs (which are essentially failed stars) emit very little visible light but are bright enough in the infrared that we now count them by the thousands in our catalogs.
Observing in these other kinds of light is critical for unveiling important details about the underlying astrophysics of these and other phenomena. It’s only by combining observations across the electromagnetic spectrum that we can truly understand how the universe works.
Still, launching telescopes into space is a lot of trouble and expense. Official work on Hubble started in the 1970s, but delays kept it on the ground for decades. It also cost a lot of money: roughly $19.5 billion total between 1977 and 2021, in today’s dollars. (Operational costs have been about $100 million per year in recent years, but Hubble is facing budget cuts.) JWST was $10 billion before it even launched, and running it adds about $170 million annually to the project’s total price tag.
Compare that with the European Southern Observatory’s Extremely Large Telescope, or ELT, a 39-meter behemoth currently under construction that has an estimated budget of under $2 billion. Building on the ground is simpler, requires less testing and is more fault-tolerant, allowing much more bang for the buck.
The capabilities of ground-based versus space-based telescopes are different, however. In general, big earthbound telescopes can collect a lot of light and see faint structures, but except for the ELT, they don’t have the resolution of their space-based counterparts and can’t see light outside the transparency window of our planet’s air. Also, not every observation needs to be done from space; many can be done just fine from the ground, freeing up time on the more expensive and tightly scheduled space telescopes.
Pitting these two kinds of facilities against each other—why have one when we can have the other?—is the wrong way to think about this. They don’t compete; they complement. Together they provide a much clearer view of the cosmos than either can give by itself. Astronomy needs both.
