The Hubble Space Telescope was launched into Earth’s orbit in 1990 over 25 years ago. The Spitzer Space Telescope, Hubble’s infrared sister, just celebrated its 15th anniversary in space. Multiple X-ray observatories, including the Chandra X-ray Observatory, XMM-Newton, and the Nuclear Spectroscopic Telescope Array (or NuSTAR) are also surveying the sky from their perches in space, high above the ground here on Earth. In the next decade, NASA plans to launch the James Webb Space Telescope, the next generation Hubble and Spitzer, which will orbit the Sun.
Putting a telescope in space has its limitations. For starters, it can’t be too big because it has to fit inside the rocket that launches it. Our ability to repair it is likewise limited should (knock on wood) anything go wrong. And lastly, to state the obvious, it's pretty expensive. So why do we even do it?
The main reason we put telescopes into space is to get around the Earth’s atmosphere so that we can get a clearer view of the planets, stars, and galaxies that we are studying. Our atmosphere acts like a protective blanket letting only some light through while blocking others. Most of the time this is a good thing. No level of SPF could protect us if we were bombarded by high energy X-rays or gamma rays whenever we went outside. But that protection means we are out of luck when it comes to collecting those forms of light for ground-based studies. We can’t exactly ask the atmosphere to make any special exceptions for light we would hope to reach our telescopes.
Wavelengths in the Earth's Atmosphere
Some forms of light—like the aforementioned gamma rays and X-rays, as well as most ultraviolet and infrared light—can’t be observed at all from the ground because they are entirely blocked by the Earth’s atmosphere. Putting telescopes in space, outside of the Earth’s atmosphere, is the only way to see what the universe looks like at these wavelengths.
So why can’t we just stick to observing at wavelengths that aren’t totally blocked by the atmosphere? That would be like trying to put a puzzle together when you’re missing half the pieces. Without observations in the infrared, for example, we wouldn’t understand how stars are formed. Stars spend much of their lives glowing at optical wavelengths which we can observe from the ground, but they are born out of collapsing clouds of gas and dust that emit mostly in the infrared. Stars remain enshrouded in that dust, and thus invisible in the optical, for the earliest stages of their lives.
Our studies of black holes would be severely limited without the ability to observe them at X-ray wavelengths. Black holes so far cannot be observed directly—although astronomers are getting close—hence their “black” moniker. But as black holes gather their surrounding material in the form of a rotating disk, that material emits at X-ray wavelengths as it joins what astronomers call the accretion disk. Observing those X-rays can provide information on the mass and energy of the black hole.
What Is Atmospheric Turbulence?
But space-based telescopes aren’t just for observing the kinds of light our atmosphere totally blocks. Although our atmosphere does allow optical wavelengths to pass through, we still put optical telescopes, like the Hubble, into orbit. That’s because of atmospheric turbulence, or movement in the atmosphere that wiggles the light from distant objects along its path to us, and our telescopes, here on the ground. This wiggling results in a blurred image that is not as crisp as the ones provided by space-based telescopes that take their images from above the clouds. You actually see this blurring in action every night—atmospheric turbulence is what makes stars twinkle.
Ground-based telescopes are also subject to weather. On the ground, we tend to put optical and near-infrared telescopes high up on mountain tops or in deserts where rainfall is minimal so that we can maximize the number of nights with clear skies. (These locations also help avoid light pollution from neighboring cities.) But there will be cloudy or rainy nights when telescopes on the ground are out of luck.
There are also some wavelengths where the atmosphere doesn’t matter at all, and observations can even be done under cloudy or rainy conditions. For example, at longer radio wavelengths, the light is not easily deterred by motions in the atmosphere or by any intercepting clouds. The more limiting factor for the crispness of images from radio telescopes, relative to optical telescopes, tends to be the size of the telescope rather than the atmosphere. So radio telescopes are built where it is convenient to put a very large dish, like in karst limestone sinkholes in the Puerto Rican jungle.
What Is Adaptive Optics?
Astronomers are developing tricks to improve observations here on Earth so that our ground-based observations might better compete with those from space-based facilities, at least in the optical and near-infrared. Adaptive optics, for example, allows astronomers to correct for the distortion or blurring of images by the atmosphere using deformable mirrors. By observing a nearby bright star while also observing the astronomical target of interest, instruments can model the motions of the atmosphere in real time. That model is then used, again in real time, to adjust the shape of the telescope’s mirror to cancel out those atmospheric distortions.
But what if the planet or galaxy you want to observe is not close to a nearby star that can be used for this modeling? Are you out of luck? In that case, astronomers just shoot a giant laser into the sky and observe that instead. So cool!
The Hubble Space Telescope helped us to measure the expansion rate and the age of the universe, to image in detail moons in our solar system, and to observe the catastrophic supernova explosions that end the lives of certain stars. The Spitzer Space Telescope has shown us galaxies from a time when the universe was still very young and told us the most we know about any star system of planets outside of our own. We can observe a lot from the ground, but our space-based telescopes are crucial for unlocking more of the universe’s mysteries.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com. Image courtesy of nasa.gov
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