IIf you’ve ever been blown away by an image of outer space, it’s a pretty safe bet that it was taken by a spacecraft. This is no surprise when we are talking about the planets of our own solar system, where probes have been returning spectacular close-ups since the 1960s. But what about all those nebulae, star clusters and galaxies that are much further away? For beautiful astrophotography, nothing beats it Nasa’s Hubble Space Telescopeor its great new successor, the James Webb Space Telescope (JWST). They are called space telescopes not only because they observe space, but because they are located in space.
The JWST, for example, is about 930,000 miles (1.5 m kilometers) away – about four times as far as the Moon and far enough that radio signals sent from Earth, traveling at the speed of light, take about five seconds to achieve this. In other words, the JWST is about five light seconds away from Earth. But many of the galaxies it photographed are hundreds of millions, or even billions, of light years away. Clearly, the reason for locating the JWST, and Hubble before it, in space has nothing to do with getting close-up pictures. They are no closer to the objects they view than telescopes here on Earth. So why do astronomers go to all the trouble and expense of putting telescopes into space?
Above the atmosphere
One reason is to get a clearer view. Of course, a space telescope will not be bothered by cloud or haze, but there is another atmospheric effect that is so familiar down here on Earth that we take it for granted. This is the way stars twinkle, rather than appearing as fixed points of light. This happens because the rays of starlight are constantly being shaken around by atmospheric turbulence and this means that no matter how good a telescope is in theory, it can never form a perfectly sharp image when it is located on the Earth’s surface. The idea of launching a telescope into space to circumvent this problem was first proposed in 1946 by American physicist Lyman Spitzer.
Of course, it was long before space travel became a practical reality. It was not until 1990, after much lobbying by astronomers, that Nasa Hubble finally launched into orbit. In terms of design, it is comparable to a medium-sized ground-based telescope, but its unique vantage point in orbit has made it far more powerful than any instrument on Earth’s surface.
The most obvious result was the stream of beautiful full-color images we’re all familiar with, but – perhaps surprisingly – this has very little to do with Hubble’s primary mission. They are basically “outreach” aimed at bringing home the wonders of astronomy to the general public and hopefully inspiring new generations of students to pursue careers in the physical sciences. As important as that is, it’s just a sideline to Hubble’s main goal, which is cutting-edge science. Over the course of three decades, its findings were reported in over 20,000 peer-reviewed scientific papers, many of them without any photographic images. For astronomers, high-precision measurements – for example of light intensity or chemical spectra – are the most important thing and this is what Hubble’s data collection equipment is really designed for.
Ultra-long exposure
The atmosphere has another adverse effect besides blurring astronomical images. The phenomenon of “skyglow”, or the scattering of light within the atmosphere, means it never gets completely dark, limiting the ability of Earth-bound telescopes to see extremely faint objects. In space, however, the background sky is completely black, making it possible to distinguish even the faintest objects with a long enough exposure.
In the case of Hubble, its longest exposure pictures – taken by repeatedly pointing at the same patch of sky and adding up the result over many days – are called “deep field” images because they go much deeper into space than any ground-based telescope can. And because light travels at a finite speed, the deep field images also search further back into the past. Simply put, the further away an object is, the longer ago its light began its journey to us.
This makes Hubble something of a cosmic time machine, with the deepest of its deep-field images drilling through 97% of the universe’s 13.8 billion year lifetime to show us what it looked like just 400m years after the big bang. And this is just the beginning; astronomers hope that the JWST will see even further, all the way back to the formation of the very first stars and galaxies.
Searching for exoplanets
Among the many scientific goals pursued by today’s astronomers, few hold just as much fascination for the public. Probing the birth of the universe, as Hubble’s deep-field images do, is one example – and the search for extraterrestrial life is another. If we are talking about life at a comparable stage of complexity to ourselves, then it is highly unlikely that we will find it here in our own solar system; we really need to look at exoplanets that orbit stars other than our own sun. This appears to be another area where space telescopes have major advantages over ground-based models.
There are several ways to discover new exoplanets, but there is one in particular that can be done on an industrial scale – as long as you use a specially designed space telescope. This is called the “transit method” and takes advantage of the fact that when a planet orbiting a distant star passes across the face of that star from our perspective, then a small part of the star’s light will be blocked out. To detect the planet, astronomers just have to look for that characteristic dip in brightness. So far so good; monitoring the brightness of a star over time—its “light curve”—is a well-established part of astronomy. Traditionally, however, it has been used to look for relatively large and frequent fluctuations, not the small ones that a transiting exoplanet might produce once every few years.
To have any hope of success, we must monitor thousands of light curves simultaneously – and continuously over a period of several years – looking for dips in brightness of just a few parts per million. This is a tremendous engineering challenge that can ultimately only be achieved with specially designed space telescopes. The first of them, Nasa’s Kepler was launched in 2009, and by the end of its working life in 2018, it had discovered at least 2,700 new exoplanets – more than two-thirds of all known at the time. Nasa’s follow-up mission, the Transiting Exoplanet Survey Satelliteis being equally productive.
Beyond the visible spectrum
The wavelengths of light that our eyes can see span a small fraction of the full range of electromagnetic (EM) waves, analogous to a single key in the middle of a piano keyboard. All those “invisible” wavelengths on either side of the visible waveband carry information of potential interest to astronomers, but many of them fail to make it through Earth’s atmosphere (one of the reasons our eyes evolved to see such a narrow spectrum).
In his prophetic newspaper of 1946, Spitzer pointed out that placing a telescope in space would open up those parts of the EM spectrum that are normally obliterated by the atmosphere. Indeed, some of the earliest space telescopes, decades before Hubble, concentrated on the shorter wavelength bands such as ultraviolet and X-ray. At longer wavelengths, infrared is particularly useful for penetrating the dust clouds surrounding star-forming regions and for seeing faint, cool objects such as exoplanets – and it’s no coincidence that the JWST’s coverage extends from the visible band into this part of the spectrum not.
Even wavelengths that do reach the Earth’s surface, such as radio, can sometimes be easier to perceive from space. A good example is the cosmic microwave background – primordial radiation that permeated the universe some 380,000 years after the big bang. It is so faint that Earth-bound telescopes struggle to detect it against competition from human-generated sources of similar wavelength, such as mobile phones and wifi – but space telescopes such as Planck, of the European Space Agency (Esa), did mapped it in detail.
In the future
From the Big Bang to exoplanets, space telescopes have played a key role in building our current understanding of the universe. But for every discovery made, there are new questions to answer – so it’s a safe bet that astronomers will always be clamoring for bigger and better space telescopes. Recent entrants into the field include Esa’s Euclid, launched last year and just embark on the ambitious task of mapping the distribution of billions of galaxies to a distance of 10 billion light years in an attempt to unravel the mysteries of dark energy (the energy in empty space that makes the expanding universe accelerate) and dark matter (which makes up about 85% of the universe’s mass).
