IIf you've ever been moved by an image of space, it's safe to assume that it was taken by a spacecraft. That's not surprising if we are talking about the planets of our solar system. Since the 1960s, spacecraft have been sending back stunning close-up photos. But what about the nebulae, star clusters, and galaxies far away? When it comes to stunning astrophotography, it's hard to beat NASA's Hubble Space Telescope, or its gigantic new successor, the James Webb Space Telescope (JWST) . It is called a space telescope not only because it observes the universe, but also because of its location. in space.
For example, JWST is approximately 930,000 miles (1.5 meter kilometers) away. This is about four times the distance of the Moon, and so far that it would take a radio signal sent from Earth about five seconds to reach it at the speed of light. In other words, JWST is approximately 5 light seconds from Earth. But many of the galaxies it photographed are hundreds of millions or even billions of light years away. Obviously, the reason for placing JWST and Hubble before it in space has nothing to do with taking close-up photos. They are not as close to the objects they are looking at as telescopes on Earth. So why do astronomers go through the trouble and expense of installing telescopes in space?
beyond the atmosphere
One reason is to get a clearer view. Obviously, space telescopes don't suffer from clouds or haze, but there's another atmospheric effect we take for granted because we're so familiar here on Earth. It's the way the star twinkles, rather than appearing as a steady point of light. This happens because the star's rays of light are constantly swayed by atmospheric turbulence. This means that no matter how good a telescope is in theory, it will never be able to form a perfectly clear image if it is placed on the Earth's surface. The idea of launching telescopes into space to circumvent this problem was first proposed by American physicist Lyman Spitzer in 1946.
Of course, that was long before space travel became a reality. After much lobbying by astronomers, it wasn't until 1990 that NASA launched Hubble into orbit. In terms of design, the telescope is comparable to medium-sized ground-based telescopes, but its unique orbital perspective makes it far more powerful than any instrument on Earth's surface.
The most obvious result was the stream of gorgeous full-color images we're all familiar with, but perhaps surprisingly, these have little to do with Hubble's main mission. They are essentially “outreach” aimed at bringing back the wonders of astronomy to the general public and hopefully inspiring a new generation of students to pursue careers in the physical sciences. While important, this is only a supporting role to Hubble's main purpose: cutting-edge science. Over 30 years, his discoveries have been reported in more than 20,000 peer-reviewed scientific papers, many of which lacked photographic images. For astronomers, high-precision measurements such as light intensity and chemical spectra are of paramount importance, and Hubble's data collection hardware is designed to do just that.
super long exposure
The atmosphere has other negative effects besides blurring astronomical images. The phenomenon of “skylight”, or scattering of light in the atmosphere, means that it is never completely dark, limiting the ability of ground-based telescopes to see very faint objects. But in space, the background sky is completely black, so even the darkest objects can be identified with long enough exposures.
In Hubble's case, the longest-exposure photos, taken by repeatedly pointing to the same part of the sky and adding the results over many days, are called “deep field” because they probe much deeper into space than any terrestrial photo. called an image. You can do it with a telescope. And because light travels at a finite speed, deep-field images can be used to investigate further back into the past. Simply put, the farther away an object is, the longer its light began its journey towards us.
This makes Hubble a kind of cosmic time machine, digging into 97% of the universe's 13.8 billion-year lifespan from the deepest depths of its deep-field images, allowing us to see what it was like just 400 million years after the Big Bang. will show you. And that's just the beginning. Astronomers hope that JWST will extend further back, to the formation of the first stars and galaxies.
Search for exoplanets
Some of the many scientific goals pursued by astronomers today have equally great appeal to the general public. Exploring the birth of the universe, such as Hubble's deep-field images, is one example, and the search for extraterrestrial life is another. If we are talking about life at a stage of complexity comparable to ours, the chances of finding it in our solar system are extremely low. We actually need to observe exoplanets orbiting stars other than our sun. It turns out that this is another area where space telescopes have significant advantages over ground-based models.
There are several ways to discover new exoplanets, but one in particular can be done on an industrial scale, as long as you use specially designed space telescopes. This method, called the “transit method,” takes advantage of the fact that when a planet orbiting a distant star passes over the star's surface from our perspective, a small portion of the star's light is blocked. I am. To detect a planet, astronomers only need to observe a characteristic dip in brightness. So far, so good; monitoring the brightness of stars over time, or “light curves,” is an established part of astronomy. But traditionally, it has been used to look for relatively large and frequent fluctuations, rather than the small fluctuations that a transiting exoplanet might produce once every few years.
Any hope of success requires monitoring thousands of light curves simultaneously, continuously over several years, and looking for drops in brightness of just a few ppm. This is an extremely difficult engineering challenge that can ultimately only be achieved with specially designed space telescopes. The first satellite, NASA's Kepler, launched in 2009 and discovered at least 2,700 new exoplanets by the end of its operational life in 2018. This represented more than two-thirds of all exoplanets known at the time. NASA's next mission, the Transit Exoplanet Survey Satellite, is set to accomplish just as much.
Beyond the visible spectrum
The wavelengths of light that our eyes can see span a small portion of the total range of electromagnetic (EM) waves, equivalent to one key in the center of a piano keyboard. All these “invisible” wavelengths on either side of the visible wavelength band carry information of potential interest to astronomers, but much of it cannot pass through Earth's atmosphere (this is why our eyes can't see it). This is one of the reasons why we evolved to use such a narrow spectrum).
In a prophetic 1946 paper, Spitzer pointed out that placing a telescope in space would reveal parts of the EM spectrum that are normally hidden by the atmosphere. In fact, some of the earliest space telescopes, decades before Hubble, focused on shorter wavelength bands such as ultraviolet and X-rays. At longer wavelengths, infrared light penetrates the dust clouds that surround star-forming regions, making it especially useful for viewing dark, cold objects such as exoplanets. And it's no coincidence that JWST's coverage extends from the visible band into this part of the spectrum.
Even wavelengths that reach the earth's surface, such as radio waves, may be easier to observe from space. A good example is the cosmic microwave background radiation, the primordial radiation that permeated the universe about 380,000 years after the Big Bang. It is so faint that ground-based telescopes have a hard time detecting it, competing with human-generated sources of similar wavelengths, such as cell phones and Wi-Fi. But space telescopes such as the European Space Agency's (Esa) Planck have created detailed maps of it.
future
From the Big Bang to exoplanets, space telescopes have played a key role in shaping our current understanding of the universe. But each discovery raises new questions to answer. Therefore, there is no doubt that astronomers will always be looking for bigger and better space telescopes. Recent entrants to the field include Esa's Euclid, which was launched last year to explore billions of galaxies 10 billion light-years away to unravel the mysteries of dark energy (the energy in the sky). We have just embarked on the ambitious task of mapping the distribution of dark matter (accounting for about 85% of the mass of the universe).
