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If you have ever been amazed by a picture of outer space, chances are it was captured by a spacecraft. This is especially true for the planets in our solar system, as probes have been providing incredible close-up images since the 1960s. However, what about the distant nebulae, star clusters, and galaxies? For breathtaking astrophotography, there is nothing that can compare to Nasa’s Hubble Space Telescope or its newer and larger counterpart, the James Webb Space Telescope (JWST). These are referred to as space telescopes not only because they observe space, but also because they are positioned in space.

The JWST, for instance, is situated at a distance of approximately 930,000 miles (1.5m kilometres) from Earth – roughly four times the distance of the moon. This means that it takes about five seconds for radio signals to travel from Earth to the JWST, as they move at the speed of light. In simpler terms, the JWST is five light seconds away from Earth. However, the galaxies captured by the JWST are typically hundreds of millions or even billions of light years away. This shows that the decision to place the JWST, and previously Hubble, in space was not motivated by the desire for close-up images. These telescopes are not any closer to the objects they observe than those on Earth. So why do astronomers go through the trouble and cost of sending telescopes into space?

Above the atmosphere

One reason for using a space telescope is to overcome atmospheric obstacles. Unlike telescopes on Earth, which are affected by clouds and haze, a space telescope is not hindered by these conditions. However, another common atmospheric effect that we often overlook is the twinkling of stars. This is caused by the constant movement of starlight due to atmospheric turbulence. Therefore, even the most advanced telescope on Earth cannot produce a completely clear image. In 1946, American physicist Lyman Spitzer proposed the idea of launching a telescope into space to avoid this issue.

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That, of course, was long before space travel became a practical reality. It wasn’t until 1990, after much lobbying by astronomers, that Nasa finally launched Hubble into orbit. In terms of design, it’s comparable to a medium-sized ground-based telescope, but its unique vantage point in orbit made it far more powerful than any instrument on the Earth’s surface.

The primary result of Hubble’s mission has been the production of visually stunning, full-color images that are well-known to the public. However, these images are not the main focus of Hubble’s purpose. They serve as a means of outreach to bring the wonders of astronomy to the general population and inspire future generations to pursue careers in science. While this is important, it is not the main goal of Hubble. Its main purpose is to conduct cutting-edge scientific research. Over 30 years, Hubble has produced more than 20,000 peer-reviewed scientific papers, many of which do not include any photographic images. For astronomers, precise measurements, such as light intensity and chemical spectra, are the most crucial aspect, and Hubble’s data-collecting equipment is specifically designed for this purpose.

Ultra-long exposures

Aside from causing blurry images of astronomical objects, the atmosphere also has another negative impact known as “skyglow”. This refers to the scattering of light within the atmosphere, which prevents it from ever becoming completely dark. As a result, telescopes on Earth are limited in their ability to detect extremely faint objects. However, in space, the background sky is completely black, allowing for the possibility of observing even the faintest objects with a lengthy exposure.

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For Hubble, its extended exposure images are referred to as “deep field” pictures. This is achieved by repeatedly focusing on the same area of the sky and combining the results over several days. These images provide a deeper look into the universe compared to what can be seen with telescopes on the ground. Additionally, due to the limited speed at which light travels, the deep field images also offer a glimpse farther back in time. In other words, the further an object is from us, the longer it took for its light to reach us.

This makes Hubble something of a cosmic time machine, with the deepest of its deep field images drilling down through 97% of the universe’s 13.8bn-year lifetime to show us how it looked just 400m years after the big bang. And that’s just the beginning; astronomers hope that the JWST will see even farther, all the way back to the formation of the very first stars and galaxies.

Searching for exoplanets

One of the main objectives of modern astronomers is to understand the universe’s origins, which can also captivate the general public. The exploration of the universe’s beginnings, as seen in Hubble’s deep field images, is one example, and the quest to find extraterrestrial life is another. However, if we are searching for life that is similar in complexity to our own, it is unlikely that we will find it within our own solar system. In order to do so, we must look at planets outside of our own solar system that orbit other stars. This is an area where space telescopes have a significant advantage over telescopes on Earth.

There are various methods for identifying new exoplanets, but one stands out as being able to be used on a large scale – as long as a specialized space telescope is utilized. This method, known as the “transit method”, takes advantage of the fact that when a planet orbiting a distant star passes between us and the star, it will block out a small portion of the star’s light. To spot the planet, astronomers simply need to observe this dip in brightness. While monitoring a star’s brightness over time, also known as its “light curve”, is a common practice in astronomy, it has traditionally been used to detect significant and frequent fluctuations, rather than the subtle changes that may occur when an exoplanet transits every few years.

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In order to be successful, we must constantly monitor thousands of light curves for several years to detect small decreases in brightness. This is a difficult engineering task that requires specialized space telescopes. The first telescope, Kepler, was launched in 2009 and found over 2,700 new exoplanets before its mission ended in 2018. The next mission, the Transiting Exoplanet Survey Satellite, is expected to have similar success.

, we cannot see

We are unable to perceive outside of the visible range of light.

Our eyes are only able to detect a small portion of the entire range of electromagnetic waves, similar to a single key on a piano. The wavelengths that fall outside of this visible spectrum contain valuable information for astronomers, but most of them are unable to penetrate the Earth’s atmosphere. This limited range is one of the reasons why our eyes have evolved to only use a narrow spectrum.

In his forward-thinking document from 1946, Spitzer noted that positioning a telescope in outer space would provide access to the parts of the electromagnetic spectrum typically blocked by the Earth’s atmosphere. In fact, many of the initial space telescopes, predating Hubble, focused on shorter wavelengths like ultraviolet and X-ray. For longer wavelengths, infrared is especially advantageous for penetrating the dust clouds around areas where stars are forming and detecting dim, cool objects like exoplanets – and it’s not by chance that the JWST’s range extends from the visible light band into this portion of the spectrum.

Some wavelengths, like radio waves, can be more easily observed from space when they reach the Earth’s surface. One example is the cosmic microwave background, which was present in the universe around 380,000 years after the big bang. This type of radiation is very weak and can be difficult for telescopes on Earth to detect due to interference from human-made sources with similar wavelengths, like mobile phones and wifi. However, space telescopes like Planck, operated by the European Space Agency (ESA), have been able to create detailed maps of this background radiation.

The future

Space telescopes have been integral in expanding our knowledge of the universe, from the beginning with the big bang to the discovery of exoplanets. However, with each new discovery, more questions arise, leading astronomers to continuously push for more advanced space telescopes. One recent addition is Esa’s Euclid, which was launched last year and is currently undertaking the ambitious task of creating a map of billions of galaxies up to a distance of 10 billion light years. This mission aims to unravel the mysteries of dark energy, which causes the universe to accelerate, and dark matter, which makes up 85% of the universe’s mass.

Source: theguardian.com