In 2018, NASA plans to launch the James Webb Space Telescope (JWST) [] to replace the Hubble Space Telescope, which has been in orbit since 1990 []. The Hubble has taken images of galaxies in all stages pay someone to write my paper of evolution, helped establish a better estimate of the age of the universe and contributed to the discovery of dark energy. Like the Hubble, the JWST will be able to take images of distant galaxies, but it will be able to see objects that are 10 to 100 times fainter than those imaged by the Hubble []. With its advanced technology, the JWST will be able to image the first stars and galaxies that formed after the Big Bang and help us understand how galaxies evolve.

Looking Into the Past

When a star or galaxy is moving away from us, the color of the star appears to be redder than it would be if it were not moving (see Figure 1). Most galaxies are moving away from us, so their light becomes more red, or “redshifted.” Galaxies that are farther away are traveling away faster, which means their light is redshifted more.  The amount of redshift can give us an indication of how far away objects in the galaxy are from us. In addition, we are really seeing the light those objects emitted long ago: the farther away an object is, the longer the time required for the light to travel to us. The distance light travels in one year is known as a “light year,” so the light that is reaching us now from galaxies billions of light years away was emitted billions of years ago.

 Figure 1. What is redshifting? In the first illustration, the star is not moving relative to the planet, so the wavelength of the light detected on the planet is the same as the wavelength of light emitted from the star. In the second illustration, the star is moving away from the planet. The star is “catching up” to the light that is emitted in the direction it is traveling (though it would have to be moving at the speed of light to actually catch up to it), so the wavelength of the light becomes smaller and therefore bluer. The light emitted by the star in the other direction is redshifted because the star is leaving it behind: it has already traveled away before the next wavelength of light is emitted, so the wavelength seems to be stretched. Shown above is a spectrum of light. Visible light, which we can see, has a wavelength of 400 to 800 nanometers (a nanometer is one billionth of a meter). Infrared light has a longer wavelength. Mid infrared light has a wavelength of greater than 5 micrometers (a micrometer is a millionth of a meter). A human hair is about 100 micrometers in diameter.

The JWST will observe light from early galaxies in the ultraviolet band, which has a wavelength shorter than visible light, that has been redshifted to the mid infrared, which has a wavelength several times larger than visible light (see Figure 1). This degree of redshift corresponds to a distance of 13.5 billion light years, which means that this light was emitted only 100-250 million years after the Big Bang, when the first stars were forming [].

Space telescopes have two main advantages over telescopes on Earth. First, space telescopes have the ability to image a wider spectrum of light than Earth-based telescopes. The atmosphere of the Earth absorbs ultraviolet and infrared radiation, making it difficult to measure signals at these wavelengths from Earth. In fact, most infrared light is absorbed by water vapor in the atmosphere, so only telescopes that are above Earth’s atmosphere, in space, are able to detect infrared light.

The second advantage of space telescopes is that light does not have to pass through the Earth’s atmosphere before reaching the telescope, a journey that can distort the image of stars. Light from a star radiates in every direction, and so the wavefront of the light, which is the surface described by all of the light that was emitted from the star at the same point in time, is a sphere. Stars are so far away that the spherical wavefront is practically flat by the time it reaches the Earth. Think of the wavefront like a balloon that expands as the light travels outward. Just like the surface of a very large balloon seems flat if you are standing on it, the wavefront will also seem flat to us on Earth when we are so far away from the star that produced it. When light has to pass through the atmosphere, which is not uniformly dense due to turbulence, some parts of the wavefront travel faster than others, and the wavefront becomes rippled instead of flat. On Earth, adaptive optics (AO) can be used to compensate for the distortions in the wavefront to some extent. For example, the Keck telescope, which is on Mauna Kea in Hawaii, uses a deformable mirror to fit the shape of the incoming wavefront []. However, AO requires a lot of computational power and moving parts.

Another challenge to interpreting light signals from space is that dust particles, such as those found in nebulas where stars and planets form, can scatter light just like the atmosphere does on Earth, making it difficult to interpret the signals. However, mid infrared light (which is what the JWST is designed to detect) is not readily scattered by dust particles, and so the JWST will be able to look inside dust clouds that are near us and learn about the formation of planets and stars as it is happening.

All On Its Own

Detecting infrared light can be very challenging because the faint signal from distant galaxies can easily be swamped by the infrared radiation given off by objects at high temperatures, including the telescope itself. To decrease the infrared radiation from the telescope, the JWST can be kept at a temperature of 50 Kelvin (or -223 degrees Celsius or -370 degrees Fahrenheit) by using a sunshield to block radiation from the sun, Earth and moon. The JWST will be in orbit 1 million miles from the Earth’s surface in what is known as the second Lagrange point of the sun-Earth system.  One reason that this orbit was chosen is that the sun and Earth are in the same direction from the JWST and can be blocked with one sunshield (see Figure 2) [].

Figure 2. Artist conception of the James Webb Space Telescope in September 2009, with the segmented primary mirror sitting on the multilayered sunshield below. Image courtesy of NASA http://www.jwst.nasa.gov/images2/013526_white_high.jpg.

Another reason the 1 million mile orbit was chosen is that the gravity of the sun and the Earth are in balance at Lagrange points, so the telescope will not need to burn much fuel to stay in place once it is there. It is hoped that the JWST will be in service for at least 5.5 years and hopefully up to 10 years [] without manned missions to repair it. Unlike the Hubble, which is in low Earth orbit and accessible by manned space flight for repairs and upgrades, the JWST will be about 760,000 miles beyond the orbit of the moon (see Figure 3), and there are no missions currently planned to send astronauts there.

Figure 3. Orbits of Hubble Space Telescope (in low Earth orbit), the moon and the James Webb Space Telescope. This figure is not drawn to scale because the distance from Earth to the JWST is almost 1000 times the distance from Earth to the Hubble. The Hubble and the moon both orbit Earth, but the Earth and the JWST both orbit the sun.

The JWST is a very ambitious engineering project because the 21-ft primary mirror and the sunshield must be able to fit into the capsule for launch and then unfurl in space without the assistance of astronauts []. However, once it is in place, the JWST will be able to give us insights into the early formation of galaxies, stars, and planets with better resolution than ever before. Just as the Hubble led to the discovery of dark matter and gamma ray bursts, the JWST might also yield surprising new information about the nature of the universe.

Kristen Sunter is a graduate student at the School of Engineering and Applied Sciences, Harvard University.

References

[] Miriam Kramer, “NASA’s Next Flagship Space Telescope Back on Track … and on Budget”, January 11 2013. http://www.space.com/19235-james-webb-telescope-on-track.html

[] “The Telescope: Hubble Essentials” http:hubblesit.org/the_telescope/hubble_essentials

[] “The James Webb Space Telescope: Frequently Asked Questions” http://www.jwst.nasa.gov/faq.html

[] “Mirror: W. M. Keck Observatory” http://keckobservatory.org/about/mirror

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