This artist’s conception of the James Webb Space Telescope in space shows all its major elements fully deployed. The telescope was folded to fit into its launch vehicle, and then was slowly unfolded over the course of two weeks after launch | Photo courtesy: NASA GSFC/CIL/Adriana Manrique GutierrezThis artist’s conception of the James Webb Space Telescope in space shows all its major elements fully deployed. The telescope was folded to fit into its launch vehicle, and then was slowly unfolded over the course of two weeks after launch | Photo courtesy: NASA GSFC/CIL/Adriana Manrique GutierrezAs with many others over the world, I was glued to my computer screen on Christmas Day as the massive Ariane 5 launched the James Webb Space Telescope (JWST), which began its million-mile journey into deep space to unlock the secrets of our universe. In a sense, we have been waiting for Webb ever since Galileo pointed his telescope at the sky and began modern astronomy. Since then, larger and larger ground-based telescopes were built, many at the top of mountains where the skies are dark and clear but were limited to visible light, just like our own eyes but much fainter.
Radio telescopes opened up a new frontier in astronomy just before WW2, but it was not until the Americans launched telescopes on captured German V2 rockets that observations in other wavelengths from gamma-rays to the infrared became possible. Not only did space observations reveal different aspects of astronomical objects, but they were able to resolve details unobservable from the ground because of the stifling blanket of the Earth’s atmosphere. The start of the satellite era with Sputnik in 1957 allowed astronomers to build more extensive and sophisticated instruments culminating in the Great Observatories, of which the best known is the Hubble Space Telescope (HST).
Hubble Space Telescope | Photo courtesy: Special arrangement
Hubble revolutionised every branch of astronomy and brought public attention to science through its beautiful pictures and outreach. In fact, Hubble had made such an impact that, in 1996, only five years after the launch of Hubble, the astronomical community, with the support of NASA, came together to plan for its successor, with even more ambitious scientific goals. This mission, then called the Next Generation Space Telescope (NGST), had a specific theme, that of “Origins”, answering fundamental questions about where we come from in an ever evolving universe.
Why is Webb called a 'Time Machine'?
Unlike Hubble, Webb would be a purely infrared (the region of the electromagnetic spectrum that is longward of red light) instrument. Understanding the science goals will clarify why the infrared was chosen, which, in turn, dictated the instrument and mission characteristics.
The first of these themes was the search for the origins of the universe. The universe began with a Big Bang 13.8 billion years ago, and the first stars and galaxies were formed only a few hundred million years later. We know that one of the stars within our own galaxy has an age of 13.4 billion years, suggesting that the Milky Way, itself, was formed not long after the Big Bang.
The young Milky Way would have looked much different from its current form and, although we cannot observe that baby galaxy, Webb will observe other galaxies at a distance of as far as 13.5 billion light-years from us. It has taken the light from those galaxies 13.5 billion years to reach us, and we see those galaxies, perhaps similar to our own, as they were in their youth. This is what is meant when Webb is called a “time machine”: we are looking into the past, into a time when galaxies and the stars therein were just being formed.
Observations of galaxies at different distances will reveal their nature at varying stages of their evolution. They will demonstrate how galaxies (and stars) change over their lifetime and how they grow into the magnificent nebulae we see in the sky. Why do we have to go to the infrared to observe these early galaxies? We know that the Universe is expanding with distant objects moving ever more rapidly away from us. The light from those distant objects will be redshifted (shifted to longer wavelengths) into the infrared, which can only be observed by a space telescope.
The Origin of Stars
From the origins of the universe, we shift to the second objective: the origin of stars. Stars are born in huge molecular clouds, such as the spectacular Pillars of Creation in the Eagle Nebula, a nursery for new stars and stellar systems. However, the stars are hidden from visible light instruments by the very gas and dust that makes for such stunning pictures. As we know from watching a sunset on the Earth, particularly on dusty days, the dust is transparent in the infrared, and JWST will be able to see the newly formed stars through the intervening dust. The Sun and the Solar System formed in a similar manner, and Webb will be able to observe the birth of new planetary systems in these stellar nurseries. Of course, time scales in astronomy are so long that we cannot actually watch the formation of planets. What Webb will do is survey a multitude of young stellar systems in different phases of their evolution. By putting together all the different systems, we will be able to understand where planetary systems, like our own, come from.
Unveiling Planetary Secrets
Exoplanets, planets around other stars, had only just been discovered when NGST was conceived and was not part of the original scientific goals but are now one of the most exciting and active areas of astronomy. Close to 5000 exoplanets have been confirmed, of all shapes and sizes, upending our theories of planetary formation. Fortunately, the very qualities that make Webb so good for studying the early universe make it well-suited for understanding the nature of exoplanets.
In particular, Webb will measure the composition of planetary atmospheres by measuring the absorption and emission of infrared light by molecules in the planetary atmosphere. The composition of the atmosphere may tell us whether life is possible, the ultimate goal of exoplanet research. There has been significant interest in Mars as a possible habitat for life, and Webb will even observe the Martian looking for organic molecules.
Although these are the primary science goals for Webb – those goals that determined the basic operating parameters for the telescope, astronomers are always excited about new tools to solve their own scientific problems. The agencies themselves welcome this input both because science should be open to all and, more selfishly, because new ideas will inevitably come when a worldwide audience community is excited about a project.
Astronomers all over the world submitted proposals to the Space Telescope Science Institute (STScI) in Baltimore that were referred in a double-blind procedure. Science has been beset by gender biases and a double-blind process – where the referees did not know who the proposers were and vice versa – ensured that only the best science was considered without being prejudiced by such irrelevant parameters as the gender or reputation of the proposer. It may turn out that the most important results will be in the most unexpected areas.
It was decided, based on the science goals, that Webb was to be an infrared telescope. The infrared is a difficult wavelength regime to operate in because all objects radiate infrared, as we know from the heat-sensing infrared goggles used by heroes (and villains) in action movies. Infrared instruments have to be kept cold, and it was decided that Webb would be sent far from the Earth to L2 (Lagrangian point 2), 1.5 million km away. The combination of the Sun’s gravity and the Earth’s gravity keeps the spacecraft in a fixed location relative to the Earth and Sun, with only minimal use of rocket fuel. Even in L2, solar radiation would heat the telescope to unacceptable levels, and Webb uses a tennis court-sized sunshield with five layers to block light from the Sun and, happily, the Earth and Moon, as well. The operating temperature of the instruments is less than 50K, 300K cooler than the sun-facing side.
As befitting the successor to Hubble, Webb’s mirror was chosen to be 6.5m in diameter. This would have been large even for a ground-based telescope and a conventional mirror would have been too large and too heavy for any rocket to carry out to space. Instead, they built the mirror in 18 segments which folded up to fit into the rocket and were then expanded and focused in space using actuators. Each segment was made out of beryllium which was lightweight yet strong and then coated with gold to be highly reflecting in the infrared. There are four science instruments on JWST, built by different institutions and countries, with a combination of imaging and spectroscopy. These are the instruments that will deliver the actual science, as determined by proposals from the international community.
It has been a long journey from conception to space flight. The final cost of Webb has been 10 billion dollars with a development time of two decades. Three space agencies – NASA, ESA, and CSA – have been involved with hundreds of scientists and engineers over the mission's lifetime.
Webb is on its way to L2 and has been spectacularly successful to date with a flawless launch and an equally flawless deployment of the sunshield and the mirror. We must now wait until it reaches L2 at the end of the month and completes a six month cooling period before science operations begin in June. We know much of the science that will result from Webb's journey, but the true excitement will be in the science that we could not have anticipated.
Dr. Jayant Murthy used to be a research scientist with Johns Hopkins University, Maryland, USA. He retired as a Senior Professor at the Indian Institute of Astrophysics (IIA), Bangalore. He received his PhD in Physics from the Dept. of Physics and Astronomy, Johns Hopkins University. His research revolves around space missions, for instance payload on the Space Shuttle (TAUVEX and UVIT missions), interstellar dust and diffuse radiation field studies, SETI (Search for Extra-Terrestrial Intelligence) project amongst others. Dr. Jayant is one of those prominent scientists of the country who are popularising science amongst the non-science oriented citizens to bring in awareness around how science can be beneficial to humanity.