A Balancing Act in Space
How the James Webb Space Telescope will observe the early universe from its home at the second Lagrange point
The James Webb Space Telescope, often referred to as “Webb”, is a space-based observatory, named after the second administrator of NASA, that aims to expand upon the discoveries made by the Hubble Space Telescope. It will have a much larger primary mirror than Hubble, allowing it to collect more light, requiring an unfolding mirror made of hexagonal segments that can fit into the rocket used to launch the telescope. Webb is being developed by NASA, with contributions from the European and Canadian Space Agencies.
Unlike Hubble, Webb will not orbit the Earth, and will instead orbit the Sun at the second Lagrange point of the Sun-Earth system, known as L2. Hubble is unusual in that it was designed to be accessible for servicing, whereas Webb will be far beyond the reach of manned vehicles. The complications due to increase in mass, cost and complexity of making Webb serviceable were deemed to outweigh the possible benefits of better reliability and a longer life. L2 is one of the points, found by 18th-century mathematician Joseph-Louis Lagrange, that solves the “three-body problem”. At each of the five Lagrange points, a small object maintains a stable or nearly stable position relative to two larger orbiting bodies (in this case the Sun and the Earth). At any other location, the small object would be pulled by gravity into an orbit around one of the larger bodies.
L2 is situated on the line between the Sun and the Earth, about 1.5 million kilometres beyond the Earth. An object at a greater distance from the Sun than the Earth would normally take longer to complete an orbit, however, at L2, the gravitational pull from the Earth decreases the orbital period so that it is able to keep up.
L4 and L5 are stable, so objects such as asteroids orbit these points with no assistance, whereas L1, L2 and L3 are only “metastable”, so require assistance, such as rocket thrust, to prevent them from drifting into their own orbits. Instead of remaining exactly at L2, Webb will use rocket thrust to stay in orbit around it. This will keep it out of the shadows of the Moon and Earth, allowing it to operate unimpeded 24/7, unlike Hubble which goes in and out of the Earth’s shadow every 90 minutes. The telescope will take around 30 days after launch to reach the start of its orbit at the Lagrange point.
Situating the telescope at L2 makes it easier for us to communicate with it, since it will always remain in the same place relative to the Earth. We can then use the Deep Space Network (DSN), to uplink command sequences and downlink data. The DSN consists of antennas situated in Spain, Australia and California which allow for continuous communication, as one site can pick up a signal from a telescope that is just about to dip below the horizon at the previous site.
Webb is designed to observe light primarily in the infrared, which falls just outside the visible spectrum and can sometimes be felt as heat. This will allow the telescope to see what is going on inside dust clouds that are opaque to visible light, and to view galaxies that formed in the very early universe. The oldest objects that we can observe are those that are the furthest away from us, as it has taken longer for their light to reach us, and they are also moving the fastest away from us. This causes the light emitted by them to be “redshifted” to longer wavelengths, which is why an infrared telescope is required to study early star and galaxy formation.
In order to observe very faint infrared signals from distant objects, the telescope must be shielded from hot sources, which include the Sun, Earth and Moon. When Webb is positioned at L2, the heat from these bodies all comes from the same direction, and can be blocked using a sunshield, which will allow the telescope to cool to temperatures below -223ºC. This means that the hot side of the shield will be almost hot enough to boil water, whereas the cold side will be cool enough to freeze nitrogen. The sunshield will even protect the telescope from the heat of its own spacecraft bus.
Webb will be launched from French Guiana in 2021 on a European Space Agency rocket. It has a predicted lifetime of 5–10 years, limited by the fuel required to maintain the orbit around L2 and the damage to the instruments caused by harsh conditions. The beryllium mirrors have been designed and tested to withstand impacts from micrometeorites.
The launch of the telescope was originally planned for 2007, but numerous holdups and budget problems, as well as a significant redesign in 2005, have delayed the launch. The cost of the telescope was initially estimated to be $1.6 billion, but has increased rapidly through the years and is now projected to be over $10 billion by the launch date, requiring an increase in the mission’s cost cap from Congress.
Despite these huge setbacks, Webb is set to provide thousands of astronomers around the world with groundbreaking technology and the possibility of answering questions about the origins of stars and galaxies in our universe. All of this is made possible by the properties of the telescope’s future home at the second Lagrange point.
