The coming generations of space telescopes are expected to be too large to fit in a single payload.
The primary mirror of the James Webb Space Telescope (JWST) is seen in the above, with a human, and the mirror of the Hubble Space Telescope (HST) on the side for scale. The mirror is 6.5 meters in diameter, an improvement over the 2.4 meter mirror of the Hubble Space Telescope. The JWST is designed to be powerful enough to resolve a single bumblebee on the face of the Moon. The JWST is scheduled for launch in 2020. It will be put into orbit as a single payload, with the components folding up. Towards the left side of the image, the gap between the hexagonal components, shows one of these folds. When launched, the JWST would be the largest astronomical instrument in space. However, engineers around the world are already coming up with ways to peer deeper into the universe, and at better resolutions.
For future space telescopes, after the JWST, with a mirrors measuring anywhere between 20 to 100 meters in diameter, astronomers would require to use a drastically different approach to deploy the telescope. Deploying such a telescope through the conventional approach of folding up the telescope in a single payload would be expensive, and would require much bigger rockets than any that currently exist. It would be more economical to launch such a telescope in parts, with modules being sent up over multiple rocket launches. Then the modules would autonomously assemble themselves in Earth orbit.
The major component of a space telescope is the primary mirror. This is also known as the aperture. In the larger space telescopes, this is invariably made up of multiple mirrors assembled together. The larger the primary mirror is, the better the telescope is. The parts of the mirror for future telescopes will have to be send up to space over multiple launches. The materials for the mirror would have to be lightweight, to keep the cost of the launches low. The modular design should allow for easy assembly in space, and a mechanism for locking the various parts together. The components would require some kind of propulsion mechanism to arrive at the rendezvous point, after the launch. Then there has to be a way to assemble these components together in space. The parts of the mirror can navigate to each other and assemble themselves. Another option is that humans or robots in space can intervene during the assembly process. In all, it is preferable if the entire assembly process can be automated. There is also a requirement of deploying a Sun shield of sorts. This is a preferably thin screen that can protect the components from solar activity, and keep the temperature of the instruments low, which are ideal for the health of the telescopes.
NASA has just approved a feasibility study for one such telescope. The proposal is that of a modular active self-assembling space telescope swarm. Dmitry Savransky from Cornell University and 15 other engineers have been tasked with developing a concept for a telescope that could autonomously assemble in space, and require a number of relatively cheap launches involving small payloads. The idea is to use a number of hexagonal modules, each 1 meter in diameter. Each module is autonomous, and is loaded with a number of subsystems such as actuators, and an edge to edge mirror assembly. Each of the payloads would be launched independently, as and when the opportunity arises, piggybacking as secondary payloads of the main launches. Once in space, each hexagonal module would use a solar sail to navigate to the Sun-Earth Lagrange point 2 (L2). The solar sails also double up as a sunshield during assembly, protecting the sensitive instruments within the hexagons from being damaged by solar radiation. At L2, the telescope will assemble itself, without any additional intervention from humans. The combined gravity from the Earth and the Sun is stable in this location, which is where the space telescope can be parked. The Sun-Earth L2 point is the chosen destination for space telescopes, and is where the JWST will be deployed as well.
The concept is targeted for the Large Ultraviolet/Optical/Infrared Surveyor, a mission on the NASA roadmap for the 2020s. The Phase I of the study has been greenlit, where the details of how the components will travel to the target location will be worked out through simulations. This will in turn be used to figure out the required surface area of the solar sails to realise the mission. The exact mechanism of assembling the mirrors will also be validated. If phase I of the study is successful, the team can apply for phase II funding. This study is for a space telescope with an aperture of 30 meters. However, there are even bigger telescopes being conceptualised, which would also require autonomous assembly in space.
Researchers from the University of Surrey and Caltech have proposed a telescope with an aperture of 100 meters. The assembly, is however more complicated. Apart from the modules, there would be additional external intervention in the form of a robot dedicated to the assembly process. The idea is to launch 300 support truss modules. These will provide a frame to support the main telescope. A six armed hexbot will first assemble the frame, and then crawl across it in the process of assembling the mirrors. Like a mountain climber, the robot would use three arms to anchor itself to the frame, while moving only one arm at a time to move. The two additional arms will be dedicated to the assembly process. The power for the robot will be drawn from the same solar panels used to power the telescope itself. The main mirror is made up of very thin, lightweight material. The materials have the ability to deform, to fit the shape of the aperture. The rest of the space observatory would be stretched out in a staggered manner. 400 meters from the main mirror, would be an optics and instrumentation unit. 400 meters beyond that would be a control unit. At this location, a 20 meter wide sun shield would provide protection to the components of the observatory, and keep the instruments from overheating.
The Autonomous Assembly of a Reconfigurable Space Telescope (AAReST) mission, is a collaboration between Caltech, the University of Surrey, NASA/JPL and the Indian Institute of Space Science. It is a technology demonstration mission, that will show that in principle, space telescopes can be assembled in orbit. The components are a number of small nanosatellites known as CubeSats. The navigation systems of the various components use the COTS PrimeSense LIDAR, which is also used in the Microsoft Kinect. The design consist of a main mirror component, and six satellite mirrors that can autonomously undock and redock to provide various configurations to the telescope.
Self assembling space telescopes are still in the nascent stage, but will provide us with unprecedented access to the skies. These kinds of telescopes will fundamentally change how humanity looks at space. The telescopes will also be useful for dark matter and dark energy research. Telescopes with such large apertures mean that we will be able to directly image the surface details of exoplanets, something that even the JWST will not be able to do.