Reinventing the mirror for the CubeSat telescope
The CubeSat programme continues to strive for affordable access to space with new nanotube optics. Ellis Davies reports.
In the development of a lightweight telescope for CubeSat – a miniaturised satellite for space research – NASA scientists may be the first to use mirrors made of carbon nanotubes and epoxy resin. The addition of the lightweight telescopes could improve the small craft‘s capabilities as an exploratory tool.
The development of this technology was supported by a NASA Small Business Innovative Research grant in 2015.
Most telescope mirrors are made from glass or aluminium, which lack many of the useful properties of the carbon nanotube optic. The team claims the optic is lightweight, highly stable, easily reproducible, and does not require polishing. The density of a nanotube optic is 1.2 grams per cm2 (g/cc), compared with glass at ~2.4g/cc and aluminium at 2.7g/cc.
Carbon nanotube resin nanocomposites belong to a class known as 'smart materials', meaning properties such as stiffness, shape and viscosity can be manipulated. The nanotubes also have electrical and heat conducting properties, which Peter Chen, research scientist at the NASA Goddard Space Flight Center, explained were both used in the optics, ‘The electrical conductivity is used in the process of making a "smart mirror telescope" – telescopes that bend or deform the optical surface so as to counteract the effects of gravity, thermal distortion, and wind buffeting. The thermal conductivity makes the optics and structures heat up and cool down quickly and uniformly, minimising the effects of thermal distortion and thermal gradients.’
To create a nanotube optic the carbon nanotubes are dispersed by ultrasound and then mixed thoroughly with the resin. The mixture is then poured onto a mandrel or mould to cast a mirror. The mould is then heated to cure and harden the resin, and left to set. Once cooled, the mirror is coated with a reflective material of aluminium and silicon dioxide. Chen explained, ‘The process is simple and cost-effective in that it does not require a high temperature or high pressure, is non-toxic, and does not need specialised equipment such as inert gases or isolation chambers.’ This manufacturing process will enable the CubeSat crafts to be equipped with identical optics and various detectors to carry out a range of experiments.
Using a similar process, complete telescopes can be produced. ‘Imagine a normal Cassegrain telescope system – it has a primary mirror, a secondary mirror, and structures to hold the two mirrors in place and in alignment’, Chen said. The converse of this system, a structure that is solid with air inside the telescope, can be put inside a container, and the container filled with the carbon nanotube resin and left to solidify. The result is a complete telescope assembly made by replication. 'This could lead to savings in both time and cost. Carbon nanotubes lend themselves to smart optics because of their electrical properties. By applying an electric field to the resin mixture before curing, a formation of carbon nanotube chains and networks can be formed. This allows the shape of the optical surface to be changed by applying power to the mirror.
‘We are focusing on very small, light systems that can be flown on CubeSats, which require the lightest possible payload’ said Theodor Kostiuk, research scientist and colleague to Chen. ‘Telescopes made from carbon nanotube composite offer the best approach with the required physical and optical characteristics.’ Kostiuk outlined the uses of the material, saying, ‘This material technology allows replication of multiple identical telescopes at low cost, which can be used on CubeSats in array or constellation formats to make unique studies of the Earth, the Moon and nearby planets and solar system objects.’
Several CubeSat missions have already been proposed, and the team is currently applying to NASA to develop the mirrors and telescopes for the projects.