To take the search for life on planets beyond the solar system to the next level will require a handful of technologies. Some of these have been perfected on ground-based telescopes and just need to be “space-qualified.” Others still require maturation to a level of readiness that will allow them to be reliably integrated on future space-based astronomical missions.
A coronagraph is an instrument attached to a telescope that is capable of suppressing the dazzling light from a star so that objects orbiting close to the star can be detected and studied. A key performance characteristic of a coronagraph is the ratio of the brightness of the planet that can be detected to the brightness of the star it orbits. This ratio is referred to as the contrast ratio. The current state-of-the-art coronagraphs can achieve contrast ratios of 10-7 to 10-9. To detect an Earth-sized planet orbiting a star like our Sun will require achieving a contrast ratio of 10-10 to 10-11 – about a factor of 10-100 times better performance than current coronagraphs. Several designs capable of achieving such ratios have been proposed. Such a high-performance coronagraph will require a telescope with very stable optical performance.
A starshade is a concept for an external spacecraft that contains a circular light shield with specially shaped petals around its circumference. A starshade is designed to be flown in formation with another space telescope. The separation between the telescope and starshade would be tens of thousands of kilometers. The starshade blocks light from a star by casting a shadow on the telescope. This then allows the telescope to observe objects orbiting close to the star that would otherwise be lost in the star’s glare. Like a coronagraph, a key property of the starshade is the contrast ratio that it can provide. Very small (5 – 10 cm) models of starshades have been tested on the ground and have demonstrated that the principle behind their light-blocking capability is valid. These small, ground-based test models have produced contrast ratios of 10-7. As these tests were done out in the open air (not in a vacuum chamber), one expects that much better contrast ratios are achievable. No starshade has yet been flown in space. The technology required to build a starshade suitable for use with an actual space telescope needs further development.
Coronagraph and Starshade Links
- “Overview of Technologies for Direct Optical Imaging of Exoplanets,” M. Levine et al.
- Interim NASA Report on a Coronagraphic Direct Imaging Exoplanet Probe
- Interim NASA Report on a Starshade Direct Imaging Exoplanet Probe
- J. Trauger and W. Traub, “A laboratory demonstration of the capability to image an Earth-like extrasolar planet,” Nature 446, 771–773 (2007)
- D. B. Leviton et al., “White-light demonstration of one hundred parts per billion irradiance suppression in air by new starshade occulters,” Proc. SPIE 6687, 66871B-1–66871B-12(2007)
A telescope capable of obtaining the spectrum of an Earth-like planet around another star similar to our Sun will need to have a primary mirror that is at least 8 meters across. This is because such an Earth-like object would be extremely faint and because the telescope will need very high angular resolution to be able to detect the gap between the star and the planet. Building such large optical systems in space will require lightweight materials for the mirror. This will allow large telescopes that weigh a fraction of what the Hubble Space Telescope weighs to be built. Indeed, some of the mirror materials that would be suitable for building very large space telescopes have already been developed. But they need to be tested at the optical wavelengths that a space telescope will use in order to search for biosignatures on another potentially habitable exoplanet.
Active Wavefront Control Systems
Lightweight, large optical systems will require their mirror shapes and separations to be precisely controlled in order to maintain the image quality expected from a large telescope in space. Active optical systems are commonplace on large ground-based telescopes that continuously update primary and secondary mirror shape and alignment to compensate for wavefront errors induced by variations in temperature, wind, atmospheric blurring and telescope attitude. Space is generally a more stable environment and current space telescopes have adopted passive designs. James Webb Space Telescope (JWST) has a wavefront sensing system onboard but will not be an actively controlled telescope (updates to the mirror actuators will be sent once every two weeks or so). The generations of large optical space telescopes that follow JWST will likely use mirrors that are less dense than those on JWST and will require more active control to keep their less rigid mirrors properly aligned and phased. The main challenge for space applications will be adapting the ground-based systems to a level of reliability that is needed for a multi-year space mission and to ensure the degree of control is adequate for the very high angular resolution and high-stability observations that will be performed. Some of the key technologies to do this were developed as part of the Space Interferometry Mission.