Advanced Optics Could Help Us Find Earth 2.0

NASA has long been interested in building bigger and better space telescopes. Its Institute for Advanced Concepts (NIAC) has funded several methods for building and deploying novel types of telescopes for various purposes. Back in 2019, one of the projects they funded was the Dual Use Exoplanet Telescope (DUET), which would use an advanced form of optics to track down a potential Earth 2.0.

So far, the largest telescope launched into space is JWST, with a 6.5m primary mirror. However, even with that big of a mirror, it is difficult to differentiate exoplanets from their stars, which may be only a few milliarcseconds away from each other. Larger telescopes on the ground have slightly higher resolutions, but they suffer from other limitations, such as atmospheric distortion and cloud cover.

A larger telescope in space would solve many of those problems, but launching one that is simply a larger version of JWST is prohibitively expensive or just plain prohibited, depending on whether it would fit in a rocket fairing. Even Starship and other next-generation launch systems couldn’t fit a 10 m assembled primary mirror.

So, researchers have started to turn to alternative optical techniques that could solve this problem. One commonly known optical phenomenon is diffraction. The best-known example is the famous “slit” experiment that many kids perform in physics class. Light bends when going around an edge, and engineers can take that principle, scale it up, and build something that bends the light from far-away stars.

That is the underlying principle of DUET – it uses a technique called primary objective grating (POG) to focus specific wavelengths that might be of interest – for example, that wavelength that would show oxygen in an exoplanet’s atmosphere. In particular, DUET uses a type of POG that results in a circular spectrogram. Although this idea is novel in astronomy, it has been used in other fields. Tom Ditto, the PI on the project, was originally an artist before converting into a technologist focusing on optics.

With the NIAC Phase I funding, Ditto and his team developed a bench-top experiment that proved the technology underlying DUET. It consists of a slatted first data collection stage that focuses the light from a star of interest on a secondary stage and, thereby, a collector, which captures the data that could be translated into a circular spectrograph. 

In particular, the researchers were interested in UV light, as Earth would appear like a bright candle from far away, at least compared to light in other spectra. They tested a violet laser on their bench setup and analyzed the resulting circular spectrograph. It showed great promise for detecting something with a spectrum like Earth’s from very far away.

But there are still hurdles to overcome. One of the bigger concerns was the efficiency of the grating structure used in the experiments. Its 20% efficiency would make it barely feasible to detect the kind of faint objects the telescope is designed for. The deployment mechanism for the grating, which requires the assistance of additional spacecraft separate from the telescope itself, would also be a challenge.

That’s where the experiment stands, as NASA has not elected to support the project with a Phase II grant so far. Given the history of exoplanet discovery, it’s only a matter of time before we find Earth 2.0. What technology we will use to do so is up in the air.

Learn more:
Ditto et al. – DUET The Dual Use Exoplanet Telescope
UT – Building Space Telescopes… In Space
UT – Future Space Telescopes Could be 100 Meters Across, Constructed in Space, and Then Bent Into a Precise Shape
UT – Using Smart Materials To Deploy A Dark Age Explorer

Lead Image:
Graphic of the DUET Space Telescope Fully Deployed.
Credit – Ditto et al.

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