Fraser Cain at Universe Today reviews the giant space telescopes that will become possible. Space capabilities from SpaceX Super Heavy Starship and being able to build in space will enable 1000 to 1 million times larger projects on the moon and in cis-lunar orbits.
A 100-meter space telescope on the moon will let us directly observe the height of mountains on exoplanets.
A giant moon telescope will let us answer three major questions in astronomy.
1) the detection of biosignatures on habitable exoplanets,
2) the geophysics of exoplanets and
Detecting Alien Life in Other Solar Systems
One of our main science objectives is the characterization of exoplanets and biosignatures. There are about ten potentially habitable planet candidates up to 10 pc. But there is no guarantee that even a single one will present biosignatures. We must enlarge the sample and go up to say 40 pc. An Earth-sized planet at 1 AU from a G star has a planet/star brightness ratio of 3.10^−9 for an albedo of 0.3. Thus, for a 8th magnitude star, it means a 32nd magnitude target. For 1 nm spectral resolution spectroscopy needed to detect atomic and molecular emission lines, consider the goal of 1000 photons detected in 3 hours. This needs a 50-meter telescope. To detect 500 photons in the bottom of absorption lines having a depth 10 times the continuum in 3 hours, one would need a 100-meter telescope.
To achieve this goal, two requirements are needed : 1/ a very large aperture to detect spectro-polarimetric and spatial features of faint objects such as exoplanets, 2/ continuous monitoring to characterize the temporal behavior of exoplanets such as rotation period, meteorology and seasons. An Earth-based telescope is not suited for continuous monitoring and the atmosphere limits the ultimate angular resolution and spectro-polarimetrical domain. Moreover, a space telescope in orbit is limited in aperture, to perhaps 15 meters over the next several decades (until we get on orbit space construction capabilities). Researchers propose an OWL-class lunar telescope with a 50-100 meter aperture for visible and infrared (IR) astronomy, based on ESO’s Overwhelmingly Large Telescope concept, unachievable on Earth for technical issues such as wind stress that are not relevant for a lunar platform. It will be installed near the south pole of the Moon to allow continuous target monitoring. The low gravity of the Moon will facilitate its building and manoeuvring, compared to Earth-based telescopes. As a guaranteed by-product, such a large lunar telescope will allow Intensity Interferometric measurements when coupled with large Earth-based telescopes, leading to pico-second angular resolution.
The Earth is 10 billion times fainter than the Sun and orbits close to its host star : viewed from 100 parsecs, the separation is only 0.01 arcsec. But this is well above the diffraction limit of a very large lunar telescope. We can study exoplanet atmospheres from a lunar platform, where there is no atmosphere to confuse our signal. Telescope size simultaneously guarantees a large number of earth-like targets. We cannot fail, if the will is there to develop known technology, with the aid of robotic resources in deep icy craters near the south pole, in permanent darkness and where temperatures approach 30K, with adjacent crater rims in perpetual sunlight to provide solar power.
Mountains and volcanos on planets
Some astronomy questions require the extremely high angular resolution from an Earth-Moon Intensity Interferometer. Telescopes on the Earth and Moon can work together to create a 380,000 kilometer telescope array.
Once an OWL-type telescope is installed on the Moon, or even a 10 meter lunar precursor, one could readily address optical Intensity Interferometry with unprecedented baselines and angular resolution. For instance it could measure the heights of mountains on transiting exoplanets. This is an important problem for the geophysics of planets. Weisskopf (1975) has shown that there is a relationship between the maximum height of mountains on a planet and its mass and the mechanical characteristics of
The issue of mountain detectability has already been addressed for transiting planets (McTier & Kipping 2018). Researchers propose a significant improvement, Based on the principle of the detection of the silhouette of ringed planets by Intensity Interferometry as developed by Dravins (2016). With a 60 meter resolution at the 1.4 parsec distance of alpha Cen, for transiting planets, mountains will appear at the border of the planet silhouette during the transit. These observations will require very long exposures. During the exposure, the planet is rotating around its axis, leading to a washing-out of the features on the exoplanet.
The planet rotation period will be well known from the periodicity of its photometric data. Therefore, the mountain silhouette will appear in a 2D Fourier transform of long series of short exposure images at the planet rotation frequency. Moreover, volcanos can be detected as a temporary excess of red emission of the planet.
Oceans and Continents
The flux received from the glint of the ocean of an Earth-sized planet around a solar-type star at 10 pc and for an ocean albedo 6 %, 7 photons/sec with 30 m telescopes. The monitoring of this image would reveal the contours of the continents.
Earth Atmosphere as a Lens to Map the Surface of Pulsars – Terrascope » detector
It has recently been proposed to use the Earth atmosphere as a gigantic annular chromatic lens (Kipping 2019). It happens that the focal length of this lens is approximately the Earth-Moon distance, depending on the wavelength. Given the size of this lense, the amplification of the source flux is 20,000 compared to a 1 m telescope meter. With a 100 meter telescope on the Moon, the amplification would thus be 200,000 compared to a 30 meter telescope. Of course the images are of very poor quality, but this terrascope would be suited for very high spectral resolution or extremely high speed photometry of extremely faint sources (e.g.
very faint, yet undetected, optical pulsars). Given the 5° inclination of the lunar orbit with the ecliptic, this terrascope could explore a ± 5° band on the sky above and below the ecliptic, depending on the season.
An array telescopes on the Moon and earth (baseline of 380.000 km on average) corresponds to an angular resolution of 200 picoarcsecond at 600 nanometers. An Earth-Moon intensity interferometer would partially resolve the Crab pulsar.
New Telescope Technology
The Nautilus project or the WAET project should soon begin. The Nautilus project has designed new technology for cheap and light 8-
meter-class telescopes. This is based on a modified version of Fresnel lenses, made in light plastic. The WAET project is a very large 10 meter x 100 meter rectangular aperture. The optical quality of these two projects would not be suited for standard interferometry, but suffices for Intensity Interferometry and high resolution spectroposcopy.
A 100-meter diameter will allow statistical searches for life on the nearest 100 or so exoplanets (many of them Earth-like).
SOURCES- Universe Today, ESA, ESA Voyage 2050 White Paper- OWL-MOON: Very high resolution spectro-polarimetric interferometry and imaging from the Moon: exoplanets to cosmology
Written By Brian Wang, Nextbigfuture.com