Very large baseline optical astronomy

Large baselines will be required in the future to perform direct imaging and, in some cases, spectroscopic observations of exoplanets. Therefore, astronomers will inevitably be led to design large interferometers, even at short visible wavelengths.

If around 2020–2030 we have found a promising biomarker candidate on a nearby planet [for instance, around Proxima Centauri, such a discovery would trigger two kinds of projects:

Direct visualization of living organisms. To detect directly the shape of an organism 10 meters in length and width, a spatial resolution of 1 meter would be required.

Even on the putative closest exoplanet, Proxima Centauri A=B b, the required baseline would be at 600 nm B ¼ 600,000 km (almost the Sun’s radius). In reflected light, the required collecting area to obtain 1 photon per year in reflected light is equivalent to a single aperture of B ¼ 100 km. In addition, if this organism is moving with a speed of 1 cm s 1, it would have to be detected in less than 1000 s. To get a detection in 20 minutes with a S=N of 5, the collecting area would correspond to an aperture B ¼ 3 million km. Laser-trapped mirrors and other hypertelescope designs could make massive optical astronomy possible.

Seth Shostak,Senior Astronomer, SETI Institute, suggests building a hypertelescope array with a baseline of 160 million kilometers.

Milliarcsecond astronomy today, microarcsecond, nanoarcsecond in the not distant future

The current state of the art in the optical is represented by phase/amplitude interferometers such as the CHARA in California, MROI in New Mexico, NPOI in Arizona, SUSI in Australia, VLTI in Chile, and others. With baselines up to a few hundred meters and typically operating in the near-infrared, these realize resolutions on the order of 1 milliarcsecond (mas). Tantalizing results show how giant stars (diameters of a few tens of mas) are beginning to reveal themselves as objects with a broad variety of properties: flattened or deformed due to rapid rotation, engulfed by shells or obscuring clouds.

Using the RadioAstron space antenna together with ground-based telescopes, a resolution of 21 µarcsec at 43 GHz has been demonstrated. Through indirect techniques, indications of the presence of even much smaller structures can be found, even if explicit images are not obtained.

Optical imaging with microarcsecond resolution will reveal details across and outside stellar surfaces but requires kilometer-scale interferometers, challenging to realize either on the ground or in space.