The Earth is a simple pale blue dot in the immensity of the Universe. A probe located six billion kilometers away can barely make out our world as a speck next to the Sun. And yet, soon we will be able to see other pale blue dots around other stars and analyze their composition in search of biomarkers. But to do this we need large space telescopes equipped with high-tech instrumentation. Although the James Webb Telescope has only begun to show us its potential, the US space agency is already planning the next project. In 2022 it was confirmed that the next large space telescope would be the Habitable Worlds Observatory or HWO . The decision was not easy, since several space observatory projects were competing for the position, with the finalists being the LUVOIR and HabEx proposals.
Preliminary (reference) design of the Habitable Worlds Observatory (NASA).
HWO was conceived as a compromise between both projects, although, despite its name, its characteristics are closer to LUVOIR ( Large UV/Optical/InfraRed [Surveyor] ) —a telescope with ultraviolet capacity and a 6- to 10-mm segmented mirror. 8 meters—that of HabEx ( Habitable Exoplanet [Observatory] ) —a telescope with a 4-meter monolithic mirror—. HWO is still very far away in time, but its features are being defined little by little thanks to the drive of the scientific community and several NASA centers, especially the Goddard Center. Like Hubble, HWO will observe in the ultraviolet, visible and near-infrared. Also like Hubble and JWST, and like the LUVOIR proposal, HWO will be a generalist telescope that will be able to observe nearby and distant celestial objects, from planets in the Solar System to very distant galaxies. But, following in the wake of the HabEx proposal, HWO will be the first telescope designed specifically to search for signs of life outside the Solar System (this is not a figure of speech, but one of the program requirements set by NASA).
The goal of HWO is to see Earth 2.0 around another star (simulation of what the Solar System would look like several light years away with a similar telescope equipped with a coronagraph) (NASA).
LUVOIR (NASA) proposals.
To do this, HWO must be able to analyze light directly reflected by a planet approximately the size of Earth located in the habitable zone of its star (that is, a potentially habitable exo-Earth). By observing in the visible and near-infrared, the Habitable Worlds Observatory will search in this light for the spectral signature of biomarkers such as methane, oxygen and water, but being sensitive to ultraviolet will also be able to detect the presence of ozone, another powerful biomarker. HWO will need an advanced coronagraph to block the brightness of the main star and thus be able to analyze the reflected light from the planets of a given star system.
HWO will be able to detect biomarkers on Earth-like planets from ultraviolet to infrared (NASA).
The main mirror of HWO must be at least 6 meters in diameter, like that of JWST, and will also be located, like JWST and other space observatories, at the L2 Lagrange point of the Earth-Sun system. Of course, HWO will be designed, if possible, to be able to be repaired 1.5 million kilometers from Earth. Unlike all large space telescopes in service, HWO will use an off-axis secondary mirror, that is, the secondary does not block the entry of light, a fundamental requirement to reconstruct the wave front and thus detect habitable exoplanets near their stars. .
Current HWO design (NASA).
The new generation of launch vehicles, SLS, Starship and New Glenn, allow launching a telescope with a primary larger than 6 meters, but no one wants to repeat the escalation of costs and delays of the JWST, so we start from a relatively conservative position in the size of the optic. In principle, HWO will take advantage of the JWST inheritance for the construction of the primary mirror, so it will be segmented. This could change in the future as the design is finalized, but at this point it must be clarified that the availability of larger launchers, such as the Starship, does not necessarily imply that the mirror has to be monobloc. And large monolithic mirrors have their own set of problems, among them, there are no companies that currently manufacture such large mirrors for space telescopes, in addition to being pieces that are very difficult to transport and test. In contrast, segmented mirrors incorporate a very capable active optics system due to the nature of the design and the segments are easy to move, test and replace during construction.
Another view of the preliminary HWO design showing the off-axis secondary (NASA).
The need to observe in the ultraviolet forces HWO optics to use layers of materials—such as lithium fluoride—capable of adequately reflecting these wavelengths, as well as the visible and infrared. Additionally, the optics will be covered by a drop-down screen to prevent the entry of external light (interestingly, this screen and segmented mirror makes the provisional design of the Habitable Worlds Observatory more similar to that of the ATLAST space telescope proposal from more ago. a decade than LUVOIR). This barrier will also serve to control the temperature of the telescope and protect it from micrometeors, a threat that JWST has been able to verify is very real (unlike JWST, HWO will not be a cryogenic observatory). For its part, the coronagraph will be the heir to the one used in the future Nancy Grace Roman telescope (WFIRST), which will have a 2.4-meter mirror, like Hubble. In total, HWO will have four main instruments: the coronagraph, a deep field camera, a specific instrument for observing in the ultraviolet and an additional instrument yet to be decided.
