Definition of Habitable Worlds Observatory, Earth Search Space Telescope 2.0

The Earth is a simple pale blue dot in the vastness of the Universe. The probe, located six billion kilometers away, can barely see our world as a speck next to the Sun. Yet we may soon be able to see other pale blue dots around other stars and analyze their composition for biomarkers. But this requires large space telescopes equipped with high-tech equipment. Although the James Webb Telescope has only just begun to show us its potential, the US space agency is already planning its next project. In 2022, the next major space telescope was confirmed to be the Habitable Worlds Observatory, or HWO.Habitable Worlds Observatory). The decision was not easy, as several space observatory projects competed for this position, with proposals from LUVOIR and HabEx being finalists.

HWO was intended as a compromise solution between both projects, although, despite the name, its characteristics are closer to LUVOIR (Large UV/optical/infrared (surveyor)) than HabEx (Habitable exoplanet (Observatory)). HWO is still very far in time, but its features are gradually being defined thanks to the efforts of the scientific community and several NASA centers, especially the Goddard Center. Like Hubble, HWO will observe in the ultraviolet, visible and near-infrared wavelengths. Just like Hubble and JWST, as well as the LUVOIR proposal, HWO will be a general purpose telescope that will be able to observe near and far celestial objects, from solar system planets to very distant galaxies. But following 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 requirements of the program established by NASA).

To do this, HWO must be able to analyze light directly reflected by a planet roughly the size of Earth located in the habitable zone of its star (i.e., a potentially habitable exo-Earth). Observing in the visible and near-infrared, the Habitable Worlds Observatory will look for spectral signatures of biomarkers such as methane, oxygen and water in this light, but being sensitive to ultraviolet, it 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 host star and thus be able to analyze the reflected light from the planets of a given star system.

HWO’s primary mirror would be at least 6 meters in diameter, like JWST’s, and would also be located, like JWST and other observatories, at the L2 Lagrange point of the Earth-Sun system. Of course, HWO will be designed, if possible, so that it can be repaired at a distance of 1.5 million kilometers from Earth. Unlike all large space telescopes in service, HWO will use an off-axis secondary mirror, meaning the secondary mirror does not block light penetration, a fundamental requirement for wavefront reconstruction and thus exoplanet detection.

The new generation of launch vehicles SLS, Starship and New Glenn allow the launch of telescopes with a main star larger than 6 meters, but no one wants to repeat the escalating costs and delays of JWST, so we are starting from a relatively conservative position. by optics size. Basically, HWO will use JWST inheritance to build the primary mirror, so it will be sharded. This may change in the future once the project is completed, but for now it needs to be clarified that having larger launchers like Starship does not necessarily mean the mirror has to be monobloc. Large monoblock mirrors have their own problems, including the fact that there are currently no companies making such large mirrors for space telescopes.

The need for ultraviolet observation forces HWO optics to use layers of materials, such as lithium fluoride, that can adequately reflect these wavelengths. In addition, the optics will be covered by a drop-down shield to prevent outside light from entering (interestingly, this shield and segmented mirror make the preliminary design of the Habitable Worlds Observatory more similar to the design of the proposed ATLAST space telescope from over a decade ago than LUVOIR). This barrier will also serve to control the temperature of the telescope and protect it from micrometeors – a threat that JWST was able to verify is very real. For its part, the coronagraph will be the successor to the one that will be used in the future Nancy Grace Rome Telescope (WFIRST), which will have a 2.4-meter mirror like Hubble’s. In total, HWO will have four main instruments: a coronagraph, a deep-field camera, an ultraviolet observation instrument and an additional instrument, the decision on which has not yet been made.

HWO could be included at a later stage, like HabEx star shadow or a “star umbrella” to increase sensitivity in detecting exoplanets. On the other hand, before choosing Starship as a launch vehicle, let’s remember that SpaceX’s vehicle requires refueling to leave low orbit, a procedure that can contaminate the telescope’s sensitive optics. In any case, since the launch of HWO will not take place until 2040, there is plenty of time to refine this technique. By then, the list of candidate exo-Earth worlds is expected to be relatively long, thanks to next-generation ground-based telescopes and other space-based instruments. Hopefully, in a couple of decades, the Habitable Worlds Observatory will start showing us the first analyzes of the composition of one of these exoplanets. And we may be surprised by the presence of several biomarkers that together are associated with life as we know it.