The proposed Fast-class ESA mission directly derives from the ERC Consolidator Grant project (ID 101089312 HYADES, PI Michał Drahus) commenced in July 2023, which investigates the concept of a space telescope designed specifically for studies of water and its isotopic composition in comets and asteroids.
Thanks to the unparalleled sensitivity to these species, the envisioned satellite will provide definite answers to some of the most profound problems of the near Universe. First, it will enable determination of the deuterium-to-hydrogen ratio in water for about one hundred comets of different types with unprecedented homogeneity and accuracy. These measurements will ultimately resolve the puzzle of cometary origin of water on Earth and unveil the link between present-day cometary reservoirs and their original place of formation in the solar nebula. Second, the satellite will enable ultra-sensitive studies of future interstellar objects passing through the Solar System, with direct implications for our understanding of orbital anomalies in the absence of visible dust (such as observed in ‘Oumuamua), as well as birth environments and physical natures of these bodies in general. Finally, the satellite will enable the first large-scale survey of water-ice sublimation in about fifty main-belt asteroids, thereby transforming our knowledge of the ice reservoir in the outer Main Belt.
It is expected that HYADES will increase the existing samples across all three domains by an order of magnitude—a level typically offered by M-class missions (e.g. Plato, Ariel), and only across one object category.
The basic principle behind HYADES is the detection and characterization of water via hydroxyl (OH), which is released from the water molecules after sublimation as a result of photodissociation. Hydroxyl provides unparalleled access to water-ice outgassing and its D/H ratio, offering strong advantages over other techniques: (1) large intrinsic brightness, (2) negligible optical depth, (3) measurement self-consistency, and (4) complete immunity to effects such as variable outgassing. Importantly, the production of hydroxyl by species other than water is negligible, making it an excellent tracer.
Normal and deuterated hydroxyl will be observed via their strong emission bands at 310 nm. HYADES will employ a 50 cm telescope and offer two observing modes: high-resolution long-slit spectroscopy and high-thruput imaging. A set of narrow-band filters mounted in a common filter wheel will be used as a band selector for imaging and order selector for spectroscopy. Thanks to the availability of multiple filters, HYADES will also allow observations of the oxygen isotope in hydroxyl as well as isotopic variants of NH and CN, providing extremely valuable context to its main scientific objectives.
With its ambitious goals, HYADES builds upon the heritage of ESA’s Giotto and Rosetta. To take this legacy to a new level, the proposed mission will be the first in the flotilla of ESA cometary space probes, including the upcoming Comet Interceptor, to adopt a fundamentally new approach: surveying a large number of targets from Earth’s orbit rather than visiting a single object. In addition, it will further extend the legacy of Rosetta by enabling observations of comet 67P over a wide field of view, which is advantageous for accurate D/H ratio determination.