Before engineers designed a single instrument for the Habitable Worlds Observatory (HWO), scientists had already figured out what the telescope needs to be able to do. It is an unusual but logical approach: define the science first, then build the hardware to match, rather than the other way around.

Why HWO matters

HWO is NASA's future flagship mission, named the top priority for large space missions by the Astro2020 Decadal Survey. Its core goal is "transformative astrophysics" and the search for biosignatures — chemical traces of possible life — in the atmospheres of roughly 25 Earth-like planets beyond the solar system. That is a more ambitious target than any previous space mission has attempted.

To turn this broad vision into concrete technical requirements, NASA formed the Science, Technology, Architecture Review Team (START). The team reached out to the wider scientific community, inviting researchers into working groups to explore the full space of discoveries the future telescope could enable.

70 science cases across four pillars

The effort produced 70 concrete science cases, grouped under four major pillars:

— growth of galaxies — 15 cases;
— evolution of the elements across the universe — 13 cases;
— solar systems in context, comparing our own to others — 32 cases;
— living worlds — 10 cases.

Together, these 70 cases address 27 of the 30 science questions and discovery areas identified by Astro2020. That means the future telescope could potentially answer nearly the entire list of top-priority questions in modern astrophysics — from how galaxies form to whether life exists beyond Earth.

Carrying out all 70 cases would require 140 separate observing programs. Each program is not a single observation but a coordinated set of measurements tailored to a specific science goal.

What instruments are needed

Analyzing the 140 programs revealed which technical capabilities matter most. Spectroscopy — splitting light into its components to analyze chemical composition — is needed for 87% of all cases, making it by far the dominant technique. Photometry, measuring the brightness of objects, is needed for 30% of cases.

More specialized technologies stand out too. High-contrast imaging, which allows faint objects like planets to be seen next to bright stars, is required for 34% of cases. Polarimetry, which measures the polarization of light to reveal the structure of matter, is needed for 27% of cases.

Access to ultraviolet wavelengths emerges as especially critical. 83% of science cases require observations below 400 nanometers, and a quarter need data below 100 nanometers, deep in the far ultraviolet. This range is poorly accessible from the ground because of Earth's atmosphere, which is exactly why a space telescope with such capability is so valuable.

At the other end of the spectrum, about a quarter of cases require observations beyond 2000 nanometers, in the near-infrared.

Beyond optics: other requirements

Besides spectral and optical capabilities, researchers identified several other needs. Precise astrometry — extremely accurate measurement of an object's position in the sky — is needed to measure planet masses. Some science cases require rapid-response capability to react to unexpected events, a large instantaneous field of regard, and non-sidereal tracking to follow objects that move unusually across the sky, such as asteroids or comets.

The list also includes strategies to mitigate detector saturation from bright targets and the need for high dynamic range — the ability to capture both very bright and very faint details in the same observation.

This study marks only an early stage in HWO's development. The mission still exists as a concept, and its final design will take shape much later. But it is already clear how broad and varied a scientific portfolio this observatory is expected to support — from the growth of galaxies in the early universe to the search for life on planets tens or hundreds of light-years from Earth.