Paul Scowen

The Habitable Worlds Observatory (HWO) mission concept is anticipated as NASA’s next astronomical flagship observatory after Roman and the top large, space-based mission recommendation in the 2020 US Astronomy and Astrophysics Decadal Survey. HWO, which combines leaps in scientific discovery space relative to the Hubble Space Telescope with the ability to find and characterize Earth-like exoplanets, is currently being matured by NASA and its potential international partners. The worldwide astronomical community is developing the scientific framework for this mission concept, and it has become widely recognized that most of the cosmic ecosystem, exoplanet, and stellar science goals of HWO require high-throughput imaging and/or spectroscopy at ultraviolet (UV) wavelengths. This paper summarizes current progress by the HWO Community Science and Instrumentation Team to identify and quantify the potential driving science cases that motivate the capabilities of the UV instrument suite for HWO. Key topics currently include the regulation of star formation over cosmic time through galactic feedback, the intergalactic ionizing radiation field, the properties of local analogs of early universe massive stars, and the evolution of chemical complexity in stars and galaxies. We will describe the current driving science cases relevant to UV observations and the measurement goals they imply for the HWO, ranging from UV high-resolution point-source spectroscopy, multi-object and integral field spectroscopy, and far-UV imaging capabilities.

The EXoplanet Climate Infrared TElescope (EXCITE) is a balloon-borne mission dedicated to measuring spectroscopic phase curves of hot Jupiter-type exoplanets. Phase curve measurements can be used to characterize an exoplanet’s longitude-dependent atmospheric composition and energy circulation patterns. EXCITE carries a 0.5 m primary mirror and a moderate resolution diffraction-limited spectrograph with spectral coverage from 0.8 to 3.5 μm. EXCITE is designed to fly from a long-duration balloon. EXCITE will observe through the peak of a target’s spectral energy distribution and through spectral signatures of hydrogen and carbon-containing molecules. In this paper, we present the science goals of EXCITE, detail the as-built instrument, and discuss its performance during a 2024 engineering flight from Fort Sumner, New Mexico.

In preparation for development of both key technologies and instrument concept studies to use those technologies, the Habitable Worlds Observatory Technology Maturation Project Office at the NASA Goddard Space Flight Center has compiled a series of statements of state of the art for those same key technologies. These statements are being provided to the public as exemplars and suggestions for possible future collaboration for those same instrument concept studies, but without mandate, to enable proposing teams to be able to find the technical solutions they need to field a compelling proposal. This information resides in the public domain and is presented without prejudice.

This dedicated journal collection will present and discuss a variety of science cases that can be used to extend our knowledge of massive stars and the influence of their rapid rotation on their subsequent evolution. The aim is to build understanding of this pivotal class of stellar objects that provides the energy and processed material driving galactic evolution and setting the stage for subsequent star and planet formation. This collection of papers offers a unique discussion of physical factors that are driven by rapid rotation and whose influence directly impact the evolution and end-of-life state of massive stars. They are presented together to give the reader a perspective that only the ensemble can provide instead of a single paper. We hope that we are successful in our goal of shedding light on the scope and outcome of this important facet of massive star physics.

We present here the current state of a collection of promising ultraviolet technologies in preparation for the Habitable Worlds Observatory. Working with experts representing a significant number of groups working in the ultraviolet, we summarize some of the leading science drivers, present an argument for a 100-nm blue wavelength cutoff, and gather the current state of the art of UV technologies. We present the state of the art of contamination control, a crucial piece of the UV instrument plan. We explore next steps with individual technologies, as well as present paths forward with system-level testing and development.

The near-ultraviolet (NUV) wavelength range contains a valuable ozone absorption feature for characterizing the atmospheric composition of Earth-like exoplanets. Both the LUVOIR and HabEx decadal mission concept studies baseline instrumentation for obtaining photometry of directly imaged exoplanets down to wavelengths of 200 nm. Both of their proposed implementations present challenges: in the case of the HabEx starshade occulter, a separate spacecraft is required; in the case of a NUV channel within the main coronagraph instrument, non-UV-optimized optics limit the performance. However, the science requirements allow the relaxation of some of the trades of a dedicated near-UV coronagraph. For instance, given that the inner working angle (IWA) of the coronagraph is a function of the wavelength, and one would only consider the characterization of planets detected in the visible, the IWA of the NUV coronagraph can be relaxed to meet the IWA of the visible coronagraph. In addition, given that a NUV coronagraph would be used for a follow-up observation after a visible detection, it could be designed to produce a half dark zone, allowing the simplification of the optical design. Here, we present the case for a standalone NUV coronagraph instrument for the Habitable Worlds Observatory based on the science requirements, developed as part of the Coronagraph Technology Roadmap study commissioned by NASA’s Exoplanet Exploration Office. We describe the trades and rationale behind the recommendation of having a separate NUV coronagraph instrument and show the evaluation of several coronagraph designs in the NUV. We assess the effect of polarization aberrations, discuss relevant technologies, and describe potential NUV detectors and wavefront sensing and control strategies.

We summarize a 3-day workshop held on May 7–9, 2024, at NASA’s Jet Propulsion Laboratory in Pasadena, CA, focused on exploring science opportunities through ultraviolet (UV) observations and the required instrumentation and technology for various platforms, ranging from CubeSats to flagship missions. The workshop supports the research community’s efforts as it prepares for the Habitable Worlds Observatory (HWO) and preceding UV mission opportunities that may be enabling of HWO. UV light is endowed with rich spectroscopic information that can inform many aspects of astrophysics, including the characterization of interstellar medium components, stellar atmospheres, transients of all types, and exoplanet atmospheres, to name just a few. The workshop featured invited and contributed presentations, panel discussions, and poster sessions. Participants discussed science priorities and possibilities through space-based UV observations, explored the necessary instrumentation for these measurements, and identified both existing and needed technologies.

Polstar is a Small Explorer mission designed to operate at L2 to study the role of rapid rotation in massive star evolution. These stars play a vital role in the creation of heavy elements and the injection of energy and material into the Baryonic Cycle that drives the ecology of galaxies. Massive stars often engage in the related activities of rapid rotation and binary mass transfer, but the detailed physics of those processes remains poorly understood because of the absence of suitably diverse observational constraints. The mission will leverage complementary information from far ultraviolet (FUV) spectroscopy and polarimetry to resolve existing uncertainties in the internal angular momentum transport and external exchange of mass and angular momentum involving massive stars. The approach will take advantage of high opacity and polarimetrically sensitive FUV resonance lines, as well as polarization from the temperature-sensitive FUV continuum. This insight will provide vital observational constraints on the impact rapid rotation has on the interior structure of stars and inform models that constrain their evolutionary tracks. The mission exploits advances in ultraviolet (UV) technologies from mirror coatings to detectors to birefringent materials to open a new window on the universe at polarimetric accuracies as good as 0.03% while achieving high signal-to-noise time-domain UV line diagnostics of bright stars at 15 km/s Doppler-shift resolutions.

The Off-Axis Far-uv Off Rowland-circle Telescope for Imaging and Spectroscopy (OAxFORTIS) is a new off-axis reconfiguration of the sounding rocket experiment, FORTIS, designed in a Gregorian fashion with a diffractive secondary to conduct multi-object spectroscopy. This redesign was motivated by the recurrence of geocoronal Lyman-α scattering off the optical train assembly during FORTIS flights, resulting in high background rates. In addition to the off-axis reconfiguration, high efficiency LiF/Al mirror coatings were included in the changes implemented for the 1st launch of OAxFORTIS, which took place on August 2024 and resulted in the successful observation of M10. With over 370 s of exoatmospheric data, this flight achieved nearly an order of magnitude improvement in background rate compared with the final FORTIS launch. We present a review of the OAxFORTIS instrument design and pre-flight characterization. We also give a review of our science mission and present preliminary M10 spectral results. OAxFORTIS enables us to field one of the first FUV explorations of the curious diversity within the hot horizontal branch in the otherwise similar globular clusters M10, M13, and M3. This exploration is intended to address a question commonly referred to as the “Second Parameter Problem” as researchers hunt for parameters other than metallicity that drive this diversity. With the next flight to observe M13 set for June 2026, we conclude with future work that will be completed before then, including the switch to XeLIF mirror coatings and a new microshutter array. The demonstration of these technologies in space systems such as OAxFORTIS supports the feasibility of future large-scale missions such as the Habitable Worlds Observatory by raising their technical readiness.