NASA’s Nancy Grace Roman Space Telescope as a planetary defense asset

Introduction. The Nancy Grace Roman Space Telescope (hereafter Roman) is NASA’s next flagship astrophysics mission, with launch currently planned for late August or early September 2026. The primary instrument on Roman is the Wide Field Instrument (WFI), which provides broad-band imaging in the near-infrared (0.48 – 2.3 μm) over a large 0.281 deg² field of view. Roman’s primary goals are to study dark energy and dark matter, evaluate the expansion rate of the universe via observations of supernovae, and push the limits of exoplanet detection. To achieve these goals, Roman must look out through our own solar system, creating an opportunity to study foreground solar system small bodies. In particular, Roman will be capable of providing photometric and astrometric observations of near-Earth objects (NEOs) for both scientific and planetary defense applications. Such observations can be combined with those from other facilities which are beginning to come online, including the Vera C. Rubin Observatory (Ivezić et al. 2019) and NEO Surveyor (Mainzer et al. 2023), to improve orbits, determine sizes, and constrain bulk compositions for some of the smallest and fastest-moving NEOs. The results presented here were published in Holler et al. (2025).

Observe smaller NEOs. The Roman Core Community Surveys are not designed with the necessary combination of coverage and cadence to discover new NEOs; that task is left to Rubin’s Legacy Survey of Space and Time (LSST) and NEO Surveyor. However, using the WFI and the wide bandpass F146 filter, Roman can detect NEOs with diameters smaller than 20 meters at the 5-σ level over a wide range of expected angular rates of motion. This diameter is roughly a third the size of the “city killer” asteroid 2024 YR₄ (Rivkin et al. 2025) and comparable in size to the Chelyabinsk meteoroid that exploded in the atmosphere in 2013, leading to widespread injuries and property damage. The ability to reliably detect objects of this size does not currently exist.

Improve orbits. The ability to observe very small, potentially fast-moving asteroids is a boon for planetary defense initiatives. The first task when any new object is discovered, and particularly for NEOs on Earth-crossing orbits, is to determine the orbit and begin to evaluate risk. NEO Surveyor is designed with the cadence to allow orbit computation for previously unidentified NEOs: two sets of four observations in a 6 – 9-hour period spaced ~13 days apart. We find that adding a single additional astrometric data point with Roman, which has a pixel scale ~30 times finer than that of NEO Surveyor (0.11” vs 3”), a week after the second NEO Surveyor measurement (for a total time baseline of only ~3 weeks) can decrease the orbital uncertainties by 95 – 99% up to a year into the future.

Determine sizes. A full assessment of an object’s risk to the Earth also includes an estimate of the diameter. Only Roman and NEO Surveyor can observe the smallest NEOs. NEO Surveyor operates in the mid-infrared in two wavelength windows, 4.0 – 5.2 μm and 6.0 – 10.0 μm, where thermal emission is strongest (Mainzer et al. 2023). Roman’s wide-band filters span the near-infrared from 0.48 – 2.3 μm, where reflected light peaks. Individually, neither facility is capable of simultaneously constraining albedo and diameter, but together, the combination of near- and mid-infrared photometric measurements can recover the albedo and diameter to high-precision (>10-σ). Constraining a potentially hazardous NEO’s orbit and diameter to high-precision provides the necessary information for more detailed risk assessment and mitigation planning.

Constrain bulk compositions. The diameter of an NEO provides a first-order approximation for the impact assessment, but what is more valuable is an estimate of the mass, which requires constraints on the composition. Information on the spectral type, and thereby composition, provides a crucial delta that can lead to a more accurate mass measurement. The Roman broad-band filters provide sufficient spectral leverage to differentiate between C-, S-, and X-type asteroids in the Bus-DeMeo system (DeMeo et al. 2009). These are the most common asteroid spectral types and represent three different bulk compositions: carbonaceous, silicate-rich, and metal-rich, respectively. Each type is associated with a range of densities, which when combined with a diameter leads to an improved mass estimate.

Roman calibration pipeline enhancements. At the time of abstract submission, none of the measurements described above could actually be performed. This is because the Roman default calibration pipeline as constructed treats moving objects, especially fast-moving objects like NEOs, similar to cosmic rays and removes them from images. We will provide an update on efforts to recover moving target streaks, fit the streaks to extract photometry and astrometry, and make that information available to the larger community.

Acknowledgements. We recognize financial support from the STScI Director’s Research Funds (DRF).

References

DeMeo, F. E., et al. (2009). Icarus 202, 160.
Holler, B. J., et al. (2025). PASP 137, 105004.
Ivezić, Z., et al. (2019). ApJ 873, 111.
Mainzer, A. K., et al. (2023). PSJ 4, 224.
Rivkin, A. S., et al. (2025). RNAAS 9, 70.