That window matters because Earth’s radiation budget is arguably the single most important number in climate science. It describes the balance between the solar energy Earth absorbs and the thermal energy it radiates back to space. When the planet absorbs more than it emits, global mean surface temperature rises; when the reverse occurs, it falls. Both the Intergovernmental Panel on Climate Change and the most recent Earth science Decadal Survey designate these observations as essential for determining the current energy imbalance and predicting future climate, and the Global Climate Observing System classifies the radiation budget as an Essential Climate Variable. Because the climate system responds over decadal to centennial timescales, only continuous, accurate monitoring from space can reveal the mechanisms driving climate shifts and inform mitigation strategies.

The gold standard for such monitoring is the Clouds and the Earth’s Radiant Energy System, or CERES, a suite of scanning broadband radiometers that has maintained the longest continuous record of Earth’s radiative energy, stretching from 2000 to the present. Six CERES instruments flying on four satellites, Terra, Aqua, Suomi-NPP, and NOAA-20, have collectively delivered more than 26 years of measurements of reflected solar and emitted longwave radiation. Yet the record faces a looming threat. The CERES instruments on Terra and Aqua are scheduled to end science operations in 2027, and S-NPP potentially in 2026. NASA’s first Earth Venture Continuity mission, Libera, is set to launch in late 2027 with a five-year prime mission, but no missions are planned beyond it. According to a recent analysis cited in the study, the probability of a gap in the radiation budget record reaches 33 percent by 2028 and climbs to 60 percent by 2035, even assuming Libera remains operational. Worse, bridging a gap produces errors roughly four times larger than those arising when successive missions overlap.

Against that backdrop, the CTIM team saw an opportunity. Because the CubeSat was designed for solar irradiance, its electrical substitution radiometers, equipped with vertically aligned carbon nanotube absorbers that absorb more than 99.9 percent of incident radiation from the ultraviolet through the infrared, were calibrated in irradiance rather than radiance. During orbital eclipse, the spacecraft was pointed at Earth’s nadir, collecting calibrated irradiance measurements that the team converted to mean radiance using the instrument’s precisely characterized angular response, which reached zero at 23 degrees off-axis. At a median orbital altitude of 453 kilometers, that translated into a footprint roughly 484 kilometers in diameter, enormous compared with the 20-kilometer nadir footprints of the CERES scanners, but well suited to averaging broad scenes of outgoing thermal emission.

To find out whether these opportunistic measurements could stand shoulder to shoulder with the established record, the researchers compared CTIM’s nighttime longwave radiances with coincident observations from CERES instruments aboard Terra, Aqua, and NOAA-20. They built a four-step matching algorithm that selected CERES footprints within 242 kilometers of each CTIM footprint centroid and within 20 minutes of each CTIM observation, restricted to viewing zenith angles of 20 degrees or less to match CTIM’s near-nadir geometry, and required at least 400 CERES samples per match. The procedure yielded 541 matches containing 261,010 individual CERES footprints. Supplemental CERES observations filled gaps within each CTIM footprint, and the team weighted every CERES radiance by the CTIM spatial response function and by the viewing-angle-dependent growth of the CERES footprint area.

One subtlety demanded extra care. When a scanning radiometer views Earth at oblique angles, the observed radiance typically decreases because radiation travels a longer path through the cooler upper atmosphere, an effect known as limb darkening. Since CERES footprints extended up to nearly 30 degrees off nadir within the CTIM footprint, the team ran 700 radiative transfer simulations with the MODTRAN code, varying viewing angle, surface emissivity, surface temperature, and atmospheric profile across tropical, mid-latitude summer, and mid-latitude winter conditions. The resulting lookup tables provided limb-darkening adjustment factors that were applied to the 17.6 percent of CERES footprints meeting a strict clear-sky criterion of at least 95 percent clear fraction, ensuring that off-nadir radiances were consistent with CTIM’s nadir-viewing geometry.

The verdict was striking. Across all 541 matches, the mean relative difference between CTIM and CERES longwave radiances was −1.19 percent, with a bootstrapped 95 percent confidence interval of ±0.35 percent and a standard deviation of 4.16 percent. A linear regression produced a coefficient of determination of 0.948 and a root mean square error of 3.02 watts per square meter per steradian, indicating a strong relationship between the two instruments. Individually, Terra yielded a relative difference of −1.18 ± 0.45 percent over 238 matches, Aqua −1.21 ± 1.08 percent over 109 matches, and NOAA-20 −1.18 ± 0.53 percent over 194 matches, all well within the respective instrument uncertainties. CTIM’s own radiance uncertainty budget, dominated by solid-angle uncertainty, totaled 1.65 to 1.80 percent, while CERES longwave calibration uncertainty is about 0.75 percent. The agreement demonstrates that a solar instrument never designed for Earth observation can nonetheless deliver scientifically credible measurements of the planet’s thermal emission.

The residual spread in the comparison told its own story. When the team stratified scenes by cloud cover, clear-sky matches showed a standard deviation of only 1.17 percent, compared with 4.26 percent for cloudy scenes, and the largest spreads of all appeared in the tropical latitude bins straddling the equator, where persistent deep convective clouds of the inter-tropical convergence zone create highly heterogeneous scenes. Because the CERES scanners view clouds along oblique paths while CTIM looks straight down, the two instruments sample cloud layers along meaningfully different path lengths, introducing sensitivity to cloud optical depth and vertical structure. Even CTIM’s gridded global map of 15 months of nighttime observations reproduced familiar features of the longwave radiation field, including the characteristic minimum over the tropics where high, cold cloud tops suppress thermal emission, qualitatively mirroring 17 years of CERES climatology.

The implications reach well beyond a single technology demonstration. CubeSats offer low launch costs, flexible deployment, and redundancy that large flagship missions cannot match; three independent CubeSats with a three-year survival probability of 63 percent each collectively achieve a 95 percent success probability. A growing family of small-satellite missions, including RAVAN, NASA’s polar-orbiting PREFIRE pair, the French Uvsq-Sat constellation and its successor Uvsq-Sat NG, and the proposed BABAR-ERI concept, is already charting the path. The CTIM results suggest a particularly elegant strategy: future total solar irradiance instruments could be intentionally designed with Earth-viewing capability built in, allowing a single low-cost platform to monitor both the Sun and the planet. With an optical system purpose-built for Earth’s wider angular spread of outgoing radiation, radiance uncertainties could shrink toward the 0.02 percent level achieved for solar measurements. Challenges remain, including long-term radiometric stability, absolute Earth-viewing calibration, and cloud anisotropy corrections, but the study makes a compelling case that fleets of small, cheap, rapidly deployable radiometers could buffer the radiation budget record against the gaps that currently threaten to open after 2027, protecting the multidecadal climate data record on which our understanding of a warming planet depends.

Subject of Research: CubeSat-based continuity observations of Earth's radiation budget from space

Article Title: From opportunity to continuity: a CubeSat implementation to enhance Earth's radiation budget observations from space

Article References: A. Hawkins, M., Watwood, M., van den Heever, M., Pilewskie, P., & Harber, D. (2026). From opportunity to continuity: a CubeSat implementation to enhance Earth's radiation budget observations from space. Atmospheric Measurement Techniques, 19(19), 6229-6250. https://doi.org/10.5194/amt-19-6229-2026

Image Credits: AI Generated

DOI: 10.5194/amt-19-6229-2026

Keywords: Earth radiation budget, CubeSat, CTIM, CERES, total solar irradiance, longwave radiation, climate data record, Libera, remote sensing, satellite calibration, limb darkening, NASA InVEST

The effect at the heart of the study is called limb darkening, and it is exactly what the name suggests: a star appears fainter at its edge than at its center. This happens because stars are not uniformly bright disks. When we look toward the center of a stellar disk, our line of sight penetrates deeper into the star’s atmosphere, reaching hotter and brighter layers. Near the edge, the light we receive originates from shallower, cooler layers, and it must pass through more stellar material on its way out. The result is a gradual fade from a brilliant center to a dimmer rim, a signature that encodes the temperature and density structure of the star’s outer atmosphere.

Lead author Narsireddy Anugu, a staff scientist at Georgia State University’s CHARA Array, emphasized that the survey goes beyond simply sizing up stars. “We are not just measuring how large these stars are,” Anugu said. “We are measuring how their light is distributed across the stellar disk, which directly tests stellar-atmosphere models.” That distinction is crucial. Stellar radii can be measured in many ways, but the distribution of brightness across a stellar surface is a far more demanding test, one that probes the physics of how energy flows through a star’s outermost layers.

The CHARA Array itself is a marvel of engineering that makes such measurements possible. Located on Mount Wilson, the facility combines light from six telescopes positioned at different sites across the observatory grounds. By interfering the light together, CHARA achieves the resolving power of a much larger telescope, allowing astronomers to resolve details on the surfaces of stars that would otherwise remain forever blurred into points of light. Rather than capturing direct images, CHARA measures how the contrast of the interference pattern changes with the spacing between telescopes. These measurements reveal both the size of a star and how brightness changes across its surface, a technique that has made CHARA one of the most productive stellar-imaging facilities in the world.

Gail Schaefer, director of the CHARA Array, highlighted the facility’s unique capabilities in enabling the survey. “This study demonstrates the powerful capabilities of our facility,” Schaefer said. “By combining light from telescopes across the mountaintop, we can image stars with enough detail to see what their surfaces actually look like.” The observations were conducted by observing the same stars simultaneously through two different near-infrared filters, allowing the team to measure how the center-to-edge fading changes with wavelength. This dual-wavelength strategy is what transformed the survey from a collection of stellar portraits into a quantitative test of atmospheric physics.

The sample consisted of 31 bright stars in late stages of their lives, stars whose outer layers have expanded outward far beyond the dimensions they had during the prime of their existence. These evolved subgiant, giant, and supergiant stars provide especially valuable tests of stellar-atmosphere models because the large convective motions churning through their extended atmospheres can produce complex brightness profiles. Unlike the Sun, whose surface is a relatively thin layer governed by well-understood radiative processes, these puffed-up stars have atmospheres where convection dominates, making them natural laboratories for testing whether theoretical models can capture the messy reality of stellar surfaces.

The quantitative result is striking. Across the sample, the limb-darkening strength decreased by about 38 percent going from near-infrared wavelengths at 1.6 microns to 2.2 microns. The atmosphere models tested in the study predicted the same overall trend, weaker limb darkening at longer infrared wavelengths, but with a smaller decrease of only about 17 to 22 percent. In other words, the models capture the broad behavior correctly, but they do not fully reproduce how strongly the wavelength dependence plays out in real stars. The discrepancy means that current models may underestimate how stellar brightness changes across a star’s surface, a systematic error that could propagate into downstream measurements that astronomers rely on every day.

Why does this matter so much? Limb darkening directly affects measurements of stellar diameters, since the apparent edge of a star depends on how its brightness fades. More dramatically, it impacts how astronomers characterize exoplanets that transit across the surface of their parent star. When a planet crosses in front of a star, the amount of light blocked during the transit depends on the brightness across the stellar disk. A planet passing in front of the bright center blocks more light than one skimming the dimmer edge. If models misrepresent that brightness distribution, the inferred sizes and other properties of transiting exoplanets inherit the error. With thousands of confirmed exoplanets and missions like TESS and the upcoming PLATO survey depending on precise transit modeling, an empirical correction to limb darkening is a gift to the exoplanet community.

The survey also delivered a reassuring null result: the team found no evidence for surface features such as large starspots or hidden companion stars anywhere in the sample. That cleanliness matters, because spots and companions can mimic or mask limb-darkening signatures, and their absence confirms that the measured wavelength dependence is a genuine atmospheric effect rather than contamination from surface asymmetries or stellar multiplicity. The result strengthens the case that the discrepancy with models is real and systematic, not an artifact of a few unusual stars.

Looking ahead, the team plans to extend the comparison to broader wavelength coverage, stretching from visible light into the near-infrared, which will further constrain how well models reproduce the full spectral behavior of stellar atmospheres. The researchers also intend to image smaller main-sequence stars, which are far more difficult to resolve but are especially important for understanding transiting exoplanets around Sun-like stars, the primary targets in the search for habitable worlds. For now, the CHARA Array’s survey of 31 fading disks stands as a reminder that even the most familiar objects in the sky still hold surprises, and that pushing observational precision to its limits remains one of the most reliable ways to find where our theories fall short.

Subject of Research: Near-infrared limb darkening measurements of evolved stars using optical interferometry

Article Title: Georgia State Telescope array reveals how nearby stars fade from center to edge

Article References: Georgia State Telescope array reveals how nearby stars fade from center to edge. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: CHARA Array, limb darkening, stellar atmospheres, optical interferometry, evolved stars, near-infrared astronomy, exoplanet transits, stellar diameters, Mount Wilson, supergiant stars, stellar convection, The Astronomical Journal