Official websites use .gov
A .gov website belongs to an official government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.
Compact Spectral Irradiance Monitor
Instrument Details
Location
Low Earth orbit (LEO), 575 kilometers (357 miles) above Earth’s surface
Purpose
The Compact Spectral Irradiance Monitor (CSIM) was a small instrument squeezed into a CubeSat the size of a shoebox. The instrument measured how solar energy is distributed across light wavelengths ranging from 200 nanometers, in the ultraviolet, to 2,800 nanometers, in the infrared.
Knowing this distribution helps scientists predict how Earth’s atmosphere responds to changes in solar output. While other satellites also measure solar spectral irradiance, CSIM aimed to do so with equal or greater accuracy at a fraction of the size and cost. Ultimately it achieved comparable accuracy to the best existing instrument at less than a tenth of the price tag.
NIST’s role
NIST researchers designed and built a new kind of chip-based bolometer to provide periodic on-board calibrations for the CSIM. The NIST bolometers are smaller, faster, cheaper and at least as accurate as their predecessors.
The chips consist of carbon nanotubes and metals patterned on silicon wafers. NIST scientists designed and fabricated them under an agreement with the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder, which built the CSIM with support from NASA.
Significant discoveries and current status
CSIM launched in 2018 and was decommissioned in 2022.
CSIM obtained “first light” results across its full scanning range. While it was operational, it took daily readings of top-of-the-atmosphere solar spectral irradiance. Its measurements were accurate to within around 0.2%.
The CSIM mission demonstrated that compact bolometers based on nanotubes could function in space and deliver measurements comparable to existing technology at a fraction of the size and cost. The current instrument used to measure solar spectral irradiance, the Total and Spectral Solar Irradiance Sensor (TSIS), has a hand-crafted cavity to trap light. Because nanotubes are so good at trapping light, CSIM didn’t need such a cavity, allowing it to be shrunk to roughly one-tenth the size of the TSIS detector.
Additionally, CSIM cost less than $10 million to develop, build and launch. By contrast, TSIS (which is mounted to the International Space Station) cost $100 million to build.
Other interesting facts
Carbon nanotubes were discovered in the 1950s, but only in the early 1990s did scientists figure out ways to synthesize them efficiently. They are the blackest known material on Earth and efficiently absorb nearly all light across a broad span of wavelengths.
The CSIM satellite took on a second life in 2025, when students at the University of Colorado Boulder figured out a way to repurpose the instrument to measure drag forces in very low Earth orbit (150 to 220 miles above Earth’s surface). The satellite finally burned up in Earth’s atmosphere in August 2025.
Collaborators
The Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder
Media
