CSIM Shines Light on New Way to Measure the Sun

CSIM Shines Light on New Way to Measure the Sun
Paul Padgett, May 2019, paul.padgett@nasa.gov


The daily rising of the sun may be a proverbial constant, but at the end of the day, the sun is a star in constant flux. From sun spots to solar flares, the sun’s energy output fluctuates, and even small variations can have large effects here on Earth.

This is why scientists have measured solar irradiance from space for 40 years to establish a continuous record of solar activity. This reliable, unbroken record is critical to our understanding of the Sun and its effects on our climate, and a new CubeSat may represent the smaller, more affordable future for continuing the measurement of solar irradiance.

The Compact Spectral Irradiance Monitor (CSIM), developed by Principal Investigator Erik Richard at the Laboratory for Atmospheric and Space Physics (LASP) with support from NASA’s Earth Science Technology Office and NASA’s flight office, launched in December 2018 on Spaceflight’s SmallSat Express, and by early March 2019, it had obtained first light results across its full scanning range with impressive accuracy.

This small satellite only measures one half of the solar irradiance question, that of solar spectral irradiance (SSI). SSI is the distribution of solar energy input across specific wavelengths, and it is this quantity that provides insight into how the earth’s atmosphere responds to changes in solar output. Though other instruments currently operating also obtain the separate measurement of Total Solar Irradiance (TSI), CSIM represents a significant technologic advance in instrument size, weight, power and cost reductions, all with a potential increased accuracy.

Big Shoes to Fill

Currently, NASA has two instruments measuring solar irradiance: the SOlar Radiation and Climate Experiment (SORCE) and the Total and Spectral Solar Irradiance Sensor (TSIS-1). Both of these missions have more capabilities than CSIM, but they also have some drawbacks that CSIM overcomes. SORCE was launched in 2003 with a planned five-year operational life. Its continued operations now 16 years later is a testament to its resilient build, but its age also presents a risk to the continued scientific record.

To provide redundancy and eventual replacement, TSIS-1 was launched in late 2017 and installed on the International Space Station (ISS), and it represents the state of the art in spectral radiometry for accuracy and stability. Being an ISS passenger adds complexity to this instrument’s mission however. As ISS supports a myriad of critical priorities, there are occasionally constraints on power and operations that periodically interrupt TSIS’s measurement.

Enter CSIM, the free-flying CubeSat with a reduction of mass to 1/10th, volume to 1/20th, and power to 1/4th of the currently operational TSIS. . In simpler terms, CSIM squeezes refrigerator-sized instrument into a 22 pound toaster oven, all with a fraction of the cost. And though CSIM only measures SSI and not both SSI and TSI that TSIS examines, current CubeSat projects are underway to provide the TSI sister measurement to complement CSIM’s SSI. The size, weight, power, and cost reductions of these CubeSats translate into faster builds, cheaper launches, and ultimately, great redundancy for solar irradiance measurements.

And with first light results now achieved, CSIM can add the most important spec to the list: absolute accuracy comparable to TSIS SIM.

CubeSats move into the big leagues

CubeSats are increasing in prevalence, but there is still some institutional hesitancy to their use. “Early on, a lot of people viewed CubeSats as student projects that really couldn’t compete with big science missions,” says Richard. “What we saw here was the rapid revolution of these small satellites that may be able to demonstrate some harder core science.”

The validation of CSIM and the characterization of how its optics degrade over time may open the door for a future where missions dependent on one large science satellite are replaced by two or three smaller, cheaper CSIM CubeSats, creating redundancy and ensuring the long-term continuity of the solar irradiation measurement.