NASA has been measuring the irradiance of our central star since 1978. Scientists at the University of Colorado Boulder want to ensure that the series of measurements, which now spans more than four decades, continues. The Laboratory for Atmospheric and Space Physics (LASP) is therefore a hive of activity. Two instruments are under construction here for the next stage of NASA's TSIS mission. TSIS stands for Total and Spectral Solar Irradiance Sensor. The TSIS-2 mission, which is now being prepared, is scheduled for launch in 2023 and will succeed TSIS-1, which has been active since 2017.

The TSIS instruments, called TIM and SIM, achieve a significantly improved measurement accuracy. TIM, the Total Irradiance Monitor, records the total irradiance—the dominant energy source for the Earth. SIM, the Spectral Irradiance Monitor, records the spectral irradiance and is intended to allow conclusions to be drawn about how the Earth's atmosphere reacts to changes in solar energy output.

TIM and SIM, unlike their predecessors at TSIS-1, do not sit on the outside of the hull of the International Space Station. Rather, they are traveling independently, flying through space on a satellite about the size of a washing machine in orbit around the Earth. David Gathright, the LASP project manager responsible for the mission's payload, sees this as a major advantage, since it allows much more data to be collected: "As a free-flying mission, we're going to be able to collect a lot more data.“

The goal of the mission: better data for climate and energy

For centuries, astronomers have observed that the number of sunspots and solar flares fluctuates cyclically. Such a cycle lasts on average just over eleven years. Both phenomena - spots and flares - signal local changes in the surface temperature of the sun and thus reduced or increased radiation output. In short, the amount of radiation emitted by the Sun is not constant, but varies.

The fluctuations affect the Earth's climate, among other things, which makes the mission's results particularly relevant to society. Scientists working on climate change need more accurate information on the amount of radiation emitted by the sun for the next generation of climate models, reports LASP team leader Erik Richard. The measured values also play a role in calculating the efficiency of photovoltaic systems.

The challenge: extreme precision

The Laboratory of Atmospheric and Space Physics is also responsible for the command and control of the instruments. As soon as the satellite enters its orbit, maximum precision is essential to ensure that the measurements are successful. The high standards naturally apply to the material, right down to the smallest screw. This will include the valves that VAT has developed together with the University of Colorado Boulder. Specifically, these are mini vacuum valves of the 01.2 series with the proven, robust VATLOCK sealing technology. The designation "mini" is no coincidence: with a nominal size of DN16, it is one of the smaller VAT valves. To meet the high cleanliness requirements of the mission, all parts of the valve are subjected to special VAT proprietary cleaning processes. There must be no particle residues or outgassing of any kind. As with other satellite components, assembly takes place under cleanroom conditions.

© Nasa

As the satellite circles outside the Earth's protective atmosphere, the equipment is particularly exposed to the harsh space environment, including thermal stress and other extreme external influences. In order to be able to solve the resulting technical difficulties with foresight, the management of the TSIS-2 instruments in Boulder must have precise information about all components. VAT project manager Dr. Wolfgang Niessner emphasizes, "For our valves, this means that every single component, from the simple screw to the valve housing, is carefully tested and documented." Coordination in the run-up to and during the project was therefore immensely important and time-consuming. Thanks to the professional cooperation with colleagues and suppliers, everything went very well, praises Niessner. The valves prepared for the mission have already arrived at the University of Colorado's space laboratory, the first stop on their journey into space.