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X-rays and UV photons are absorbed by the Earth's atmosphere, so Arcus requires a space mission. It will be ready to launch in December 2031.

Arcus's instrumentation flows directly from the requirements imposed by the science goals, and was designed to be highly modular and redundant. It will use well-tested, solid technology.


The X-ray telescope has four identical, independent optical channels that feed into two identical focal planes, thus optimizing manufacturing, assembly, and testing. Each optical channel is composed of 24 co-aligned silicon pore optics (SPO) mirror modules, each of which focuses their collected X-rays through the Critical Angle Transmission (CAT) grating windows. The gratings disperse the X-rays through a 12 meter boom onto two independent detector arrays. Videos about the making of the SPOs at Cosine Measurement Systems can be seen here and here. The CAT gratings are made at MIT's Space Nanotechnology Lab.

Costs are minimized and robustness is increased by building on and adapting techonologies from other missions. For instance, the detector array electronics are based on those from Suzaku and TESS; the boom is derived from the GEMS boom development, which was finished prior to that mission's cancelation (see Figure 1, left). Costs are kept even further in check — and the production time is reduced — by the roboticized, production line fabrication of the optics modules.

The gratings have already completed environmental testing with pre- and post-X-ray performance testing. The individual grating windows have been tested and are ready for use with Arcus, as seen in Figure 1 (right).

Figure 1. Left: A full-size boom, which was derived from the GEMS boom development, has been built and tested to TRL 6. Different parts of the boom are indicated. Right: A CAT grating with mount, with a quarter for scale.

The far-UV spectrometer will be designed and built by Dr. Kevin France and his team at the University of Colorado. Dr. France is the PI of numerous UV experiments (see Figure 2, left and right) including the Ultraviolet Astrophysics Rocket Group and the Colorado Ultraviolet Transit Experiment cubesat mission. He was on the Hubble Space Telescope-COS instrument team, and was the science lead for the Hubble Space Telescope-COS Guaranteed Time Observing program’s protoplanetary disk, cool star, and exoplanet programs. He is currently on NASA's LUVOIR Science and Technology Definition Team.

Figure 2. Left: The University of Colorado team has experience working with large format, photon-counting detectors. This image shows the SISTINE sounding rocket detector with an active area of 220mm x 40mm. The SISTINE sounding rocket flew large format detectors and advanced UV optical coatings on three NASA missions from 2019–2022. Right: The University of Colorado team has coated and flight-tested 50cm optics using a range of FUV sensitive coatings. This photo shows the coating of the INFUSE rocket primary mirror.

Spacecraft and Orbit

The spacecraft will be produced by Northrop Grumman Innovative Systems, which has a long and distinguished history of providing spacecraft platforms for science missions. Arcus will use the LEOStar-2 platform, a versatile, robust, high-quality platform that is particularly well-suited to missions of Arcus's class; in fact, NuSTAR, OCO-2, ICESat-2, and TESS all use this as their spacecraft.

Similar to TESS, this platform will be designed for high-Earth orbit, with solar arrays that are capable of withstanding long eclipses, simple thermal design, and propulsion systems that are proven by other missions. Further, the platform is 3-axis stabilized with fine-balanced wheels and no actuations during science observations, thus providing a low-jitter base for the instrument.

Arcus will use a highly-elliptical insertion orbit with phasing loops through lunar swingby to achieve its final high-Earth science orbit (see Figure 3). This orbit was selected because it will provide a thermally stable environment and benign disturbance torques, yielding continuous, efficient (efficiency > 85%) science operations.

Figure 3. The science orbit is a high-Earth orbit similar to that of TESS. It provides thermal stability and low disturbance torques, allowing easy, efficient operations.