Advancing Space Domain Awareness: MIT Lincoln Laboratory's Innovative Mirror Technology and the Role of Entegris SUPERSiC®-SP

Enhancing our understanding of the space environment is crucial to navigating the ever-evolving landscape of space exploration. As part of collaboration efforts between the U.S. and Japan to boost space domain awareness through cutting-edge mirror technology, MIT Lincoln Laboratory (MIT LL) built payloads hosted on Japanese satellites.

MIT LL built two identical space payloads for the Japanese QZS-6 and QZS-7 satellites and selected Entegris’ SUPERSiC-SP silicon carbide (SiC) material to serve as a substrate for their mirrors. They also chose our chemical vapor deposition (CVD) SiC solution to serve as a mirror cladding. These payloads are part of a collaborative effort between the U.S. and Japan to augment space domain awareness of objects in or near geosynchronous orbit (GEO). This initiative not only showcases international cooperation but also highlights the innovative engineering challenges and solutions involved in creating functional space payloads.

The Challenge of Geosynchronous Orbit 

GEO is a vital zone for satellites, where they maintain a fixed position relative to Earth. However, this region is congested with a variety of objects, from active satellites to space debris. The need for enhanced monitoring capabilities has prompted the development of sophisticated payloads capable of observing these objects accurately.

Mirror Technology: The Heart of the Payload

The mirror system is central to the payload’s functionality, and it needs to withstand the harsh conditions of space. The unique thermal environment and mass budget requirements led the MIT LL team to select SiC as the mirror material. This choice was motivated by SiC's impressive stiffness-to-weight ratio, which allows for lightweight mirrors that maintain their shape even under the stresses of launch and space conditions.

Entegris was chosen as a SiC vendor due to our unique manufacturing process, which allows us to fabricate SiC into extremely complex shapes with tight tolerances. 

Why Silicon Carbide?

SiC offers several advantages for aerospace applications:

• Low Mass: The high stiffness of SiC enables the design of lightweight mirrors, crucial for space applications where every gram counts.
• Thermal Stability: SiC exhibits low thermal expansion and high thermal conductivity, reducing the risk of distortions due to temperature fluctuations. This is a common issue in space environments.
• Surface Quality: The selected SUPERSiC-SP material, produced by Entegris, ensures that the mirrors can achieve the precise surface figure required for optimal optical performance.

Addressing Silicon Carbide Manufacturing Challenges 

Creating these mirrors involved an intricate fabrication process. Entegris’ unique SUPERSiC process starts with a block of purified synthetic graphite, which is converted into SiC through a high-temperature chemical vapor conversion process. This method not only ensures structural integrity but also allows for the creation of parts with tight tolerances.

Creating the desired finish, however, meant cladding the mirror with SiC via CVD. This introduced potential complications. Specifically, the difference in thermal expansion between the cladding and the substrate could lead to deformation under extreme temperature changes. This necessitated extensive testing.

Rigorous Testing and Results

To ensure reliability, the MIT LL team conducted thorough risk reduction testing. This involved measuring the coefficient of thermal expansion (CTE) of the materials. Using a thermo-mechanical analyzer, the team assessed the thermal strain of the SiC bars across a temperature range from -70°C to 70°C.

The results indicated a notable difference in CTE between the SUPERSiC-SP and the CVD SiC cladding, approximately 0.14 ppm/K. This measurement is crucial as it provides insights into how the mirror will behave in the cold vacuum of space. Furthermore, the team suggests a conservative estimate of 0.2 ppm/K for future design considerations, accommodating potential uncertainties in manufacturing and environmental conditions.

Optical Testing: Ensuring Performance

In addition to thermal tests, optical performance was evaluated through interferometric measurements. A pathfinder mirror was subjected to rigorous testing in a thermal vacuum chamber, simulating the conditions it would face in orbit. The data showed that as temperatures decreased, the mirror became slightly more convex — consistent with the anticipated behavior of the different materials.

Conclusion: A Step Forward in Space Exploration

The collaborative effort between the U.S. and Japan represents a significant advancement in our ability to monitor space. By harnessing innovative materials and rigorous testing methods, this project not only enhances our understanding of the space environment but also lays the groundwork for future missions.

Meanwhile, because the experimental results confirmed that the Entegris SiC mirror and cladding would perform as expected, our materials were approved for use. As of this writing, the QZS-6 satellite containing Entegris components is scheduled for launch on February 1, 2025. Meanwhile, U.S. Space Systems Command reports that the second satellite, QZS-7 will be launched early in fiscal year 2026.

As we continue to face challenges in space domain awareness, initiatives like these remind us of the importance of international collaboration and technological innovation. The successful development and testing of these mirrors are vital steps toward ensuring safer and more efficient use of GEO, ultimately benefiting all of humanity as we explore beyond our planet.

To learn more, download our study, “CTE Effects of CVD Silicon Carbide Cladding on a Silicon Carbide Optic.”