Building spacecraft is no walk in the park. However, when it comes to designing a craft for space, one aspect stands out: aerodynamics. At the altitudes where most satellites fly, atmospheric drag is minimal. This is why many satellites resemble large refrigerators adorned with solar panels and antennas.
But there’s a flip side. Satellites orbiting high above the Earth can drift in space for decades without proper means to slow down. If they don’t have propulsion systems to decrease their speed, they could remain in orbit indefinitely.
Once upon a time, the focus was purely on getting to space without future worries. Today, the low Earth orbit is becoming crowded. With lower launch costs, many companies are deploying thousands of satellites. This creates potential issues with space congestion.
One promising solution is very low Earth orbit (VLEO). At this altitude, satellites experience enough drag to require active thrusters, which means they won’t stay in orbit for long if propulsion fails. An interesting benefit of VLEO is that it can allow for quicker communications due to lower latency.
Satellites designed for VLEO look quite different from traditional ones. They require more aerodynamic designs, similar to aircraft. This hybrid approach helps them deal with the unique challenges of low Earth orbit.
The European Space Agency (ESA) led the way with the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE), launched in 2009. GOCE operated at an altitude of about 255 kilometers (158 miles) and had to continuously power its ion engine to maintain its position due to significant drag. It wasn’t just a typical satellite; its design resembled a torpedo with solar panels integrated into its structure.
GOCE’s mission lasted four and a half years, ending when it ran out of xenon propellant. After that, it quickly descended and burned up upon re-entering the atmosphere.
Another significant VLEO mission comes from Japan’s Aerospace Exploration Agency (JAXA). In 2017, JAXA launched the Super Low Altitude Test Satellite (SLATS). Over time, SLATS was brought down to 167.4 kilometers (104 miles) using its ion engine and chemical thrusters to counteract drag. In terms of size, SLATS was smaller than GOCE but also required more energy to maintain its orbit.
Once SLATS turned off its propulsion in 2019, it fell to Earth within a day. This mission has since been recognized for achieving the record for the lowest operational orbit for an Earth observation satellite.
The successful operation of GOCE and SLATS shows the potential for a new class of satellites. Future designs may focus exclusively on VLEO, utilizing air-breathing electric propulsion systems. These hybrid vehicles could maintain altitudes as low as 90 kilometers indefinitely, as long as they generate enough solar power.
Research from agencies like DARPA and ESA aims to explore these innovative propulsion systems. Projects like Redwire’s SabreSat highlight this trend. If successful, these VLEO satellites could alleviate future congestion in space, ensuring our continued use of space remains sustainable.
