VLEO?

I couldn't begin to guess how many times we've referred to Low Earth Obit or LEO over the years. I'm sure there's a precise definition somewhere, but I've come to conceptualize LEO as starting at the minimum height that can remain stable for some amount of time, probably more like months than years. This weekend was the first time I've seen the term Very Low Earth Orbit. 

LEO is getting crowded, but there’s plenty of real estate available closer to home if you can make it work. California-based Viridian Space is aiming to do just that.

The company is developing air breathing electric propulsion (ABEP) technology to fly future satellites in VLEO, and the US government is lending a hand to get the tech off the ground.

Viridian Space CEO Slava Spektor announced that the company has signed a five-year cooperative research and development agreement (CRADA) with the US Air Force to co-develop Viridian’s ABEP tech, and to work together on studies leveraging VLEO. The CRADA will be run out of Albuquerque, New Mexico's, Kirtland Air Force Base. The Air Force will assist Viridian with testing and characterizing the power systems they develop, while Viridian will concentrate on the technology and how it can be used to create new operational concepts for the US military. 

Viridian is expecting to launch their first mission within three years. Among its first priorities will be to collect data on the atmospheric conditions in VLEO, offering the DoD a better understanding of atmospheric drag at different altitudes, and how space weather events impact satellite life spans.  

VLEO has the obvious advantage that being miles closer to the ground than higher orbits, it will offer faster up and downlink speeds, and less optical magnification will be required to image targets, but it goes beyond that. Their air breathing electric proposal isn't just replacing the oxidizer in a chemical engine with oxygen taken out of the air, the air itself is the fuel they'll use. No, the short article on Payload offers no details on that. I'll get back to that in a little while.

Viridian’s aim is not just to get around the growing traffic in LEO, but to create a satellite system that is more capable than chemical prop systems flying in higher orbits. An ABEP sat in VLEO has multiple benefits, according to Spektor: [NOTE: read prop as propellant - SiG]

• Viridian’s sats are aiming to stay in orbit for up to 10 years. Because the system is designed to use air as fuel, the sat will be able to maneuver without worrying about burning through onboard fuel reserves.
• Viridian also expects its sats to provide more maneuverability to its customers. The sats are expected to fly at altitudes as low as 150 km, and to dip in and out of the atmosphere to reach multiple orbital altitudes on a single flight.

Viridian's website offers a link to a paper, but there's nothing there. Some searching led me to a more useful site, which links to a pdf version of a paper published in Acta Astronautica in 2022. The top-level introduction says this about the fuel.

The concept is based on the ingestion of rarefied atmospheric particles to be used as propellant for an electric thruster, thereby removing the need for onboard propellant

Viridian offers this diagram of the engine, but no other details. 

Viridian's ABEP engine conceptual diagram. Image credit: Viridian Space

While I'm always suspicious of things that might come across as "something for nothing," the engineers at Viridian have been working on this for years and I think it's worth experimenting to see if it lives up to the experimental results and analysis so far.