DARPA Will Fly Satellites at Lower Orbits

DARPA wants to fly satellites closer to the earth where there is more drag and resistance. The thermosphere (90 km or 56 miles) up to 500 and 1,000 km (311 to 621 miles) above our planet. DARPA wants Very Low Earth Orbit (VLEO) as orbits less than 450 km, or roughly 280 miles, in altitude. Low Earth Orbit starts around is 2,000 km or 1,200 miles but SpaceX Starlink satellites go as low as 550 km (342 miles). The benefits of being in Very Low Earth Orbit are improved resolution for optical imaging, higher signal-to-noise ratios for radar and lidar systems, improved geospatial position accuracy.

It becomes about ten times harder to maintain an orbit of 450 kilometers versus 550 kilometers.

There have been Super Low Altitude Test Satellites (SLATS). Tsubame was a JAXA satellite intended to demonstrate operations in very low Earth orbit (VLEO, below 200 km), using ion engines to counteract aerodynamic drag from the Earth’s atmosphere. It had 4 newtons of thrust for its ion engine and stayed in orbit for about 2 years. Super Low Altitude Test Satellites were twice as close the where DARPA wants to fly.

DARPA must want to achieve years of operation without using a lot of fuel. Tether wires can be used to generate electricity while passing through the atmosphere.

NASA and Italy had the space tether experiment. A large, spherical satellite deployed a conducting cable (tether) 20 km (12.5 miles) long. The shuttle would drag the tether across the Earth’s magnetic field, producing one part of a dynamo circuit. On February 25, 1996, the experiment began as planned. Miles of tether unrolled while the observed dynamo current grew at the predicted rate. The tether broke and ended the experiment. Electric current had melted the tether. Later vacuum-chamber experiments showed that unwinding of the reel uncovered pinholes in the insulation. This would not have initially caused a major problem, because the ionosphere around the tether, under normal circumstance, was too rarefied to divert much of the current. The air trapped in the insulation bubbled out of the pinholes, the high voltage (“electric pressure”) of the nearby tether, about 3500 volts, converted it into a plasma. The instruments aboard the tether satelite showed that this plasma diverted through the pinhole about 1 ampere, a current comparable to that of a 100-watt bulb but at 3500 volts.

Additional Info from Nextbigfuture Commenter Asteroza

DARPA daedalus program for VLEO is the project name.

There was an earlier low altitude GOCE satellite.

Electrodynamic tethers would be helpful for propellantless propulsion, but those favor an equatorial orbit, while earth observation sats prefer polar/SSO orbits.

Likely candidate for Daedalus is Earth Observant’s Stingray VLEO craft, being built for the USAF spaceWERX incubator program and also being built commercially. Uses a 45 degree mirror, sorta similar to old Coronoa spysats

https://eoi.space/

Blue Canyon might also try.

ESA is leading the Skimsat group for a VLEO demo.

ESA is also studying air breathing electric propulsion (ABEP) to augment VLEO sats with no-refueling propulsion.

Others, like Albedo, have committed to refueling VLEO sats as a strategy.

KAIST (Korea) is looking at something similar to Earth Observant as well for a demo VLEO sat.

A korean outfit, skeyeon, wants to try a slimmer profile VLEO sat using concentric optics to compress the cross section of a downward pointed telescope.