SpaceX is about to test a system aimed at overcoming the communications blackout that happens when a spacecraft re-enters Earth鈥檚 atmosphere from orbit.
Almost every spacecraft that has returned to Earth has suffered a break in communications during re-entry as the atmosphere slows it down. The same friction that decelerates the spacecraft also heats the air below the craft until it ionises into a glowing, conductive plasma. This quickly forms a sheath around the spaceship that blocks radio frequency signals.
Historically, blackouts have lasted several minutes, during which the fate of any astronauts on board was unknown. In the ill-fated Apollo 13 mission, there was an agonising 6-minute blackout on its emergency return to Earth. Blackouts still happen today: astronauts leaving the International Space Station on SpaceX鈥檚 Crew Dragon capsule can lose communications for up to 7 minutes.
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The problem isn鈥檛 limited to spacecraft. Any vehicle travelling at speeds above Mach 5 will experience similar communications issues, including hypersonic missiles. .
Now, SpaceX thinks it might have a solution. Instead of trying to punch communication signals through hot plasma down to the ground, it will send signals up to orbiting satellites instead.
Sometime in the next few months, a 70-metre-tall Falcon 9 rocket will blast off from Cape Canaveral in Florida with a Starlink system on its second stage. This won鈥檛 be another satellite to join the approximately 5300 already in orbit, but a standard user terminal to access them, modified to fit in the rocket.
According to a filed with the US Federal Communications Commission, the experiment will start once the Falcon 9 deploys its commercial payload and will continue until the second stage burns up on re-entry.
鈥淪paceX鈥檚 satellite constellation can provide unprecedented volumes of telemetry and enable communications during atmospheric entry when ionized plasma around the spacecraft inhibits conventional telemetry frequencies,鈥� reads a . 鈥淭hese tests will demonstrate its ability to improve the efficiency and safety of future orbital spaceflight missions.鈥�
The previous success of a similar idea for NASA鈥檚 hints the technique could work for SpaceX, at least in some situations. From the late 1980s, the space shuttle avoided re-entry blackouts of up to a quarter of an hour by connecting to one of NASA鈥檚 , a network that provides communications to many NASA spacecraft.
That NASA satellite was in a distant geostationary orbit 鈥� meaning it appeared to be fixed in the sky because its orbital velocity matched Earth鈥檚 speed of rotation. The Starlink satellites are in much closer low-Earth orbits, however, where their orbital velocity whips them across the sky in minutes. This means the Falcon system will have to switch from one satellite to another in quick succession.
Even if the experiment works on the second stage, which is about 14 metres long, it may not translate to the smaller , says , a professor emeritus at the University of Arizona who has worked on technology to prevent blackouts. 鈥淭he space shuttle was humongous,鈥� he says. 鈥淧lasmas are notoriously fickle in the sense that if you have a little change in shape, you get pretty large changes in the plasma.鈥�
While the problem of communicating with craft moving at very high speeds isn鈥檛 trivial, the SpaceX experiment is definitely one worth trying, says Ziolkowski. 鈥淚t鈥檚 a reasonable go at understanding the problem better.鈥�
SpaceX hasn鈥檛 responded to a request for comment.
