(BBC News) There is a new race in space, but it is not where you might think. It is happening close to home – in the nearest bit of space, right on the edge of Earth’s atmosphere.
Just above the boundary where space begins, companies are working to create a new class of daring satellites. Not quite high-altitude planes and not quite low-orbiting satellites, these sky skimmers are designed to race around our planet in an untapped region, with potentially huge benefits.
Roughly 10,000 satellites are orbiting our planet right now, at speeds of up to 27,000 km/h. Every one of these delicate contraptions is in constant free-fall and would drop straight back down to Earth were it not for the blistering speeds at which they travel. It is their considerable sideways momentum, perfectly stabilised against the Earth’s gravitational pull, that keeps satellites in orbit.
A new class of satellites is aiming to push the limits of this balancing act and plough a much more precarious, lower orbit that would skim the top of Earth’s atmosphere. Known as Very Low Earth Orbit (VLEO), spacecraft at these altitudes have to battle against the significantly greater drag from the air in the upper reaches of the atmosphere than their loftier cousins, lest they get pushed out of the sky. Should they manage it, however, such satellites might achieve something even more jaw-dropping – they could potentially fly forever.
“When you start describing it to people, it starts to sound like a perpetual motion machine,” says Spence Wise, senior vice-president at Redwire, an aerospace firm in Florida. A perpetual motion machine is not meant to be possible. But it almost is, in this instance.
A handful of pioneering companies have begun work on designs for satellites that may be able to orbit the planet at these unusually low altitudes while simultaneously harvesting air and using it to make propellant – literally on the fly. This new generation of orbiters could enable ultra-high-definition surveillance of activities on the ground, or superfast satellite-based communications.
If you want to send something into orbit, you have to decide how high your satellite is going to fly. Earth orbits are generally described in terms of altitude and are categorised into different sections. The highest, at some 36,000 km and above, is called High Earth orbit. Here, satellites enter a geostationary position, meaning they are always above the same location on Earth below. This is useful for telecommunications and weather monitoring, for example. Next is Medium Earth orbit, which spans from roughly 36,000 km down to 2,000 km above the planet’s surface. Below this is Low Earth orbit, which stretches down to altitudes of 400 km, where the International Space Station (ISS) is found.
Even further below this lies VLEO, loosely defined as anything below the ISS and down to an altitude of about 100 km. Operating here is difficult because of the influence of Earth’s atmosphere. “The atmosphere will increase exponentially as you come down,” says Hugh Lewis, a professor of astronautics and a space debris expert at the University of Southampton in the UK.
That creates more drag on your satellite, which can spell doom. As molecules in the atmosphere smash into the satellite, they rob the vehicle of its momentum, causing the tug of our planet’s gravity to drag it towards the ground.
A satellite left in medium Earth orbit or above would carry on circling our planet for millennia. In VLEO, however, it would last barely months, weeks, or even days depending on its speed, shape and mass, dictating the amount of drag it produces and thus its lifetime. Once a satellite dips to an altitude of about 100 km, the end is imminent. The intense friction created by the thicker atmosphere subjects the satellite to temperatures of thousands of degrees, ultimately tearing it apart.
All satellites pass through VLEO on their way up or down, but not many have purposefully tried to stay there. One such spacecraft, however, was the European Space Agency’s Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite. Launched in 2009, it orbited at an altitude of around 250 km, using an ion propulsion system to fire out charged particles behind the spacecraft. This gave it a constant level of thrust that could counteract the drag of the atmosphere.
GOCE was intended to measure Earth’s gravitational field with extreme precision, which it achieved. But it also demonstrated the design choices that were necessary for operating in VLEO. It had a sleek, elongated form that helped it overcome atmospheric drag. “It looked like a dart,” says Lewis. GOCE ultimately ran out of fuel and burned up in the atmosphere on re-entry in 2013.
Several companies are now trying to do something even more impressive. They are developing technology to harvest molecules from the thin layer of air that is present in VLEO in order to actually propel satellites here. Such a system, called Air-Breathing Electric Propulsion (ABEP), has been made possible by advancements in electric and ion propulsion in recent years. In essence, it involves fixing a large bucket or opening to the front of the satellite, into which gas molecules from the atmosphere flow before they are ionised to create plasma that generates thrust.
“The idea is to use the same air slowing down your satellite as a propellant,” says Francesco Romano, a scientist at the Swiss Plasma Center in Lausanne, Switzerland. Using electric and magnetic fields, the engine would ionise gas from the atmosphere, taking away one electron from each molecule, to produce a free electron and an ion. Then, using magnets, the electrons and ions are pushed out the back of the spacecraft, producing thrust. “Theoretically, if you can generate a thrust that is the same as your drag, you stay at this altitude for an infinite amount of time,” says Romano.
To date, an assortment of experimental ABEP systems have been able to produce relatively small amounts of thrust at ground level, but their feasibility in orbit has yet to be properly tested.
https://www.bbc.com/future/article/20250207-sky-skimmers-the-race-to-send-satellites-into-very-low-earth-orbits
