The skies may soon be full of unpiloted solar-powered planes, employed by the military to surreptitiously watch what is happening far below. But in an era in which private companies like SpaceX can launch more than 3000 satellites into orbit, with thousands more planned, why would countries or military establishments rely on aircraft for this?
A clue comes from the US Central Command鈥檚 admission that there are , following a classified surveillance and reconnaissance conference at a US Air Force base in Tampa, Florida, in October.
While satellites once had the advantage of being invulnerable to enemies, it is now thought that the US, China and Russia all have systems that can take them down, which is one reason countries are seeking alternatives. Such anti-satellite weapons have been tested, but aren鈥檛 thought to have been used aggressively.
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The conference wasn鈥檛 open to the public, but what we do know is that one of the options on the table was a lightweight, slow-flying solar plane that can stay aloft for weeks or months and effectively act as a geostationary satellite, which seems from the ground to have a fixed position in the sky, at a fraction of the cost.
Staying in the air for so long is no longer pie in the sky. In August, the record for uncrewed flight duration was smashed by exactly that sort of aircraft, the Zephyr, made by Airbus. In tests over Arizona, this 75-kilogram vehicle traced out a series of circuits and patterns above 60,000 feet during a 64-day flight. China is testing a similar craft called .
at Brunel University London says one reason to focus on solar-powered planes is adaptability: they can be moved from place to place at will, whereas satellites take time and planning to move to a new orbit.
鈥淭hese drones are pseudo-satellites that offer you greater operational flexibility than a satellite to respond to situations,鈥� he says. 鈥淔or example, real-time imagery that can be used to plan operations and monitor the activities of adversaries. Such capability makes sense for China, for example, if it鈥檚 monitoring sea lanes in the Pacific region. Having something like the Zephyr gives you lower costs per flight, or task, and greater flexibility.鈥�
This means large numbers of solar aircraft could be deployed at once for the same cost as a satellite. For instance, the research and development contract for developing one solar aircraft programme was just $5 million, while the generation of spy satellites thought to be in operation now was expected to at the turn of the century.
at the University of Warwick, UK, says solar planes may be less exciting than other military tech like supersonic spy aircraft, but they have ideal characteristics for surveillance. 鈥淭hey鈥檙e just up there in perpetuity because they鈥檙e solar powered and there are never any clouds [at that altitude],鈥� he says. 鈥淚f you鈥檝e got something that high, it might also actually be totally invulnerable because even if you can see it, you can鈥檛 do anything about it.鈥� Air defence systems are only good for strikes on things below about 30,000 ft, he thinks.
Solar-powered planes are also better suited to the task than conventionally powered drones, say, which need to regularly return to base to refuel.
Solar planes could be useful for other purposes too, which may see them deployed more widely.聽 at Airbus says aircraft like Zephyr could deliver mobile phone connections and internet access. 鈥淪atellites will always have to contend with the very large distance between user devices and the satellite,鈥� he says. 鈥淚n order to achieve any meaningful bandwidth, either the satellite or the antenna on the ground have to be very large. This is not always possible, practical or economical.鈥� Solar planes may be a decent alternative because they aren鈥檛 nearly so far away.
