Companies are shifting away from traditional large satellites towards small satellites. cubesats in LEO have been increasingly used in space since the 1990s. In recent years, advances in camera technology and computer miniaturization have allowed companies to do optical imaging or radar observations using smaller and smaller satellites. This has led to the present day, where companies ranging from OneWeb to SpaceX and Planet have been deploying large fleets of satellites (fleets that could eventually include thousands of individual satellites) for applications ranging from telecommunications to Earth observation.
LEO presents a number of advantages. By being in a lower orbit, satellites can, despite their smaller sensor size, have better resolution and detection as well as shorter transmission delays between space and Earth. Less power is also needed to transmit signals to and from LEO as opposed to higher orbits. Small satellites in fleets are also easier and cheaper to launch than single large satellites sent to a higher orbit. Moreover, it is easier to replenish and update a constellation of satellites as technology changes.
For surveillance purposes, LEO also allows for satellites to flow in orbit around the world, instead of being fixed in one spot. That might be a natural fit for the United States military, Northern Sky Research analyst Brad Grady told Space.com. "Because the U.S. is everywhere, it has a global footprint, global scale and interests all over the world," he said. "That global requirement is creating an LEO-based architecture."
And the U.S. military is paying attention, as both the military and the Defense Department's new Space Development Agency (SDA) are assessing using constellations of LEO satellites for applications that could include communications, advanced missile defense or an alternative navigation system to GPS, George Nacouzi, a senior engineer with the RAND Corporation, told Space.com in an email. The Defense Advanced Research Projects Agency's (DARPA) tactical technology office also started a project in 2018 called Blackjack, which seeks to test how useful LEO constellations could be for the military.
"If done properly, LEO satellites would not necessarily pose a security risk. It would actually increase the resilience of the system — again, if done properly," Nacouzi said. "On an individual basis, these satellites may be more vulnerable since they're easier to access. However, their large number and ease to replace and replenish gives them a measurable advantage over GEO [geosynchronous satellites]," he said.
Military LEO satellite fleets might not be created right away, as one of the military's future constellation plans still calls for a fleet of geosynchronous-based satellites. The Next Generation Overhead Persistent Infrared (OPIR) system is currently in the design phase and the first delivery is expected by 2025, Nacouzi said. Additionally, the SDA and the Defense Department's Missile Defense Agency are eyeing a hypersonic and ballistic tracking space sensor that could include a constellation of small satellites in low Earth orbit, Nacouzi added.
The rise of commercial constellations presents another option to the military to rent space on these satellites (a cheaper option than buying and building them), as long as security is not compromised.
"Space Force is one of those catalysts of new thinking," Brady said, referring to the new branch of the U.S. military that is focused on the space domain. "This enables it [the military] to say, 'I don't need to own it in order to trust it. There are other ways I can verify the accuracy of this information.' "
One example of renting space on a satellite could take place in commercial satellite communications, RAND's Nacouzi said. He pointed to a "more concerted effort" to leverage commercial space capabilities under national space policy guidelines.
There are challenges to LEO, however.
For satellites in LEO, sensor resolution isn't as high as the geosynchronous satellites and the field of view is smaller, which can be a disadvantage if you want to cover the large swath of territory geosynchronous satellites can see from a higher orbit. Low Earth satellites also operate in constellations, which require handover periods if you're trying to have stable communications. Luckily, that technology has matured as a big player in the space industry, Iridium, is already doing it, Nacouzi said.
One of the worst-case scenarios for a set of LEO satellites could be something like high-altitude nuclear detonation that would knock out a large set of satellites at once. Nacouzi said that this is a "doomsday" scenario that, of course, the military would discuss as it is the military's job to keep operating under adverse conditions. In this case, he added, one way of preventing issues in this dire, possible situation would be making sure LEO constellations are not the only way of executing a critical mission.
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