Back in February an Indian Space Research Organization rocket deployed over a hundred miniature spacecraft into Earth orbit. This constituted the largest stream of petite satellites, called CubeSats, ever dispensed into space courtesy of a single heave-ho booster—but not by much. Seventy more are slated to hitch a ride onboard a Russian rocket later this week. In launches large and small, on the order of 700 CubeSats have made their way into space since the late 1990s, and the pace is picking up.
CubeSats are a distinctively low-cost class of nanosatellite that can weigh less than three pounds and come in a standard size and shape. The average CubeSat size is "one unit" or "1U" measuring 10 X 10 X 10 centimeters, but can be upsized to 1.5, 2, 3, 6 or even 12U. This flexibility makes CubeSats a bargain compared with full-size satellites, with a wide range of costs. For instance, a simple CubeSat built by students or hobbyists might cost roughly $50,000 whereas more advanced projects from professional aerospace companies can range from $250,000 up to $2 million.
A CubeSat's utility in space seems limited only by its size and the imagination of its designers and users—governments, universities and private companies increasingly rely on them for everything from broadband remote monitoring of Earth to performing on-orbit science experiments as well as test-flying novel propulsion and communications technologies. [CubeSats: Tiny, Versatile Spacecraft Explained (Infographic)]
As the number of CubeSats and other orbiting nanosatellites continues to rise, so too do debates about their most important effects: Are CubeSats really the vital educational, scientific and technological tools that their staunchest proponents insist they are — or are they mere indulgent toys irresponsibly adding to the menacing shell of litter already encircling the planet?
It is already a well-known mess. Orbital debris is the term for any object in Earth orbit that no longer serves a useful function. This clutter of the commons is a mix of dead spacecraft, spent rocket stages, steel bolts, paint chips and even frozen bits of nuclear reactor coolant. A hypervelocity run-in with space rubbish can take out or cripple an expensive satellite or even threaten the safety of a vehicle's crew.
A recent study led by Hugh Lewis, an aerospace engineer at the University of Southampton in England, reports the growth of small satellite traffic is expected to have a "significant impact" on the space environment. "From my perspective, there are two sides to the story…at least," Lewis says. "CubeSats are being launched in greater numbers partly because they represent a cost-effective solution to an engineering or commercial problem. In particular, there are growing demands for Earth observation data and communications, which can be met through the use of CubeSats at relatively low cost. The challenge for regulators is to develop and apply rules that enable these services and benefits to be realized in a sustainable manner. I think it is fair to say that this is not an easy task." Lewis's study suggested a need to consider additional space debris mitigation guidelines for CubeSats.
Under an international agreement called the "25-year rule" countries acknowledge they should not launch objects whose life span in Earth orbit will go beyond 25 years after their mission concludes. Compliance, however, is voluntary; countries can choose to implement it (or not) for launch vehicles lifting off from their territories.
The U.S. government does not approve any launch violating the 25-year deorbit rule. For U.S.-launched CubeSats, this usually means placing them in an elliptical or low-altitude orbit from which they will drift downward and burn in the atmosphere after seven years or less. But the big picture—for both big-time and small-time space players—is there is no enforcement of this rule. Evidence from the past decade has shown satellite operators to have a "patchy record" of compliance, at best.
The study advises that enforcement could help mitigate the impact of small satellites on the space environment—but the researchers add a caveat: Imposing restrictions on small satellite missions might inhibit creativity and perhaps have a cost impact that could derail the commercial viability of missions. [Worst Space Debris Events of All Time]
CubeSats have proliferated because of their low cost, says Holger Krag, head of the European Space Agency's Space Debris Office based in Germany. Krag adds, however, this is not a problem as long as everyone follows space debris mitigation guidelines like the 25-year rule. "Our simulations have shown that even [an] increased amount of CubeSats can be absorbed by the environment if launched into low-enough orbits," Krag says. "Of course, there is a natural limit to this, which has not been quantified as yet, and increased traffic in low altitude means more interference with the operations of the International Space Station."
NASA and the international community are concerned about the environmental damage CubeSats could cause, says Don Kessler, a retired NASA senior scientist whose name will forever be enshrined in the pantheon of space junk specialists. In 1978 Kessler detailed how debris-creating satellite collisions and explosions in orbit could kick off a chain reaction that exponentially increases the amount of space junk whirling around the planet. Such a "Kessler syndrome" could easily render spaceflight too hazardous to conduct.
Kessler says CubeSats' small size is a plus, because they simply contain less material that could be transformed into spacecraft-threatening debris. But this is offset by other factors including their great numbers and their limited reliability and maneuverability. Their proliferation may require new mitigation rules, particularly because some might be too small to be adequately tracked from the ground. "The services they could provide at a low price are positive attributes," he says. "But they also represent a new challenge to maintaining a sustainable environment." On one hand, the spacecraft community is aware of these challenges and is attempting to address them. On the other, "my additional concern is that the modeling of the large constellations of CubeSats is inadequate," he adds, referring to computer simulations used to predict and analyze potential technical problems.
Although at least one model includes space junk items as small as one centimeter, most long-term models predict growth only in the 10-centimeter and larger category, Kessler explains. "However, CubeSats are highly vulnerable to much smaller debris, with collisions resulting in even more nontrackable debris and potentially contributing to cascading within the constellation as well as with any nearby large constellation." Kessler notes he has already voiced his concern over this issue. [Watching a Spacecraft Wreck Tumble Through Space]
There has been a noteworthy increase in the use of CubeSats and other small satellites for a variety of space missions, says George Nield, associate administrator of the Federal Aviation Administration's Office of Commercial Space Transportation. And for good reason, Nield says: because of their lower costs, CubeSats offer a major opportunity for increased innovation. They can fly more frequently and for less money to hone novel concepts, potentially unlocking new products, services, customers and markets faster and cheaper than any traditional satellite system. "However, it is important to note that there are several challenges that need to be addressed in order to take full advantage of those potential benefits," he suggests. "For example, because of their size the satellites themselves can be difficult to track, and they are frequently not maneuverable. At the same time, because they are intentionally designed to be low in cost, most of these satellites have limited or no redundancy and only marginal reliability." As a result, many of these systems become orbital debris shortly after they are launched, he says. "If we want to be responsible stewards of the space environment," he adds, "we need to think about how we can enable these systems to operate in space while ensuring that they do not become hazards to other space operators." Nield points out that some ideas already exist for minimizing debris risks from the proliferation of CubeSats, such as:
—Encouraging the use of orbital altitudes and inclinations that decrease the chance of collisions with the International Space Station and other high-value facilities.
—Identifying the best methods for reducing CubeSat orbital lifetimes.
—Adding beacons, transponders or corner reflectors to each spacecraft so they become easier to track.
CubeSats offer a number of challenges, says Brian Weeden, a former officer in the U.S. Air Force with a focus on space security and current director of program planning for the Secure World Foundation.
First, CubeSats are now relatively hard to track and conclusively identify. Often deployed in clusters, most lack distinguishing features and have limited or no maneuverability to actively avoid collisions, Weeden explains. Moreover, CubeSat operators may have little to no experience with using satellites, he continues, noting some CubeSats are being launched by countries that may not have much national regulation or oversight in place. He thinks the identification challenge is more pressing than the tracking one—although the two are interlinked. A 10-centimeter CubeSat is trackable with existing sensor networks, he says, "but if you dump dozens of them out at the same time, it can be really tough to tell which one is which."
Several ways to establish satellite "ID tags" have been proposed, Weeden says, but he is unaware of any effort to assess which ones are the most effective or to develop them into working solutions. "I'm skeptical of getting much progress from the government side in the near term," he says. "The zeitgeist in Congress and the [Pres. Donald] Trump administration right now is all about getting rid of regulations and reducing the size of government. In fact, there have been prominent members of Congress openly questioning whether the U.S. government needed to provide any regulation of space activities at all." That said, Weeden adds that the CubeSat and commercial space community seems to be taking these issues seriously. This community includes CubeSat operators who are "keen to be responsible space actors," he says.
T. S. Kelso, a senior research astrodynamicist for Analytical Graphics' Center for Space Standards and Innovation, views CubeSats as a positive development — especially in encouraging university participation and stimulating creativity. "And I think some of the 'big data' applications have the potential to be truly game-changing for our society," he adds.
Kelso notes that the creators of the CubeSat standard chose the 10-centimeter unit size based largely on what the community understood the Pentagon's Space Surveillance Network could track—because knowing a satellite's location is key to using it. "Unfortunately, the Department of Defense resistance to discussing capabilities and pointing out the difficulties of tracking objects of this size has left us in a situation where this is still a challenge," he says.
When that challenge is combined with projects that deploy 30 or more CubeSats, "trying to associate observations with tracks and generate good orbits becomes even more difficult," Kelso explains. It can take military space watchers weeks to sort out each CubeSat's orbit, he says, and failure to be able to track and identify objects can prevent operators from being able to perform key activation tasks and result in the loss of a satellite. "Unfortunately, most operators are unaware and unprepared for this problem, since they believe that the U.S. has it 'all under control.' They often don't understand the problem until after launch when they are scrambling for help," he says. "The community needs to be informed of this challenge, and then work together to find creative ways to address it. And the U.S. government—or even nongovernmental entities that operate large networks of satellites—might want to fund use of a solution to prevent these educational tools from simply adding to the debris population."
For now CubeSats' popularity is clearly on the upswing. First used as teaching tools and for technology demonstrations, their utility to perform more complex science duties and serve as the backbone of commercial services is gaining traction. What remains to be seen is whether or not their proliferation adds to the heavenly headache of dealing with the escalating hazard of Earth-orbiting debris.
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