It was Jan. 11, 2007 when China launched a ballistic missile at its defunct weather satellite, Fengyun 1C, setting off an explosion that marked the largest manmade creation of space debris in the history of space exploration. The destruction sent more than 3,000 pieces of trackable debris and an estimated 150,000 debris particles encircling the Earth.
China’s antisatellite missile test wasn’t the first of its kind. The United States had conducted a similar test in 1985, when it used an antisatellite missile to destroy its Solwind P78-1 satellite at 555 kilometers altitude. Nevertheless, China’s destruction of Fengyun 1-C occurred at about 850 kilometers altitude, well above the Earth’s atmosphere, ensuring that most of the fragments would remain in orbit for decades.
While historic, the Fengyun event is just one conspicuous feature of an increasingly complex international problem, one with no easy solution. The space debris population has grown in size since the earliest days of spaceflight, when the U.S. Naval Research Laboratory's (NRL's) Minimum Trackable Satellite (Minitrack) receivers began tracking the first manmade objects to go into orbit—Russia’s Sputnik in 1957, then NRL’s own Vanguard satellite in 1958.
Today, the operational community is tracking more than 20,000 objects larger than 10 centimeters in the official satellite catalog, while the estimated population of objects between one and 10 centimeters in diameter runs to 500,000, according to NASA’s Orbital Debris Program Office. Though the probability of a collision between a spacecraft and debris today remains incredibly low, such a collision with an object as small as a paint fleck has the potential to disrupt or even end a space mission.
The rate of collisions also has the potential to increase exponentially, according to retired NASA scientist Donald J. Kessler, who founded the Orbital Debris program office. In 1978, he proposed a theory now known as the "Kessler Syndrome" that debris generated by a collision could cause even more collisions, leading to a runaway cascade of collisions that would create a massive debris field in low Earth orbit, posing a real threat to the future of space operations. The catastrophic consequences of such a series of events were dramatically illustrated in the 2013 movie Gravity.
At NRL, aerospace engineers, astrodynamicists, research physicists, and others are working in concert with the Air Force, NASA, and other partners to approach the problem in a number of ways, beginning with NRL’s Mathematics and Orbit Dynamics Section. There, researchers are collaborating with government partners and the private sector to track orbital debris while preparing for the completion of a new “space fence” that will radically transform our view of orbital debris from Earth.
Meanwhile, engineers with NRL’s Geospace Science and Technology Branch are developing a space-based optical instrument that can detect orbital debris. At the Blossom Point satellite tracking facility in Maryland, NRL’s high-precision, orbit-determination software is contributing to reliable “lights out” operations that ensure spacecraft can maneuver in orbit safely.
All of this work represents a continuation of NRL’s long tradition of research and development for space operations, which extends back to the creation of the Minitrack system and the launch of the Vanguard satellite, today the oldest manmade object in space.
The Big Data of Space Debris
The U.S. Air Force maintains two operations centers that manage the satellite catalog (SATCAT), the catalog of 23,000 well-tracked objects in Earth orbit that includes both debris and controlled objects. One operations center resides at Vandenberg Air Force Base, California, and the other in Dahlgren, Virginia, at the site of the old Navy Space Surveillance Center; today it’s the Air Force’s Distributed Space Command and Control- Dahlgren.
For decades, NRL has worked with the operations center in Dahlgren, developing software products and advanced methods for space situational awareness (SSA). These days, researchers with NRL’s Mathematics and Orbit Dynamics Section are helping the Air Force harness the potential of the ever-growing trove of private-sector SSA data collected by telescope networks and radars operated by universities, small startups, and other commercial entities.
“In the past, the government produced its own data using the Space Surveillance Network,” said Christopher Binz, aerospace engineer with the Mathematics and Orbit Dynamics Section. “There was no concept of even a commercial provider of this data in the first place.”
In recent years, this small industry has grown up around commercial SSA data, selling it mostly to the government but also to private companies. According to NRL research physicist Liam Healy, they’re only now beginning to understand its value. Healy and other NRL researchers are helping to evaluate the industry itself, its sensor infrastructure, and the data it’s producing for the Air Force’s space acquisition agent, the Space and Missile Systems Center.
“I view it as being a consumer reports for the government purchaser,” Healy said. “We need to evaluate what vendors have. But if it doesn’t already exist, we need to do the research and development or sponsor the R&D to get to the point where we have a good idea of what we should purchase.”
To do that, they're drawing on an asset that’s unique to NRL: a complete database of the existing satellite catalog. According to Alan Segerman, who heads the Mathematics and Orbit Dynamics Section, the database is a product of NRL’s long history of partnering with the operational community, compiled in fits and starts over a period of several years. It allows NRL researchers to test algorithms and run comparisons using the actual SATCAT data, rather than simulated data.
“Historically, we’ve developed a lot of the tools [used by the operational community], and from time to time we would get copies of the catalog,” Segerman recalled. “Eventually, we were able to develop the infrastructure to collect the data as a copy of the operational archive every day.”
At NRL’s Space Situational Awareness Laboratory, they maintain the database of the operational catalog and Space Surveillance Network data in one server, and a database of the private sector data in another—just as is the case at the operations centers. Neither have integrated the two sets yet.
“We keep them separate, but we certainly have the ability to work with them together,” Segerman said. “That’s part of the analysis that we do.”
A more recent area of their work today involves preparing for the completion of the Air Force’s new space fence on Kwajalein Atoll in the Marshall Islands. A replacement for the Naval Space Surveillance System, space fence—which the Navy transferred to the Air Force in 2004—has been years in the making. It’s forecast to become operational in 2019 and is expected to dramatically increase the number of objects we can track in orbit.
Right now, the Space Surveillance Network and the private sector can track objects in low Earth orbit that are approximately 10 centimeters in diameter and larger (the size limit of trackable objects grows larger the farther away you get from the Earth). A new fence, however, could reduce that size limit to single digits of centimeters.
The new fence also will use a mode of communication called netcentric data, rather than the Space Surveillance Network’s current aging mode of communications, the 9600 BAUD modem, which was state of the art decades ago.
“The new space fence’s data will still be radar observations, but it will come in a different form and contain more information than what the Air Force is used to processing,” Segerman noted.
Segerman expects that when the new fence comes on line later this year, the number of warning messages of potential collisions will likely increase.
“Every time a better sensor like this new space fence is built, we can track smaller and smaller things,” Segerman explained. “That means the tracked debris population increases—not because there’s anything more up there, but because we’re able to track more.”
It’s still unclear how the completion of the new fence will affect the nascent private sector industry, but no one thinks it will put private sensors out of business. Healy believes that, even though the new space fence will bring greater precision and increase the population of objects we can track, the Department of Defense will still need privately produced data.
“There are some smaller objects that only the new space fence will be able to detect initially,” he said. “We may recognize that we can use data from the commercial vendors to get more observations of these objects.”
Binz speculates that the previously unknown smaller objects discovered by the new fence could push the industry away from using telescope networks and toward adopting radar technology to track them.
“There’s just one company now doing commercial radar,” Binz said. “More of those might pop up. Or companies might start building more powerful radars.”
But the marshalling of private sector SSA data isn’t just about observing objects we can’t see, Segerman explains; it’s also about increasing the frequency of observation of such objects, thereby freeing up government sensors to collect other data.
“The Air Force may have sensors that can already track these objects, but if we know the [private] vendor can provide good data on them, too, we can use our sensors to detect other objects that nobody else can,” Segerman said. “And we can make that our priority instead.”
Today, the Air Force’s Space Surveillance Network has approximately 25 sites around the world with ground-based radars and optical sensors continuously collecting space situational awareness (SSA) data, which it makes available to the public online at Space-track.org. The network, however, is pumping out all the data it can—it’s operating at capacity. And even with the completion of the new space fence, we still won’t be able to see everything.
“There will still be the possibility that you’ll be unable to detect something that could take out your spacecraft,” Liam Healy said. “So then the question is: What’s your known risk from tracked objects and what do you do about unknown risks? For the International Space Station, they took the approach of shielding for the smaller stuff.”
Over the years, crew members on the International Space Station have even had to shelter in docked Soyuz capsules upon receiving word of passing debris that could potentially breach the station’s shielding. The most recent incident occurred in March 2015, when astronauts learned only 90 minutes in advance of an approaching fragment from an old Russian satellite. The incident marked the fourth time in the station's history that astronauts had to seek shelter because of debris.
“This is likely a problem that will rapidly increase—especially since we’re putting more and more satellites in orbit,” said Chris Englert, who heads NRL’s Geospace Science and Technology Branch. “The commercial sector is talking about putting thousands of satellites into Earth orbit in the not too distant future.”
With Andrew Nicholas, section head of Sensor Development and Applications, Englert has been developing a method to detect orbital debris and micrometeoroids. Their project is called LARADO, for Light-sheet Anomaly Resolution and Debris Observation, and it’s a space-based design concept for using satellite and laser technology to detect orbital debris in sizes that currently aren’t detectable from the ground.
The idea is deceptively simple: Traveling on a satellite in orbit, LARADO aims a laser on an axicon, a specialized mirror with a conical surface, which distributes the laser’s photons, creating a sheet of light in front of the satellite. Any object that passes through the sheet will scatter photons, creating a flash marking its passage that will be seen by a camera behind it with an ultrawide angle lens.
The device doesn’t weigh much, and its scalability relies merely on the size of its camera and the power of its laser. The more powerful the laser, the “wider” the sheet. On a satellite several meters long, a laser with the power of your average laser pointer would cast a sheet a couple of meters around the satellite, said Nicholas. The device should be capable of detecting debris at sizes as small as 0.01 centimeters—including debris that strikes the satellite.
Spotting debris in this way would have a number of applications, among them anomaly attribution—in other words, discovering what happened to your satellite. At low Earth orbit, objects that are too small to track are still traveling at orbital velocities of several kilometers a second. That means that, even though they’re tiny, they pack enough kinetic energy to damage a multimillion-dollar satellite.
“What happens is that you fly a satellite and everything works great, and all of a sudden it doesn’t, and no one knows why,” Englert explained. “Objects in low Earth orbit are faster than a bullet coming out of a gun, so really small things can be pretty destructive.”
With no way to detect tiny pieces of debris (smaller than about 10 centimeters in size) in low Earth orbit, satellite operators today have little way of knowing when such a piece has damaged a critical component of a satellite. Failed satellites are seldom recovered for analysis, and engineers and operators may attribute a failure caused by space debris to unrelated factors such as the satellite’s design or parts.
Right now, a satellite operator might detect a collision when a piece of debris strikes the side of a satellite, changing its attitude or position. However, if a piece of debris passes through its center, or if a small piece of debris strikes a particularly large satellite, doing little to disturb its attitude, a damaging collision might go completely undetected.
“It’s up there, so you can’t take a look. You can’t look at your electronics box and see there’s a hole in it,” Englert said. “But if you have something onboard that detects [the object] before it destroys something, you’ll be able to say, ‘Oh yeah, so we got hit by something.’”
With its potential for spotting tiny objects, LARADO might also be used to study the population density of space debris in low Earth orbit, capturing data on objects so tiny that even the new space fence won’t be able to see them—objects like the many thousands of particles and fragments distributed by the Fungyun antisatellite test and the Iridium-Kosmos collision in 2009.
With the estimated number of objects orbiting the Earth smaller than one centimeter exceeding 100 million, Nicholas expects that when the new space fence comes on line there will still be a considerable gap in observation that LARADO would be able to capture data on. He envisions collecting data on the growing debris population by flying LARADO on host satellites over a range of altitudes (300 to 1,000 kilometers) over the period of a year.
“And if you want to really understand the density population of debris in low earth orbit, you might want a more powerful laser that makes a laser sheet that’s tens or hundreds of meters across,” Nicholas said.
Such data on previously undetectable space debris, he believes, would be a valuable source for the Space Surveillance Network as well as NASA’s Orbital Debris Environmental Model, which NASA already updates regularly.
“You don’t have to make a comprehensive measurement covering every square inch to make progress in that field, because there’s so little data in that regime,” Nicholas explained.
At this stage, Englert and Nicholas also are considering a number of design variations for LARADO. In one, the device would determine the direction of debris by projecting two sheets, one behind another, creating two parallel planes of light for the debris to cross. In another, LARADO would use a cone of light to sweep areas of space to detect nearby debris and other objects.
Nicholas believes that LARADO would pair well with NASA’s own Space Debris Sensor (SDS), which uses resistive grids and acoustic signal information to distinguish the impact characteristics of different kinds of debris.
“You could do that exquisite [SDS] measurement that would give you the actual mass density and then fly a co-aligned version of LARADO,” Nicholas said. “You’d get a really big area where you collect the population density on, then you’d get this smaller area where you get really exquisite data on the mass density as well. That would be very, very useful.”
Since patenting the concept for the LARADO device in 2015, Nicholas and Englert have conducted preliminary lab tests with a number of everyday objects—marbles, erasers, pens. For further testing and, ultimately, the construction of a space-qualified unit, they plan to partner with a sponsor such as the Air Force or NASA’s Orbital Debris program office.
They are proposing more sophisticated tests of a demo of the device with high speed objects, such as testing at the NASA Ames Vertical Gun Range, a facility that can simulate high-speed impacts by firing projectiles at orbital velocities up to seven kilometers a second.
OCEAN – ‘We hit go and it works.’
In space, fuel is life. Though NASA and NRL are researching options for servicing and refueling satellites, satellites currently aren’t refueled—ever. The fuel a satellite takes into space is all the fuel it will ever have. So anytime a satellite has to maneuver, whether it’s to adjust its orbit or evade collision with space debris, its lifespan shortens.
No one knows this better than Joshua Brooks, who heads NRL’s Blossom Point Tracking Facility, which today is actively flying five satellites, among them WindSat, Upper Stage (MITEx), and eXCITe, a satellite for DARPA that launched in December 2018. Managing operations for multiple satellites at Blossom Point once involved crews of operators manning terminals on shifts, 24 hours a day, seven days a week. Today, nearly all of that is automated, managed by a handful of operators.
That is thanks in part to NRL’s high-precision, orbit-determination software suite called OCEAN (for Orbit/ Covariance Estimation and Analysis). Brooks, who began his space operations career with the Air Force as a ground radar operator doing space object monitoring, credits OCEAN as a huge contributor to the facility’s evolution.
The software suite is a space operations project that NRL has been running for more than two decades. One of the first programs to use it was NRL’s 1994 Deep Space Program Science Experiment mission to map the moon (better known as Clementine).
“OCEAN is so much better than the orbit-determination tools that I was using in the Air Force,” Brooks said. “If we had been using OCEAN, or if the Joint Space Operations Center, who used to do the traffic management function, had used OCEAN as the system of record for orbital analysis there would have been a lot fewer collision avoidance warnings and potential maneuvers.”
Aerospace engineers Evan Ward and Greg Carbott aren’t based at Blossom Point, but as OCEAN’s main developers with NRL’s Astrodynamics and Navigation Section, they visit the facility fairly regularly. Achieving a high-precision orbit, they will tell you, means reducing the uncertainty around a spacecraft and the uncertainty around the objects around it.
Think of the area of uncertainty around an object as a bubble. Should a satellite’s bubble of uncertainty threaten to pass through the bubble of uncertainty around a debris cloud, the satellite will have to maneuver out of the way, expending its limited fuel in the process, or risk being damaged by a collision.
“If you have a small bubble, you don’t have to maneuver,” Ward said. “You can say for sure that debris won’t hit my satellite because you’ve reduced the area where you know where the debris is. That’s where most of our efforts have been focused on, producing high-quality orbit determination and ephemeris products for operational satellites. We can produce better knowledge of where our own satellites are and where the debris is, which means that you don’t have to use as much fuel to maneuver your satellite to avoid a collision, and that you have a longer life for your satellite.”
Using data from GPS, the Air Force Satellite Control Network, and other sources, OCEAN determines where a spacecraft is located in space and there it will be located in the future. Here's how it works: When the ground antennas at Blossom Point initiate contact with a spacecraft with a radio signal, the satellite retransmits the signal, and Blossom Point begins receiving telemetry data. That data flows onto the computer systems and into OCEAN, which calculates the time it took for the signal to reach the satellite and return, using that information paired with the angle of the antenna to determine where the satellite is located in the sky.
“That gives us a pass overhead, and then we can use that to fit an orbit to it,” Ward said. “Typically, we get multiple passes to produce an orbit. And then we use that orbit to predict where the satellite’s going to be in the future, so we know where to point the antennas for the next pass.”
OCEAN then provides the ephemeris (the predicted position for the spacecraft) to the Joint Force Space Component Command (JFSCC), the Air Force operational arm for satellite command and control. JFSCC compares that position to the objects in its catalog. If a maneuver is necessary to avoid a potential collision, OCEAN uses that position data to calculate a maneuver plan for the spacecraft.
“When JFSCC thinks you’re getting too close to another operational satellite or piece of space debris, they’ll send what’s called an ‘orbital conjunction message,’” Ward explained. “‘Conjunction’ is just another technical term that means you’re getting close to something.”
“We would then produce a maneuver plan, and provide that to the [satellite’s] operators, who would then command the satellite to perform that maneuver,” Carbott said.
Orchestrating the whole process is Neptune, NRL’s multi-mission command-and-control software, which handles all the communication with the spacecraft antennas and the command of the spacecraft. Neptune and OCEAN are both “lights out”—fully automated—which means that much of the time this extremely complex process unfolds without the assistance of a human being.
“You have some preconfiguration that you need to do, and there’s some hiccups that can happen," Carbott said. “But for nominal operations we configure it, we hit go, and it works.”
As engineers, Ward and Carbott’s role consists of working to advance the heritage software, continually improving OCEAN’s reliability, precision, and accuracy, as well as configuring the software for new missions, each of which come with its own set of unique demands. When the new space fence comes on line and begins pumping out higher-quality data, they may also have to update the software.
When it comes to orbital debris, however, though these operations are in place to ensure precise and safe maneuvers to avoid it, Ward and Carbott have not yet experienced any issues. Neither has Brooks. Partly, that’s because right now some of their satellites fly in the subsynchronous orbital regime, where space debris is relatively uncommon.
Their others satellites—such as WindSat—fly in the polar orbital regime. That’s where the Fengyun antisatellite missile test occurred, inserting most of that debris into polar orbit. According to Ward, WindSat flies a similar orbit, placing it at relatively high risk of collision with Fengyun debris. So far, though, WindSat hasn’t encountered any of it.
“I agree that [orbital debris] is a good thing to focus on,” Ward said. “My position is that at the moment it’s not a big issue for us, but we need to pay attention to make sure it doesn’t become one.”
In 2011, the National Research Council (NRC) released a report outlining findings from its study of NASA’s meteoroid and orbital debris programs. In it, the NRC warned that debris from Iridium-Cosmos collision and the Fengyun event may have already pushed the orbital debris environment toward a tipping point where the cascading collisions predicted by Donald Kessler had already begun.
The report nonetheless noted the measures the operational community has taken over the previous 50 years to protect critical components of spacecraft and mitigate the generation of new debris: satellites redesigned, orbits monitored, and new protective shielding added to the International Space Station.
Indeed, studies by NASA’s Orbital Debris program have led to US policies and international agreements limiting the net growth of debris in low Earth orbit—policies that order the venting of propellant tanks to prevent them from exploding (tank explosions were one of the earliest causes of orbital debris) or deorbiting defunct satellites to burn up in the Earth’s atmosphere or enter a graveyard orbit.
As NRL’s work continues, we can take solace that today, eight years after the release of NRC’s report, the accumulation of orbital debris remains a manageable problem that we still have time to solve. For their part, Ward and Carbott are consoled by one indisputable fact of space operations.
“Space is big,” Ward said.
There’s a lot of room up there. For now.