We are closer than ever to witnessing the “Kessler syndrome,” a scenario proposed in 1978 by NASA scientist Donald Kessler in which the high density of objects and debris in low Earth orbit creates a cascade of collisions that renders space travel and satellite use impossible for decades. However, how close we really are is a matter of debate.
The United States Space Surveillance Network, operated by the Air Force, estimates there are more than 500,000 pieces of debris larger than 1 centimeter orbiting Earth today, including 21,000 pieces larger than 10 cm that are actively tracked. And that’s ignoring the millions of smaller bits that are also up there. The average speed at which space junk would collide with a satellite is approximately 10 kilometers per second, meaning collisions with debris as small as 0.2 millimeters can still do damage.
In March, engineers will gather at the 2015 IEEE Aerospace Conference, in Big Sky, Mont., to figure out what the real dangers are and what, if anything, we can do about them. The organizers of the conference’s session on space debris, Kaushik Iyer and Doug Mehoke from Johns Hopkins Applied Physics Laboratory, are themselves cautious about ringing the alarm bells.
“Space is a very, very big place,” says Iyer. “When you do the probability analysis of when spacecraft will collide with debris and how much damage will occur, generally you see that it’s an extremely unlikely event.”
To deal with space junk today, space agencies use ground-based radar to track debris larger than 10 cm and supply that information to the owners and operators of satellites that may be in the way. Those people can choose whether or not to carry out collision-avoidance maneuvers—if they have that capability. Jer Chyi Liou, NASA’s chief scientist for orbital debris, argues that the biggest concern is posed by small pieces of debris, which cannot now be tracked but can still destroy a satellite.
Apart from disasters like the 2009 Iridium-Kosmos satellite collision, we haven’t seen anything calamitous happen yet. Raymond Sedwick, an aerospace engineer at the University of Maryland’s Center for Orbital Debris Education and Research, thinks what is being done right now is effective for the most part, but he recognizes a growing concern that will have to be dealt with soon. “Some may argue that it’s already here or that it’s still decades away, but if you let it go until then, it might be too late,” he says.
According to Iyer, we are years away from really changing our approach to orbital debris. Most proposals have focused on three areas: improving protective measures, removing debris already in orbit (remediation), and preventing further accumulation of orbital junk (mitigation). Here’s a quick look at some of the technologies that may help.
What: Tracking systems and collision avoidance
How: The U.S. Air Force is replacing its current tracking system with one that Lockheed Martin is building for US $914 million. Dubbed Space Fence, it will use an array of S-band radars spread around the world to track debris as small as 2 to 3 centimeters.
Experts say: Technologies like Space Fence will help, “but it’s not the solution,” says Donald Kessler, a retired NASA space debris expert. Little satellites are vulnerable to even smaller, undetectable objects as tiny as a millimeter. While Space Fence will be able to track more debris, Raymond Sedwick, an aerospace engineer at the University of Maryland, notes it will not really do so with any greater precision than the tracking done now. Kessler also says improved tracking will not keep two nonfunctioning satellites from colliding.
What: Increased shielding
How: The most well known form of improved shielding is the Whipple shield, a thin outer bumper placed a certain distance away from the spacecraft. The bumper wall can shock the incoming particle and cause it to disintegrate during impact, leading to a distribution of momentum and allowing the spacecraft’s hull to better withstand the impact. Whipple shields are used in many different configurations on the International Space Station. They’re effective in protecting spacecraft from roughly centimeter-size pieces of debris racing at 3 to 18 kilometers per second.
Experts say: Increased shielding like Whipple shields makes a craft heavier and bulkier, raising both production and launch costs, notes Kessler. Unless there is some extremely expensive equipment on board, manufacturers are likely to opt against it.
How: High-powered lasers—fired either from the ground or from space—could slow down debris until it’s unable to maintain orbit and burns up in the atmosphere. A 1990s NASA project called Orion explored this possibility, but international agreements banning the use of lasers in space prevented any real testing.
Experts say: Sedwick says such a system might work well for smaller debris, but like most other scientists, he believes the whole idea is still not technologically practical, let alone economically feasible. Another concern is that a laser could unintentionally break a piece of debris into several pieces, generating more junk.
What: “Catchers” or “tugs”
How: The idea is to use spacecraft to physically relocate space junk, either to lower-altitude orbits where debris would burn up in the atmosphere or to higher-altitude “graveyard” orbits. A spacecraft could use a claw or harpoon to tether debris and tug it away. That’s the concept behind Nantucket, Mass.–based Busek Co.’s proposed Orbital Debris Remover (Order) spacecraft.
Experts say: Sedwick believes such systems might work but would be too costly. Also, any spin on the debris would make capture and tugging difficult. What’s more, these proposed tethers would also be several kilometers long and could easily collide with something.
What: Increased enforcement of the “25-year rule”
How: The 25-year rule argues that any satellite launched into space must not be allowed to stay in low Earth orbit more than 25 years past the completion of its mission. Owners and operators of satellites need to make plans to send the spacecraft out of orbit—most likely by reserving enough propellant for the job.
Experts say: Jer Chyi Liou, NASA’s chief scientist for orbital debris, believes that this is the “most cost-effective and most efficient plan for dealing with orbital debris and the first line of defense against generating more debris in the future.”
This article originally appeared in print as “Averting Space Doom.”