Johnson: These objects vary in terms of size from about 10 cm or so up to many tens-of-meters. That’s in terms of what can be tracked. Actually there are many smaller objects in orbit which DoD can’t track; they’re down to millimeter, or even micron, size. Their masses, of course, range anywhere from a sub-gram up to many metric tons. In low earth orbit the speeds of orbital debris are 7-8 km/s; in geosynchronous earth orbit the speeds are much less.
NASA relies on the U.S. Space Surveillance Network to track objects larger than about 10 cm in low Earth orbit, up to 2000 km altitude. NASA is responsible for statistically defining the debris environment for smaller objects. Special ground-based radars, including the 70-meter-diameter radio telescope at Goldstone, CA, can detect orbital debris as small as a few millimeters. Returned spacecraft surfaces provide insight into the population of orbital debris smaller than 1 mm.
NTB: Should one of these objects impact the Space Station or a Space Shuttle, what kind of damage could it do?
Johnson: The damage could be negligible, mission-threatening, or catastrophic, depending upon the size of the debris and the location of impact. Debris 10 cm and larger have the potential for completing destroying a spacecraft and creating large amounts of new debris. The accidental collision of two intact spacecraft on 10 February 2009 resulted in the creation of more than 600 large pieces of debris. Debris smaller than 1 mm normally do not affect the operation of a spacecraft.
The Space Station is the most heavily protected vehicle ever launched. It can withstand hits by particles up to about 1 cm. The Space Shuttle is a little bit more vulnerable because of its nature and the fact that it has to conduct a reentry successfully. But these particles, if they were to strike either the Space Shuttle or the International Space Station, would typically hit at somewhere around a speed of 10 km/sec, so a very small particle could do a lot of damage.
NTB: What is currently being done to protect these craft from being damaged by space debris, and is there new technology being developed for the future that will provide even better protection?
Johnson: It depends on the vehicle. We’re trying to design robotic spacecraft more robustly. We can shield against particles as large as about 1 cm, although most robotic spacecraft don’t have quite that much shielding onboard.
The primary near-term protection for spacecraft is the limitation of new orbital debris. The U.S. and the international aerospace community have developed specific orbital debris mitigation measures. But when possible, the design of spacecraft can be improved to protect against particles up to about 1 cm. For particles larger than 10 cm, such as those tracked by the Space Surveillance Network, collision avoidance maneuvers are the primary protection. The entire Space Station maneuvered around a piece of debris just last year. For other countermeasures, it’s all in how you fly the vehicle. Debris normally comes from specific directions, so if you put your more sensitive components away from that direction, you’ve got a better chance of surviving.
NTB: Is NASA working on any new technology that will a) reduce the amount of space debris currently floating around out there, and b) prevent future missions from turning into more space debris?
Johnson: Well, we’re looking at “a,” but we can’t do that yet, and we certainly are doing “b” very, very well. What that means is that we’ve been looking at ways to remediate the space environment, but it turns out to be a significant technical challenge as well as an economic challenge. The International Academy of Astronautics is completing a comprehensive study of concepts for the remediation of the near-Earth space environment. When that study is completed, NASA will reevaluate debris removal proposals, but to date, no technique has been found to be both technically feasible and economically viable. It’s hard to find a way to go up and remove debris once it’s in orbit, so NASA and the international community have been focusing for the last 10 to 20 years on better operations and better vehicle designs so that we don’t create debris unnecessarily.