However, in ejecting the core from the main body of a RORSAT, a plumbing problem plagued the satellite design. Faulty seals permitted the NaK coolant to leak, leaving thouands upon thousands of droplets to spill freely in to space.
Hazard to spacecraft
RORSATs were orbited by the Soviets starting in 1967 and stopped in 1988.
There is evidence from ground-based radar measurements that 16 of a total of 31 RORSAT nuclear reactors orbited lost coolant following core ejection into disposal orbits.
Paula Krisko, a space debris specialist working for Lockheed Martin at NASA’s Johnson Space Center (JSC) in Houston, Texas, noted in a recent newsletter dedicated to orbital debris research that the NaK droplets have been observed over the last decade.
Not only have the spheres come under scrutiny by several NASA radars, Krisko explained, they were also found to have dinged the Long Duration Exposure Facility, better known in NASA lingo as LDEF. This school bus-sized spacecraft floated through space for well over 5 years before being plucked from orbit and returned to Earth by shuttle Columbia in 1990.
Krisko explained that a study of the NaK coolant droplets circling Earth, floating about in varying sizes, is estimated to be 110,000 to over 115,000 in number.
"This population represents an orbital debris hazard to spacecraft in low Earth orbit," she stated.
Credited with first flagging the Soviet leaking reactor problem was Don Kessler. He has over 40 years of experience in the scientific study of human-made space trash, a large chunk of that time spent at the NASA JSC delving into the problem of space debris.
Kessler is now an orbital debris and meteoroid consultant in Asheville, North Carolina.
There has been a significant change in the RORSAT story since Kessler and his colleagues first reported the problem in 1995. "It is now accepted by all, even by the
as being correct and is now part of all orbital debris models," he said.
NASA's main source in spotting the drifting droplets was the Haystack radar. That facility is operated by Massachusetts Institute of Technology’s (MIT) Lincoln Laboratory and has been collecting orbital-debris data for NASA since 1990 under an agreement with the U.S. Air Force.
Kessler led the effort to discover the cause of an abnormally high concentration of objects that was detected by the Haystack radar. These small objects were zipping about the Earth between roughly 530 miles (850 kilometer) and 620 miles (1,000 kilometer) altitude.
Something strange going on
"Our first clue that something strange was going on," Kessler said, was the high concentration of objects near 600 miles (900 kilometers) above Earth. That intensity of objects could not be explained by an explosion, which would have dispersed the debris over a larger altitude range, he added.
"The concentration was so high that, whatever the source, it represented the most significant impact hazard for spacecraft operating at those altitudes... and still does today," Kessler said.
Early work involved techniques using the Haystack radar
to sample the environment
by simply counting objects as they passed through the radar beam. The advantage of this was that researchers were able to statistically sample the environment of much smaller objects, Kessler explained. Altitude and radial velocity of the droplets could be accurately measured. But only rough measurements of motion direction and size of the objects could be ascertained, he said.
Stare and chase
The final proof came when NASA asked MIT to develop a "stare-and-chase" procedure where they could track some of the larger objects long enough to develop orbits.
"We used all this information, plus other sources of information, to determine the source," Kessler said. That source of spheres made of NaK coolant was found to come from the seeping RORSATs, he said.
Kessler told SPACE.com that follow-on NASA work has pegged the total mass of leaked NaK as over 360 pounds (165 kilograms) -- greater than he had orginally estimated.
A report on the RORSAT leakage in 1997, led by Alessandro Rossi, a researcher at the Centro Nazionale Universitario di Calcolo Elettronico (Electronic National University Center for Calculation) in Pisa, Italy, pointed out another issue.
Rossi stated that, although the NaK coolant leakage may have been confined to a specific class of satellite no longer launched, "the probability that a debris impact might puncture the radiator of one of the RORSATs still in graveyard orbit, inducing a new leak from the secondary cooling circuit, is far from negligible."
Well-defined and publicized
"The issue has been well-defined and publicized," said Nicholas Johnson, Program Manager and Chief Scientist at the NASA JSC’s Orbital Debris Program Office.
There is a large population of NaK droplets primarily around 560 miles (900 kilometers) altitude above Earth, Johnson explained. Some of the largest of these are in the roughly 2 inches to 3 inches (5 to 7 centimeters) diameter category. They have been cataloged by the U.S. Space Surveillance Network and are routinely tracked, he said.
The vast majority of the droplets are smaller, down to less than an inch in diameter, Johnson said. "They are not decaying rapidly, although some very small particles have reached lower Earth orbits."
Johnson told SPACE.com that "they pose potential mechanical damage to other spacecraft, just as more conventional debris of the same size."
There is one added element to the RORSAT reactor coolant saga. Are those droplets radioactive?
Any object, large or small -- a paint fleck or a tiny radioactive sphere -- whizzing about Earth at high speed is troublesome to both piloted and automated spacecraft. Furthermore, eventually those NaK spheres will nose-dive into the upper atmosphere.
There is no doubt that the NaK coolant was radioactive when a RORSAT was running, said Gerald Kulcinski, associate dean for research in the College of Engineering and a professor of nuclear engineering at the University of Wisconsin at Madison. In the process, both Sodium-24 and another isotope, Argon-39 would have been created, he said.
While the radioactive Sodium-24 is short-lived, any Argon-39 released would have a half-life of 270 years, Kulcinski noted.
That being said, however, just how much of that Argon-39 radioactive isotope is encapsulated within a space-frozen NaK coolant droplet is not immediately clear, Kulcinski added. Specific engineering details of how the RORSAT reactor was designed and functioned would be required. Yet another unknown factor is what impurities could have been resident within the NaK coolant, he said.
The NaK droplets represent only the "short-term" issue, Kessler said. He underscored another concern -- terming it a "runaway threshold". That is, collisions in space would increasingly produce large quantities of smaller debris over the next 50 years or so.
"Since we began looking, we have measured debris not
only from sources like the NaK, but from solid rocket motors that eject large
amounts of centimeter-to-micron-size objects, paint flecks from painted
spacecraft surfaces, copper needles from U.S. Air Force
communication tests, and more fragments than expected from explosions in space," Kessler said.
"In the long-term, debris resulting from collisions is
still the major problem, and will be the most expensive to control," Kessler
said. "We are on the threshold, if we have
not already exceeded it, of reaching a ‘critical density’ of objects in low Earth orbit, where collisional fragmentation will cause the debris environment to slowly grow even if all other sources are eliminated."