The Soviet Union placed a series of nuclear-powered, radar-equipped ocean reconnaissance satellites, known as RORSATs in the west, into Earth orbit beginning in 1967. Employing powerful radars and working in pairs, these spacecraft kept an eye on U.S. ships for Soviet naval forces. Image
Spinning around the Earth for more than two decades, an old Soviet satellite, replete with a nuclear reactor, has acted up.
Launched by the former Soviet Union in February 1987, Cosmos 1818 was the first of two vehicles designed to evaluate an advanced nuclear power supply in low Earth orbit.
But ground-watching surveillance gear has picked up dozens of small particles spewing into space from the 21-year-old satellite. Why the unexpected debris cloud? It's still what industry types call an unexplained debris generation event.
Information on the event, first spotted in July 2008, has been highlighted in the January issue of the Orbital Debris Quarterly News - produced by the NASA Orbital Debris Program Office at the space agency's Johnson Space Center in Houston, Texas.
The newsletter notes that Cosmos 1818 and its sister spacecraft, Cosmos 1867 both toted into orbit a thermionic nuclear power supply. That nuclear power gear was more advanced than earlier thermoelectric nuclear devices that energized the well-known Radar Ocean Reconnaissance Satellites (RORSATs) during the 1970s and 1980s.
The most infamous RORSAT was Cosmos 954. It made an out-of-control nose dive in 1978, raining a mess of radioactive debris over Canada.
Unlike their RORSAT cousins that operated in very low orbits, Cosmos 1818 and Cosmos 1867 were directly inserted into much higher orbits, thereby eliminating any threat of premature reentry, the Orbital Debris Quarterly News notes.
Russian space authorities have said in the past that the nuclear reactors onboard Cosmos 1818 and Cosmos 1867 functioned for roughly five and 11 months, respectively. For the next two decades, the two inactive spacecraft orbited the Earth without significant incident.
But on or about July 4, 2008, the dormant Cosmos 1818 satellite seemed to be involved in its own Independence Day fireworks. The U.S. Space Surveillance Network spotted debris of various sizes being shed from the spacecraft.
"Special observations" of a few of the debris revealed characteristics generally indicative of metallic spheres – perhaps bubbles of sodium potassium reactor coolant, according to the space debris newsletter.
One idea floating around is that a coolant tube on Cosmos 1818 became thermally stressed and breached after coasting between sunlight and dark temperatures over the two decades.
"Alternatively, the hyper-velocity impact of a small particle might have generated sufficient heat to melt some of the NaK, which then would have formed spheres with metallic properties," the newsletter explained.
Trail of droplets
This is not the first case of former Soviet satellites casting off a trail of droplets in Earth orbit – dendrites of a dangerous kind to other spacecraft.
Back in March of 2004 I reported on the case of the leaking RORSATs – and whether or not the drips of NaK were, indeed, still radioactive.
Meanwhile, according to the space debris newsletter, more analysis of the Cosmos 1818 debris is underway in hopes of pinning down the nature of the debris and the possible cause of their origin. "To date, no similar debris generation by Cosmos 1867 has been observed," the newsletter advised.
"I can only guess for now what may be going on with RORSATs," said Don Kessler, a former NASA expert on orbital debris and now an orbital debris and meteoroid consultant in Asheville, North Carolina.
These RORSATs were placed at an altitude above 500 miles (800 kilometers), he added, in the hope that their orbit would not decay until after their radioactivity had decayed hundreds of years from now. However, this also placed the RORSATs in a region of space that has the highest collision probability with other debris.
"Most of the small debris in this region is NaK droplets, released from the RORSATs prior to 1990. Consequently, as a result of collisions with other debris, RORSATs are not likely to remain intact before they reenter," Kessler told SPACE.com. The most frequent type of collision would be with the older NaK droplets, impacting with very high velocities, he said.
"These smaller impacts would penetrate the RORSAT radiators, and release some of the remaining NaK. Impacts with larger debris would cause the entire RORSAT satellite to fragment," Kessler advised.
As for the wandering droplets of reactor coolant being radioactive, Kessler said. "I have never resolved the issue of whether these droplets are radioactive or not....they were certainly exposed to the RORSAT radiation. A specialist in radioactive would best answer the question as to how long NaK would remain radioactive."
Any experts out there capable of talking on this question? If so, give me a shout.
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Leonard David has been reporting on the space industry for more than four decades. He is past editor-in-chief of the National Space Society's Ad Astra and Space World magazines and has written for SPACE.com since 1999.