Monday (April 3, 2006) is the 41st anniversary of the launch of SNAPShot, still, as far as I know, the only working nuclear powerplant, designated SNAP-10A (SNAP: System for Nuclear Auxiliary Power), ever orbited by the USA. (Russia, on the other hand, has orbited a number of nuclear-reactor-powered spacecraft in its RORSAT and other series. We're not talking about mere RTGs here, which the USA uses on certain interplanetary missions, but full-fledged reactors). In going through the launches for 1965, I noted a moderately lengthy series of "debris" associated with this launch and I became curious as to what those objects might be. SNAPShot is not listed among the known orbital breakups and I presume it remains largely intact. Googling up "SNAP-10A" uncovered a number of Internet references, among which were several conflicting accounts of the mission. Perhaps the best account is a short one published in Russian that provides some good clues as to the nature of the objects orbited from this launch. I hope I've reconstructed the mission fairly accurately; I'd appreciate having errors and misconceptions corrected by more knowledgeable list members. According to the Summer 1965 TRW Space Log, SNAPShot was launched aboard an Atlas Agena D on April 3, 1965 from Vandenberg AFB. The payload was a conical structure about 1.5 meters in diameter at the base and about 3.7 meters tall, permanently attached to the front of the Agena D second stage. The reactor was a mechanism with a mass of about 115 kg located at the apex of the cone; the entire assembly had a mass of about 440 kg. The reactor comprised 37 fuel-moderator elements with fissionable U-235 fuel and zirconium hydride moderator. A cutaway diagram in TRW Space Log indicates the cone was hollow, with internal plumbing running just beneath the surface. The function of the reactor was to generate heat that would be converted to electricity via thermoelectric (T/E) elements. These elements were situated at various subsurface locations inside the cone below the reactor, and the reactor's heat was carried to them via molten sodium-potassium (NaK) alloy, pumped hot from the reactor at the cone's apex to a manifold at the cone's base. There the cooler but still molten alloy was pooled and pumped back to the reactor. The pump was located at the cone's apex alongside the reactor. Although it must have had a temperature of several hundred degrees C, the circulating alloy cooled the reactor through the conical radiator while transmitting its heat to the T/E elements. The electricity thus generated powered the various reactor components, such as the coolant circulation pump and the motors that controlled the moderator elements, and the spacecraft's radio transmitters, and it was also fed to a small 1 kg experimental cesium-ion engine to test the feasibility of this propulsion method. The ion engine is said to have been attached to the "side" of the Agena D, but I think it was mounted on the Agena's aft rack, which is where such secondary payloads and structures were usually positioned on other Agena-based spacecraft. At startup, I presume the essential mechanisms were powered by batteries or a fuel cell until the reactor went critical; there were no solar cells. A lithium hydride shield was located close beneath the reactor to prevent radiation damage to the spacecraft's electronics hardware from subatomic particles radiated from the reactor, and all the SNAP-10A components were radiation-hardened. Most of the electronics were located in a compartment at the base of the cone, as far as possible from the reactor. Also attached to the aft rack of the Agena D was a secondary geodetic payload, SECOR 4 (SECOR: Sequential Collation of Range, also called EGRS 4: Electronic Geodetic Ranging System). This was a rectangular box about 23x28x38 cm covered with solar cells and bristling with nine antennas to be ejected from the main spacecraft into a separate orbit. For a while Space Log listed two secondary payloads for this launch, but I have found no hard evidence of a second secondary payload, and later issues of Space Log delisted it. It has been suggested to me that there was one and it failed to release, but what with the SECOR and the ion engine, the Agena's aft rack seems to have been pretty full. Perhaps some reports counted the ion engine as that second secondary payload? At launch, the SNAPShot spacecraft was dormant. The command to go critical was issued on the second orbit, and it took six hours and 25 minutes for the reactor to start up, and another two hours and 15 minutes to achieve full power (500 watts). The earliest orbit I have for the SNAPShot, which became object 1965-027A (SatCat #1314), has period 111.52 minutes, inclination 90.03 degrees, and apogee/perigee of 1312x1280 km. The SECOR 4 was likely ejected well before the reactor went critical, becoming object 1965-027B (#1315). Unfortunately, it failed to turn on and thus became just another piece of space debris rather than an active satellite. I do not know whether anyone ever figured out how and why it malfunctioned. The earliest orbit I have for it has period 111.42 minutes, inclination 90.20 degrees, and apogee/perigee 1315x1271 km, a bit lower but not significantly different from that of the main payload, the inclination suggesting it was released posterolaterally from the Agena aft rack. In starting up the reactor, it was necessary to liquefy the NaK alloy. Most of the startup time was probably consumed in melting it. I guess this was done by the reactor itself as it heated up during its "going critical" phase, when it did not need to be cooled. I don't imagine the satellite was launched loaded with already-molten alloy(!). The Russian account mentions that there was an insulating cover or shield around the reactor to trap its heat until the alloy melted and began to circulate. The shield was jettisoned after the reactor went critical and became self-sustaining, so this must be object 1965-027C (SatCat #1316), which was observed and catalogued at about the same time as the spacecraft and the SECOR subsatellite were. Its earliest orbit has period 111.51, inclination 90.21 degrees, apogee/perigee 1311x1280 km, almost exactly the same as those of the main payload and suggesting it was released mainly laterally rather than forward or backward. Once the coolant liquefied, the reactor ran continuously for 43 days to May 16, 1965. I guess the conical radiator, or certain parts of it, had an operating temperature of hundreds of degrees C, because the coolant would have had to remain liquid at all times; it would not do to have the stuff solidify in the conduits! Initially the reactor put out 600 watts, but as the experiment progressed this decreased to about 530 watts. The total energy output during the duration of the experiment is said to have been more than 500,000 watt-hours, which is in the right ballpark (43 days at 24 hours per day is 1032 hours, times 530 to 600 watts). The ion engine was turned on when the reactor achieved full power, but noise from the ion plume scrambled data transmission and the device had to be turned off. That part of the mission didn't work out too well. It had been hoped the reactor would run for at least 90 days, but a faulty voltage regulator triggered premature shutdown. According to the Russian report, shutdown entailed jettisoning two key reactor components without which the reactor could not function. These were called "reflectors," and I think their purpose was to reflect neutrons emitted during the chain reaction back into the reactor core. Without the reflectors, the neutrons would dissipate into space and the reaction would quickly cease. Indeed, two more objects were catalogued in late May and early June 1965, a couple of weeks after shutdown, as 1965-027D (#1389) and 1965-027E (#1399), and I presume these are the reflectors. Space Track flags the latter with an asterisk, indicating uncertain identification, but their earliest orbits (D: 111.50, 90.19, 1323x1267; E: 111.51, 90.19, 1313x1278) are so similar to those of the main payload and the other objects in the launch that there can be little doubt that E is correctly identified. TRW Space Log listed E as a classified secondary payload in several issues but, as noted above, eventually delisted it. The primary SNAPShot mission is considered a great success, although the US didn't follow it up with more such space reactors. Some sources (e.g., Wikipedia) say that reactor shutdown was accomplished by reactor core jettison "into a higher orbit," but there is no object in such an orbit attributed to this launch. The Wikipedia account is probably based on how orbiting Russian reactors (as in the RORSATs) are deactivated, but this did not take place with SNAPShot. The story doesn't quite end here. As reported in the 13th edition of the History of On-orbit Satellite Fragmentations, SNAPShot suffered a series of seven "anomalous fragmentation events" beginning in November 1979. These resulted in 51 more objects in the 1965-027 series, catalogued as 1965-027F through 1965-027BH. These were catalogued intermittently but fairly continuously from late November 1979 through late October 2002. More may yet turn up. What might these things be? One suggestion has been that these are all pieces broken off the radiator cone. But such chips would be flat and light, with a high surface area to volume ratio (to efficiently radiate heat) that would make them susceptible to "solar sailing." The orbits of 1965-027F through BH are remarkably stable. Some 27 years following the first fragmentation event their orbits have lost only a few kilometers in major axis. This tells me that they're dense objects, likely solid metal, not light and flaky. Some Russian RORSATs have leaked NaK coolant that solidified in the cold vacuum of space into blobs of hard metal, but leakage while the coolant was molten doesn't seem to have occurred during the 43 days the SNAP-10A reactor was operating. Rather, the coolant must have rapidly solidified inside the SNAPShot conduits once the reactor turned off. Broken chunks of solid coolant and plumbing could have bounced around inside the radiator cone soon after the reactor cooled down. Then, perhaps because of a micrometeoroid strike or years of thermal stress in the space environment, a crack opened up in the side of the radiator. The crack gradually widened and allowed the debris within to float gently out and away from the main payload into nearby orbits. Most of the debris objects are quite small, with radar cross sections of less than 0.1 sqm, but two are substantial, with cross sections of about 0.2 and 2.0 sqm (these are 1965-027X [#18268] and AG [#18670], respectively). These happen to be the only two objects in the entire 1965-027 series listed as decayed, but neither one's TLEs show signs of significant orbit diminution around their decay dates, and I consider those dates administrative, not real. So as far as I know, all 56 objects from this launch remain in orbit. It is peculiar that the two largest fragments were administratively decayed while the many tinier fragments are still being tracked. In view of their relatively large radar cross section, I imagine the two big fragments might be pieces of the radiator that broke away where the radiator cracked open. They didn't break free right away but probably dangled from the satellite for a few years, since despite their size they were catalogued quite a bit later than the earliest fragments of the series. Repeated thermal stressing from time to time may have broken off more coolant chunks inside the radiator, perhaps with pieces of stainless steel conduit still attached. (Note the distinction between coolant >chunks< and coolant >blobs<: the former freeze inside the spacecraft's plumbing before breaking free, whereas the latter leak out while molten and freeze outside the spacecraft.) I can picture the satellite slowly tumbling and releasing a piece or two now and then from the gaping crack in the cone.. I don't know whether this scenario explains how 56 objects came to be orbiting from this launch, but it fits the facts as I know them and is not obviously implausible. If anyone has a better idea or firmer knowledge of the nature of the secondary objects from this launch, I'd sure like to hear it. ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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