Nuclear Powered Space Missions - Past and Future by Regina Hagen 11/8/98
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5. Conclusion

Currently, hundreds of kilograms plutonium and almost a metric ton of uranium circle the Earth on board of U.S. and Soviet spacecrafts - many of them on orbits which are all but safe. It is hoped, that the list in Chapter 0, 3 Past Missions – a Chronology creates an awareness of just how huge these amounts are and how many accidents occurred. On an average, there is one accident for each seven space missions - this is also true for nuclear powered space missions.

Another obviously ignored problem might yet seem ahead. In the Orbital Debris Quarterly News, Volume 2, Issue 4 (1997), NASA’s Johnson Space Center pointed to a ‘mystery’: the slow disintegration of spacecrafts many years after launch. The author of this article can not evaluate the implications this might have for the nuclear powered U.S. satellites orbiting Earth. It should, however, not be excluded that disintegration could happen for such satellites also – with a sharp increase in the likelihood of RTGs decaying into the Earth atmosphere almost any time.

As degradation seems to be a serious and ‘mysterious’ spacecraft behavior, the beginning of the article is quoted here:

"Naval Space Operations Center Finds New Evidence of Debris Separations from Three Spacecraft
by Nicholas Johnson

During July and August personnel of the Naval Space Operations Center, which serves as the alternate Space Control Center for the US Space Surveillance Network, detected five new debris from three spacecraft, each more than 15 years old. The causes of these ‘anomalous events’, which involve very low separation velocities, remain a mystery, although material degradation or small particle impacts are probable agents.

At least six polar-orbiting Transit satellites have generated debris more than 20 years after launch. Sometime between 20 and 23 July, a single object was released from Transit 17 (1967-92A, Satellite Number 2965), marking at least the fourth such event for this spacecraft since 1981. The last previous debris release was in December 1996 (see Orbital Debris Quarterly News, January 1997). The five debris previously cataloged with this source all exhibited high area-to-mass ratios and have decayed from orbit.

Another newly discovered debris has been traced to Transit 10 (1965-109A, Satellite Number 1864) which was also involved in a late 1996 release. The debris was found in early August, but orbital analysis could not determine when it had been created. The two debris pieces remain in orbits very similar to that of the parent.

In late August the NOAA 7 spacecraft (1981-59A, Satellite Number 12553) spawned at least three new debris, one of which was cataloged as Satellite Number 24935. The debris appear to have been released, perhaps at the same time, during 23-24 August. The spacecraft had previously released two debris on 26 July 1993, three years after spacecraft deactivation, but both decayed the following year.

The mechanism behind the generation of anomalous event debris large enough to be tracked by ground-based sensors remains poorly understood. Some space objects, e.g., U.S. Transit spacecraft and Soviet Vostok upper stages, seem predisposed to such incidents and, therefore, are probably related to the design or materials selection of the vehicles. Transit spacecraft are likely to exhibit multiple events, whereas the Vostok upper stages appear limited to a single event. However, only a small percentage of vehicles in these families are involved in anomalous events." [JSC/b]

For information about nuclear powered Transit missions, see Chapter 3, Past Missions – a Chronology Chronology.

In addition to the past accidents and the recently observed ‘anomalous events’, the burden left to future generations has so far been completely neglected. Orbits of 900 km altitude for the Soviet RORSAT satellites are safe a few hundred years – but eventually they will fall back to Earth. Responsibility demands to think about methods to prevent re-entry and burn-up of these generators and reactors today, rather than leave the problem to our descendants. And of course, alternatives to nuclear powered space missions should be found. The innovative approach of the European Space Agency described in Section 0, 2.3 Other Nations - "RTG Technology Is Not Available" gives an example for future-oriented solutions. In addition, NASA documents clearly show that for four of the eight suggested nuclear powered space missions the solar alternative is feasible.

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