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Another drawback to the use of radioisotopes in rockets is an inability to change the operating power. The radioisotope constantly generates heat that must be safely dissipated when it is not heating a propellant. Reactors, on the other hand, can be throttled or shut down as desired.
==Technology Development==
[[TRW Inc.|TRW]] maintained a fairly active development program known as '''Poodle''' from 1961 to 1965, and today the systems are still often known as '''Poodle thrusters'''. The name was a play on the larger systems being developed under '''Project Rover''', which led to NERVA. In April 1965 they ran their testbed engine for 65 hours at about 1500 °C, producing a specific impulse of 650 to 700 seconds (6.5 to 7 kN·s/kg).
==Photon Pressure==
The inadvertent construction of a radioisotope rocket is one of the suggested solutions to the [[Pioneer anomaly]]. The Pioneer space probes are powered by radioisotope thermal generators located on the end of a long arm to keep their radiation away from the spacecraft electronics. In this position the back of the main radio dish is preferentially exposed, meaning that radiation (primarily infra-red) scattered by the dish would tend to be scattered to the rear of the spacecraft. This could lead to a ''tiny'' amount of net thrust being generated, although all calculations to date suggest it is not enough in itself to fully explain the effect.{{Citation needed|date=November 2008}}▼
Even without an exhaust, the [[Radiation pressure|photon pressure]] of the energy emitted by a thermal source can produce thrust, although an extremely tiny amount.
▲The inadvertent
A similar phenomenon occurred on the [[New Horizons]] spacecraft; photons (thermal infrared) from the RTG, reflected from the spacecraft's antenna, produced a very small thrust which propelled the spacecraft slightly off course.<ref>[http://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_05_21_2010 New Horizons official website article mentioning the thrust from the RTG]</ref>
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