producing a specific impulse of 650 to 700 seconds (6.5 to 7 kN·s/kg). Photon pressure Even without an exhaust, the photon pressure of the energy emitted by a thermal source can produce thrust, although an extremely tiny amount. A famous example of spacecraft thrust due to photon pressure was the Pioneer anomaly, in which photons from the onboard radioisotope source caused a tiny but measurable acceleration of the Pioneer spacecraft. 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. See also Spacecraft propulsion Thermal rocket Nuclear thermal rocket Radioisotope heater unit References External links AIAA meeting paper study comparing Poodle thrusters to a chemical (hydrogen/fluorine) option for upper stage propulsion United States Patent 3315471; Direct cycle radioisotope rocket engine; 1967; Lee, Dailey Charles, Verdes, Estates Palos United States Patent 3306045; Radioisotope rocket; 1967; Buford Jr., William H. Thomas Jr., Arthur N Nuclear spacecraft propulsion