so. Attitude control and propulsion Because of the energy required to achieve a Jupiter trajectory boost with an payload, the spacecraft included a propulsion module made of a solid-rocket motor and eight hydrazine monopropellant rocket engines, four providing pitch and yaw attitude control, and four for roll control. The propulsion module was jettisoned shortly after the successful Jupiter burn. Sixteen hydrazine MR-103 thrusters on the mission module provide attitude control. Four are used to execute trajectory correction maneuvers; the others in two redundant six-thruster branches, to stabilize the spacecraft on its three axes. Only one branch of attitude control thrusters is needed at any time. Thrusters are supplied by a single diameter spherical titanium tank. It contained of hydrazine at launch, providing enough fuel until 2034. Scientific instruments For more details on the Voyager space probes' identical instrument packages, see the separate article on the overall Voyager Program. Mission profile Launch and trajectory The Voyager 2 probe was launched on August 20, 1977, by NASA from Space Launch Complex 41 at Cape Canaveral, Florida, aboard a Titan IIIE/Centaur launch vehicle. Two weeks later, the twin Voyager 1 probe was launched on September 5, 1977. However, Voyager 1 reached both Jupiter and Saturn sooner, as Voyager 2 had been launched into a longer, more circular trajectory. Voyager 1s initial orbit had an aphelion of , just a little short of Saturn's orbit of . Voyager 2s initial orbit had an aphelion of , well short of Saturn's orbit. In April 1978, a complication arose when no commands were transmitted to Voyager 2 for a period of time, causing the spacecraft to switch from its primary radio receiver to its backup receiver. Sometime afterwards, the primary receiver failed altogether. The backup receiver was functional, but a failed capacitor in the receiver meant that it could only receive transmissions that were sent at a precise frequency, and this frequency would be affected by the Earth's rotation (due to the Doppler effect) and the onboard receiver's temperature, among other things. For each subsequent transmission to Voyager 2, it was necessary for engineers to calculate the specific frequency for the signal so that it could be received by the spacecraft. Encounter with Jupiter Voyager 2s closest approach to Jupiter occurred at 22:29 UT on July 9, 1979. It came within of the planet's cloud tops. Jupiter's Great Red Spot was revealed as a complex storm