about and traveling at a velocity of 900 kilometers per hour (600 mph), the parachute deployed, the aeroshell released and the lander's legs unfolded. At an altitude of about 1.5 kilometers (5,000 feet), the lander activated its three retro-engines and was released from the parachute. The lander then immediately used retrorockets to slow and control its descent, with a soft landing on the surface of Mars. At landing (after using rocket propellant) the landers had a mass of about 600 kg. Propulsion Propulsion for deorbit was provided by the monopropellant hydrazine (N2H4), through a rocket with 12 nozzles arranged in four clusters of three that provided thrust, translating to a change in velocity of . These nozzles also acted as the control thrusters for translation and rotation of the lander. Terminal descent (after use of a parachute) and landing utilized three (one affixed on each long side of the base, separated by 120 degrees) monopropellant hydrazine engines. The engines had 18 nozzles to disperse the exhaust and minimize effects on the ground, and were throttleable from . The hydrazine was purified in order to prevent contamination of the Martian surface with Earth microbes. The lander carried of propellant at launch, contained in two spherical titanium tanks mounted on opposite sides of the lander beneath the RTG windscreens, giving a total launch mass of . Control was achieved through the use of an inertial reference unit, four gyros, a radar altimeter, a terminal descent and landing radar, and the control thrusters. Power Power was provided by two radioisotope thermoelectric generator (RTG) units containing plutonium-238 affixed to opposite sides of the lander base and covered by wind screens. Each Viking RTG was tall, in diameter, had a mass of and provided 30 watts continuous power at 4.4 volts. Four wet cell sealed nickel-cadmium 8 Ah (28,800 coulombs), 28 volt rechargeable batteries were also on board to handle peak power loads. Payload Communications were accomplished through a 20-watt S-band transmitter using two traveling-wave tubes. A two-axis steerable high-gain parabolic antenna was mounted on a boom near one edge of the lander base. An omnidirectional low-gain S-band antenna also extended from the base. Both these antennae allowed for communication directly with the Earth, permitting Viking 1 to continue to work long after both orbiters had failed. A UHF antenna provided a one-way relay to the orbiter using a 30 watt relay radio. Data storage