lander with a 15-meter umbilical. Two small metal rods were used for autonomous obstacle avoidance, as radio signals from Earth would take too long to drive the rovers using remote control. Each rover had both a densitometer and a dynamic penetrometer, to test the density and the bearing strength of the soil. Because of the demise of the landers, neither rover saw action. The Mars 2 and 3 orbiters sent back a large volume of data covering the period from December 1971 to March 1972, although transmissions continued through August. It was announced that Mars 2 and 3 had completed their missions by August 22, 1972, after 362 orbits completed by Mars 2 and 20 orbits by Mars 3. The probes sent back a total of 60 pictures. The images and data enabled creation of surface relief maps, and gave information on the Martian gravity and magnetic fields. Mars 3MS Kosmos 419 was launched on May 5, 1971. It consisted of only an orbiter, and was intended to become the first spacecraft to orbit Mars (areocentric orbit), thereby beating the American Mariner 8 and Mariner 9 spacecraft. Due to an upper stage malfunction, it failed to depart low Earth orbit. The Mars 4 and Mars 5 orbiters, launched in 1973, were designed to orbit Mars and return information on the composition, structure, and properties of the Martian atmosphere and surface. The spacecraft were also designed to act as communications links to the Mars 6 and 7 landers. Like earlier heavy spacecraft, they were launched by Proton-K rockets. Mars 4 Launch date/time: Mars 4: July 21, 1973 at 19:30:59 UTC On-orbit mass: Dry: 2270 kg Fully fuelled: 3440 kg The Mars 4 orbiter reached Mars on February 10, 1974. Due to a flaw in the transistor 2Т-312 which resulted in its degradation during the voyage to Mars, the retro-rockets designed to slow the craft into Mars orbit did not fire, and Mars 4 flew by the planet at a range of 2200 km. It returned one swath of pictures and some radio occultation data which constituted the first detection of the nightside ionosphere on Mars. It continued to return interplanetary data from heliocentric orbit after the flyby. Mars 5 Launch date/time: Mars 5: July 25, 1973 at 18:55:48 UTC On-orbit mass: Dry: 2270 kg Fully fuelled: 3440 kg Mars 5 reached Mars on February 12, 1974 at 15:45 UT and was