Pathfinder, United States spacecraft that landed on the planet Mars on
July 4, 1997. Pathfinder was the first U.S. Mars lander since the Viking
probes of the 1970s and the first in a series of small, inexpensive
spacecraft scheduled to reach Mars every 26 months until 2005. The lander was
a closed container with four triangular sides until it landed on Mars. On the
planet's surface it opened up, exposing three solar panels (see Solar
Energy), instruments, and a small, wheeled rover called Sojourner. Mars
Pathfinder tested technologies for future robotic Mars exploration and
explored its Martian surroundings. Pathfinder and Sojourner studied rocks and
Martian weather at the landing site in an ancient flood channel called Ares
Vallis to gather clues to Mars's geologic and climatic history.
rover Sojourner had six wheels, a mass of 10 kg (22 lb), and measured 65 cm
(26 in) long by 48 cm (19 in) wide and 30 cm (12 in) high. A rectangular
solar panel on top provided electricity. The rover carried the Alpha Proton
X-ray Spectrometer (APXS) device for determining the composition of rocks.
APXS aimed radiation in the form of alpha particles (helium atoms with two
neutrons and no electrons) at a rock, then measured the radiation (mostly X
rays) that bounced back. Different elements react to alpha radiation in
different ways, so scientists could determine which elements were present in
the rock by analyzing the results.