A space probe is a scientific space exploration mission in which a robotic spacecraft leaves the gravity well of Earth and approaches the Moon or enters interplanetary or interstellar space (see list of probes by operational status for a list of active probes); The space agencies of the USSR (now Russia and Ukraine), the United States, the European Union, Japan, India and China have in the aggregate launched probes to several planets and moons of the solar system as well as to a number of asteroids and comets.

. To reach another planet, the conceptually simplest means is to execute a Hohmann transfer orbit maneuver. More complex techniques, such as gravitational slingshots, can be more efficient, though they may require the probe to spend more time in transit. A technique using very little propulsion, but possibly requiring a considerable amount of time, is to follow a trajectory on the Interplanetary Transport Network.

[edit] Some notable probes

Mariner 4:
The Mariner 4 space probe in 1965 was the first operational one to fly by Mars (on June 14-15), to take pictures of its surface, and to return them (21) to the Earth via radio telemetry. These pictures confirmed that there were no canals on the surface of Mars, and it observed no signs of any kind of life. The unexpected thing that it did reveal was the presence of many impact craters on the surface of Mars, thus confirming the extreme thinness of its atmosphere.

Mariner 9:
Upon its arrival at Mars on November 13, 1971, Mariner 9 became the first space probe to go into orbit around another planet. After 349 days in orbit, Mariner 9 had transmitted to the Earth 7,329 images, which covered about 80% of the martian surface. With the depletion of its supply of attitude control propellant, this space probe was turned off on October 27, 1972.

Huygens probe:
The Huygens probe was a lander constructed by the European Space Agency (ESA) and launched as part of the Cassini-Huygens mission to Saturn's moon Titan. The Huygens probe separated from the Cassini orbiter on December 25, 2004, and landed on Titan on January 14, 2005. It radioed back 350 pictures from the surface.

Spirit and Opportunity:
The Mars Exploration Rovers, Spirit and Opportunity landed on Mars to explore the Martian surface and geology, and searched for clues to past water activity on Mars. They were each launched in 2003 and landed in 2004. As of January 24, 2009, both Spirit and Opportunity have lasted for more than five years on Mars—when they were intended to last only three months. On February 6, 2007, Opportunity had traversed more than 10 km (6 mi) on the surface of Mars.^[1]

Voyager 1:
Voyager 1 is a 733-kilogram probe launched September 5, 1977. It is currently^[update] still operational, making it the longest-lasting mission of the U.S. National Aeronautics and Space Administration (NASA). It visited Jupiter and Saturn and was the first probe to provide detailed images of the moons of these planets.

Voyager 1 is the farthest human-made object from Earth, traveling away from both the Earth and the Sun at a relatively faster speed than any other probe. As of May 9, 2008, Voyager 1 is over 15.89 terameters (1.589 × 10¹³ meters, or 1.589 × 10¹⁰ km, 106.26 AU, 14.72 light-hours, or 9.87 billion miles) from the Sun. From that distance, radio signals from Voyager 1 take more than fourteen hours to reach its control center at the Jet Propulsion Laboratory. Voyager 1 and Voyager 2 have both achieved solar escape velocity, meaning that its trajectory will not return it to the solar system.

Along with Pioneer 10, Pioneer 11, and its sister space probe Voyager 2, Voyager 1 is now an interstellar probe.

[edit] See also

[edit] References

• Deep Space: The NASA Mission Reports / edited by Robert Godwin (2005) ISBN 1894959159