A space probe is an unmanned spacecraft that leaves Earth orbit and explores space. It may approach the Moon; enter interplanetary space; fly by, orbit or land on other planetary bodies; or approach interstellar space. Space probes are a form of robotic spacecraft. 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, China and India 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. More than twenty missions are currently extant. Once a probe has left the vicinity of Earth, its trajectory will likely take it along an orbit around the Sun similar to the Earth's orbit. To reach another planet, the simplest practical method is a Hohmann transfer orbit. More complex techniques, such as gravitational slingshots, can be more fuel-efficient, though they may require the probe to spend more time in transit. Some high Delta-V missions (such as those with high inclination changes) can only be performed, within the limits of modern propulsion, using gravitational slingshots. A technique using very little propulsion, but requiring a considerable amount of time, is to follow a trajectory on the Interplanetary Transport Network. Do you remember names like Pioneer, Explorer, Mariner, Venera, Luna, Ranger, Voyager, Zond, or Surveyor? Many of the early probes launched by the United States and the former Soviet Union have borne these names. Soon after the Soviet Union launched the first artificial satellite, Sputnik 1 on 4 October 1957, both the former Soviet Union and the United States began to launch a flurry of probes to the Moon, Mars, and Venus. These pages attempt to chronicle the unmanned probes which have been launched into deep space, from the early Pioneers to the Mars Rovers. So, just what is a space probe, anyway? A space probe is an unmanned spacecraft that is used to make observations and send information back to Earth regarding these observed objects. While many satellites are also space probes, these pages are dedicated to the deep space probes, those which have escaped Earth's gravity. So, just what kind of equipment does a space probe carry? Exactly what equipment is on any deep space probe, of course, depends upon its mission. All probes need a power supply. Some probes use solar arrays to generate power and batteries to store that power, but solar arrays are useless in missions which carry the probe far away from the sun. For this reason, nuclear power sources are often used. These sources may be either a small reactor or use some kind of radioisotope batteries, which are not reactive. There needs to be propulsion and attitude control systems. In many cases, these will be ion thrusters, which are essentially an electrical propulsion system. Ion propulsion is virtually an inexhaustible system as long as the probe has power. The drawback is that ion thrusters only provide small amounts of thrust, so rapid changes in velocity, attitude, or direction are not possible. Environmental controls may be present to protect equipment in the probe from temperature or pressure extremes. They may also control and protect against cosmic radiation and magnetic exposure. Communication systems are required so that the probe may transmit and receive data. This will entail the presence of transmitters, receivers, amplifiers, and antennas. Guidance controls are required so the the probe will know where it is and where it wants to go. Engineering systems will be in place to monitor and maintain the health of all the other systems. And none of this would be possible without the computer systems to monitor and direct it all. The sophisticated scientific equipment on board provides purpose to the probe. The scientific instrumentation can be quite varied, and as mentioned before, will be mission specific. Cameras are commonly provided to provide an variety of imagery, still, motion, close-up, panoramic, etc., and use a variety of different filters. There may be instruments such as infrared sensors (to measure the temperature of an object), radars (to see planetary surfaces through clouds), ultraviolet sensors (to analyze atmospheric conditions), magnetometers (to measure magnetic fields), soil analyzers, spectrometers (to measure properties of light), and sensors to study wind velocities, or chemical compositions. A gravitometer may be present to determine specific gravities. If the probe includes a lander, that lander may have scoops or drills to collect surface samples. This is just a sampling of the information gathering devices which may be present. Many probes have their own set of web pages provided by the group or agency which controls their mission.

• published: 12 Apr 2015
• views: 1