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Pioneer 10 was first to leave the Solar System
The Pioneers Are Way Out There After 30 Years

NASA artist's vision of Pioneer 10 beyond the edge of our Solar System
NASA artist's vision of Pioneer 10
beyond the edge of our Solar System
When the interplanetary probe Pioneer 10 left Earth on March 2, 1972, it was indeed off on a pioneering mission. In fact, the spacecraft led the way to all exploration of deep space.

Pioneer 10 was the first probe to travel through the Asteroid Belt and the first craft to explore Jupiter. On June 13, 1983, it became the first space probe ever to travel further than the Sun's most distant planet.

The Asteroid Belt is a Sun-orbiting span of rocky debris floating in space between Mars and Jupiter. The most distant of the nine known major planets, at the far edge of the Solar System, are Neptune and Pluto.

Since passing the outer planets, despite dwindling power onboard, Pioneer 10 has been sending back information by radio as it heads into interstellar space. Eventually it will fly into the so-called heliopause, that part of space where the Sun's influence ends and true interstellar space begins. No spacecraft from Earth has reached there yet.

Nothing heard. Unfortunately, a signal from Pioneer 10 was not received back home on Earth by NASA's Deep Space Network (DSN) during the last contact attempt February 7, 2003.

After more than 30 years, the venerable Pioneer 10 spacecraft may have sent its last signal to Earth. The last time Pioneer's very weak signal had been received by DSN was on January 22, 2003.

NASA engineers reported that Pioneer 10's radioisotope power source had decayed, and the distant spacecraft now might not have enough power to send transmissions all the way home to Earth.

The previous three contacts, including the January 22 signal, had been very faint with no telemetry received. The last time a Pioneer 10 contact returned telemetry data was April 27, 2002. Now, NASA has no additional contact attempts planned for Pioneer 10.

The launch of Pioneer 10 in 1972
1972 launch of Pioneer 10
Fastest departure. Pioneer 10 was built by TRW Inc., Redondo Beach, California, and was launched March 2, 1972, from Kennedy Space Center, Florida, on a three-stage Atlas-Centaur rocket. Pioneer 10 reached the speed of 32,400 mph needed for the flight to Jupiter, making it the fastest human-made object to leave the Earth — fast enough to pass the Moon in 11 hours and to cross the orbit of Mars 50 million miles away from Earth in just 12 weeks.

Asteroid Belt. On July 15, 1972, Pioneer 10 entered the Asteroid Belt, a vast area of the Solar System orbiting like a huge doughnut around the Sun. The Belt is 175 million miles wide and 50 million miles thick. The material in the Belt travels at speeds up to 45,000 mph and ranges in size from dust particles to rock chunks as big as Alaska.

Prior to its passage through the Belt, scientists feared Pioneer 10 might not be able to negotiate its way through the hazardous dust and rocks. Some thought the materials comprising the Asteroid Belt might be so thick they would destroy the spacecraft from Earth. However, Pioneer 10 demonstrated that was not the case, opening the door for exploration of the five outer planets of the Solar System. Pioneer 10's journey, as the first spacecraft from Earth ever to pass through the Asteroid Belt, was considered a spectacular achievement at that time.

After Pioneer 10 emerged from the Asteroid Belt, its companion probe, Pioneer 11, was launched from Earth on a similar trajectory.

Where none had gone before. After it passed the Asteroid Belt, Pioneer 10 flew on, beyond Mars, on its long journey into deep space. After Mars, Pioneer 10 was venturing into places where nothing built by humanity had ever gone before. The probe headed on toward the planet Jupiter.

At Jupiter. Accelerating to a speed of 82,000 mph, Pioneer 10 passed by Jupiter on December 3, 1973. It was the first spacecraft from Earth to make direct observations and obtain close-up images of Jupiter.

Pioneer also charted the giant gas planet's intense radiation belts, located the planet's magnetic field, and established Jupiter as predominantly a liquid planet.

After Pioneer 10 successfully visited Jupiter, the following Pioneer 11 was re-targeted in mid-flight toward an eventual encounter with Saturn.

Feeling the solar wind. Following its encounter with Jupiter, Pioneer 10 continued flying outward to explore the outer regions of the Solar System, where it would study energetic particles from the Sun — the solar wind — and cosmic rays entering the Solar System from the region of our Milky Way galaxy beyond the Solar System.

Passing Pluto. In 1983, Pioneer 10 became the first human-made object to pass the orbit of Pluto, the most distant planet from the Sun.

Officially off duty. Pioneer 10 continued to make valuable scientific contributions in the outer regions of the Solar System until its science mission ended officially on March 31, 1997. That's when funding was cut off in favor of more scientifically productive heliospheric missions. However, as part of a training program, American controllers of the Lunar Prospector spacecraft in orbit around the Moon were given permission to contact Pioneer 10 once again in 1998.

Since that time, Pioneer 10's weak signal has been tracked by the DSN as part of a study of advanced communication technology for future interstellar probes.

Despite low battery power, Pioneer 10 had been sending back valuable data about the faraway region of unexplored space. The low-power Geiger-Tube-Telescope instrument and the Charged Particle Instrument still had been yielding valuable data before spacecraft power ran out.

An artist's vision of Pioneer 10 passes Jupiter
NASA artist's vision of Pioneer 10 passing Jupiter
Astronomical Unit. Today, the spacecraft is traveling at a distance of 7.6 billion miles from Earth. To astronomers, that distance is about 82 AU away. That's 82 times the nominal distance between the Sun and the Earth.

AU is short for Astronomical Unit. One AU is equal to the distance from the Earth to the Sun -- about 93 million miles (150 million kilometers).

At that distance, it takes more than 11 hours and 20 minutes for a radio signal, traveling at the speed of light, to reach Earth.

Opposite Voyager 1. Pioneer 10 is the farthest object from Earth flying in the opposite direction to which the Sun moves. Of course, the Voyager 1 interplanetary probe passed Pioneer 10 in mileage out of the Solar System in 1998, but Voyager 1 is travelling in a direction opposite to the course taken by Pioneer 10.

Pioneer 10's speed relative to the Sun is 27,380 mph (12.24 km/sec).

Is the Sun tugging at Pioneer 10? A team of planetary scientists and physicists spotted a tiny unexplained acceleration towards the Sun in the motions of the Pioneer 10, Pioneer 11 and Ulysses spacecraft. About 10 billion times smaller than the acceleration we feel from Earth's gravitational pull -- the unexplained acceleration was seen in detailed analyses of radio data from the spacecraft.

Scientists have been thinking about a variety of causes, such as the gravitational attraction of planets or even unknown dark matter; radiation pressure when photons hit the spacecraft; interaction between the solar wind and the spacecraft; wobbles in Earth's rotation; and gas or thermal radiation from the spacecraft.

In the end, the engineers guessed that the unexplained changes in the accleration of Pioneer 10 may be due to heat escaping from its RTG nuclear power plant radiators.

Where is Pioneer 10 going? Pioneer 10 was the first man-made object to leave the Solar System and travel on a flight path into interstellar space.

Pioneer 10 is heading away from our Sun generally in the direction of the red star Aldeberan. That's the main star seen as the eye of The Bull in the constellation Taurus in Earth's night sky. Aldebaran is about 68 light years away. Pioneer 10 should arrive at Aldeberan in 2 million years.

What if some living thing out there finds it along the way or at Aldeberan? Pioneer 10 has that gold plaque telling those creatures out there about the creatures back home that built it back home on Earth.

Pioneer 11 is Heading for the Eagle

Pioneer 11, built to be a backup if Pioneer 10 failed, was launched from Kennedy Space Center in Florida on April 5, 1973, on an Atlas-Centaur rocket, on a trajectory similar to Pioneer 10.

After Pioneer 10 completed the first ever successful encounter with Jupiter, the following spacecraft Pioneer 11 was re-targeted, even while it was flying outward, for an eventual encounter with Saturn after its visit to Jupiter in December, 1973.

Gold plaques. Like Pioneer 10, the Pioneer 11 spacecraft carried a gold plaque with messages designed to make contact with possible alien civilizations. The late Dr. Carl Sagan helped devise the plaques that bear the illustration of a man and a woman as well as a diagram identifying Earth's location in the galaxy. Like a message in a bottle, these plaques will journey out into interstellar space possibly to be found one day by an extraterrestrial civilization.

On December 2, 1974, Pioneer 11 flew by Jupiter, snapping dramatic images of the Great Red Spot, making the first observation of the vast polar regions, and determining the mass of Jupiter's moon, Callisto.

Looping high over the ecliptic plane and across the Solar System, Pioneer 11 raced toward its appointment with Saturn on September 1, 1979. Pioneer 11 flew within 13,000 miles of the giant ringed planet and took the first-ever close-up pictures of the planet.

Pioneer 11 then flew on to explore the outer regions of our Solar System. It, too, studied the solar wind of energetic particles from our Sun and cosmic rays entering from outside the Solar System in the Milky Way galaxy.

The end. The probe's mission ended in November 1995 when its last communication was received. Its RTG nuclear power source is exhausted. At that time, it was four billion miles (6.5 billion km) from Earth. Without power, Pioneer 11 no longer could make scientific observations so NASA terminated routine mission operations. There have been no communications with Pioneer 11 since November 1995.

The other outward bound Pioneer is headed into the constellation Aquila — The Eagle — seen in Earth's night sky northwest of the constellation Sagittarius. In four million years, Pioneer 11 will pass near one of the stars in that constellation.

NASA artist's vision of the Pioneers
NASA artist's vision of the Pioneers
Learn more about the Pioneers and other deep space probes:

Read more about the Solar System . . .
Star: The Sun  
Inner Planets: Mercury Venus Earth Mars  
Outer Planets: Jupiter Saturn Uranus Neptune Pluto
Other Bodies: Moons Asteroids Comets Kuiper Belt  
Beyond: Pioneers Voyagers NASA's Vision

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