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Voyagers set to enter interstellar space »
More than 30 years after they left Earth, NASA's twin probes are now at the edge of the Solar System.
Voyager's golden record »
NASA's Voyager probes carry messages for extraterrestrial civilizations:
greetings from humans and whales, some of Earth's greatest music, brainwaves of a woman in love.
 Click to enlarge NASA artist concept of Voyager 1 and 2 |
Its twin, Voyager 2, has traveled to more planets than any other in history.
The spacecraft twins, Voyager 1 and Voyager 2, were launched by NASA during the summer of 1977 from Cape Canaveral, Florida.
Barring any fatal equipment failures, the Voyager twins are likely to survive and relay data from beyond the outer planets for many decades into the 21st century.
Today, in a dark, cold, vacant neighborhood at the very edge of our Solar System, NASA's Voyager 1 deep space probe holds the record as the Earth explorer that has traveled farthest from home.
Voyager 1
When Voyager I was launched in 1977 to study and photograph the giant outer planets of the Solar System, the robot ship was expected to survive just four years. However, like the battery advertising icon, the Energizer Bunny, the little spacecraft kept on going.
For 25 years, the Pioneer 10 spacecraft led the way outbound, pressing the frontiers of exploration, but in 1998 the baton was passed from Pioneer 10 to Voyager 1, according to NASA's Jet Propulsion Laboratory (JPL), Pasadena, California.
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The twins, Voyager 1 and 2, opened new vistas for the human race by expanding our knowledge of Jupiter and Saturn. Voyager 2 then extended our great planetary adventure when it flew by Uranus and Neptune, becoming the only spacecraft ever to visit these worlds. (None has ever visited Pluto.)
Voyager 1, now the most distant human-made object in the Universe, and Voyager 2, close on its heels, continue their ground-breaking journey with their current mission to study the region in space where the Sun's influence ends and the dark recesses of interstellar space begin.
Voyager 1 is almost 70 times farther from the Sun than the Earth. Out there, the Sun is only 1/5,000th as bright as here on Earth. It is extremely cold, and there is little solar energy to keep the probe warm and to provide electrical power.
The probe can continue to operate at such great distances from the Sun because it has radioisotope thermal electric generators (RTGs) that create electricity. The fact that the spacecraft is still returning data is a remarkable technical achievement.
Voyager flight path. Voyager 1 was launched from Cape Canaveral on Sept. 5, 1977. It flew by Jupiter on March 5, 1979, and then Saturn on Nov. 12, 1980. Because its trajectory was designed to fly close to Saturn's large moon Titan, Voyager 1's path was bent northward by Saturn's gravity. That sent the spacecraft out of the Solar System's ecliptic plane -- the plane in which all the planets, except Pluto, orbit the Sun.
On Feb. 17, 1998, Voyager 1 was departing the Solar System at a speed of 39,000 miles per hour. At the same time, Voyager 2 was 5.1 billion miles from Earth and was departing the Solar System at a speed of 35,000 miles per hour.
Voyager 2 is heading in the opposite direction of Voyager I and traveling at a slightly slower speed.
Pioneer 10 had been launched earlier, on March 2, 1972. Its official mission ended on March 31, 1997. However, NASA's Ames Research Center, Moffett Field, CA, intermittently receives science data from Pioneer as part of a training program for flight controllers.
Low power. Voyager 1 was so far from Earth in 1998 that it took 9 hours 36 minutes for a radio signal traveling at the speed of light to reach Earth. Voyager's signal, produced by a 20 watt radio transmitter, is so faint that the amount of power reaching NASA's antennas is 20 billion times smaller than the power of a digital watch battery.
Deep space environment. Having completed their planetary explorations, Voyager 1 and its twin, Voyager 2, are studying the environment of space in the outer Solar System. Although beyond the orbits of all the planets, the spacecraft still are well within the boundary of the Sun's magnetic field -- the heliosphere.
Science instruments on both spacecraft sense signals that scientists believe are coming from the outermost edge of the heliosphere, known as the heliopause.
Heliosphere. The Sun emits a steady flow of electrically-charged particles called the solar wind. As the solar wind expands supersonically into space, it creates a magnetized bubble around the Sun -- the heliosphere. Eventually, the solar wind runs into the electrically-charged particles and magnetic field in the interstellar gas. The solar wind abruptly slows down from supersonic to subsonic speed, creating a termination shock. Before both spacecraft travel beyond the heliopause into interstellar space, they must pass through that shockwave. Voyager may pass through the termination shock sometime soon. Then, within ten years, Voyager should penetrate the heliopause -- the outermost edge of the Sun's magnetic field -- and enter into true interstellar space for the first time.
The low-energy charged-particle detectors aboard both Voyagers continue to detect ions and electrons accelerated by the Sun and huge shock waves, tens of astronomical units (AU) in radius, that are driven outward through the solar wind.
Using data sent to Earth by the Voyager cosmic ray detectors, NASA scientists are predicting the termination shock to be in the range of 62 to 90 AU from the Sun. They estimate the heliopause is located from 110 to 160 AU from the Sun. Most estimates are about 85 AU. Voyager 1 is moving outwards at 3.5 AU per year, so it may encounter the termination shock in the early 2000s.
Reaching the termination shock and heliopause will be major milestones for the mission because no spacecraft from Earth have been there before and the Voyagers will gather the first direct evidence of the termination shock and heliopause. Flying through the termination shock and heliopause have been long-sought goals for many space physicists. What space is like around the two boundaries is a mystery.
Receiving data. Science data from the Voyagers is returned to Earth in real-time to NASA's 34-meter Deep Space Network antennas in California, Australia and Spain. Both spacecraft have enough electricity and attitude control propellant to continue operating until about the year 2020. It is estimated that electrical power produced by the RTGs then will no longer support science instrument operation. At that time, Voyager 1 will be 150 times farther from the Sun than the Earth is -- almost 14 billion miles away.
Voyager 2
Voyager 2 was launched first, on Aug. 20, 1977, and Voyager 1 was launched shortly thereafter, on Sept. 5, on a faster trajectory.
Initially, both spacecraft were only supposed to explore two planets-Jupiter and Saturn. But the incredible success of those two first encounters and the good health of the spacecraft prompted NASA to extend Voyager 2's mission to Uranus and Neptune. Since 1989, when Voyager 2 flew by Neptune, both spacecraft have been studying the environment of space in the outer Solar System. Voyager 1 is now more than twice as far from the Sun as Neptune and their journey is only half over.
The planetary encounters
Voyager 1 at Jupiter. Voyager 1 encountered Jupiter on March 5, 1979, and Saturn on Nov. 12, 1980, and then, because its trajectory was designed to fly close to Saturn's large moon Titan, Voyager 1's path was bent northward by Saturn's gravity sending the spacecraft out of the ecliptic plane, the plane in which all the planets but Pluto orbit the Sun.
Voyager 2 at Jupiter. Voyager 2 arrived at Jupiter on July 9, 1979, and Saturn on Aug. 25, 1981, and was then sent on to Uranus on Jan. 25, 1986, and Neptune on Aug. 25, 1989. Neptune's gravity bent Voyager 2's path southward sending it also out of the ecliptic plane and on toward interstellar space.
Voyager science
As the spacecraft flew across the Solar System, remote-control reprogramming of their computers gave the Voyagers greater capabilities than they possessed when they left Earth. Four science instruments still are collecting data as part of the Voyager Interstellar Mission.
Faster solar wind. The plasma detectors, which measure the protons in the solar wind, observed a slow, year-long increase in the speed of the solar wind which peaked in late 1996. Now they are reporting a slow decrease in solar wind velocity. The velocity peak may have coincided with the recent solar minimum. As a new solar maximum arrives in the year 2000, the solar wind pressure may decrease. That might draw the termination shock and heliopause inward towards the Voyager spacecraft.
Weakest magnetic fields. The magnetometers onboard the Voyagers measure the magnetic fields carried out into interplanetary space by the solar wind. The Voyagers have measured the weakest interplanetary magnetic fields ever detected and those are responsive to charged particles that cannot be detected directly by any other instruments on the spacecraft.
The other science instruments still collecting data include the planetary radio astronomy receiver and the ultraviolet spectrometer.
Spacecraft electrical power is supplied by radioisotope thermoelectric generators (RTGs) that produced some 470 watts of power at launch. Due to the natural radioactive decay of the plutonium fuel source, the electrical energy output continually declines.
By 1997, the power generated by Voyager 1 had dropped to 334 watts and Voyager 2 to 336 watts. However, those levels were better than had been predicted before launch.
There might have been four
NASA originally planned a Grand Tour of the outer planets, including two launches to Jupiter, Saturn and Pluto in 1976-77 and two launches to Jupiter, Uranus and Neptune in 1979. There would have been four Voyagers.
However, space exploration financing was not much different then from now. NASA was not allocated sufficient funds to carry out the complete plan.
The Grand Tour project underwent a dramatic revision down to two spacecraft, each of which would travel only to Jupiter and Saturn.
The name of the mission was changed to Mariner Jupiter/Saturn, or MJS. Later, six months prior to launch, it was renamed Voyager. The downsized mission was estimated to cost only a third of what the Grand Tour design would have cost.
Where will they end up?
NASA says both Voyagers are headed towards the outer boundary of the Solar System in search of the heliopause — the region where the Sun's influence wanes and the beginning of interstellar space can be sensed.
- Voyager 1 is departing the Solar System at a speed of 39,000 miles per hour.
- Voyager 2 is departing the Solar System at a speed of 35,000 miles per hour.
After reaching the termination shock, the Voyagers will continue on to cross the heliopause in another 10 to 20 years.
| One Astronomical Unit (AU) is equal to the distance from the Earth to the Sun – about 93 million miles (150 million kilometers). |
The Voyagers have enough electrical power and thruster fuel to operate at least until 2020. By that time, Voyager 1 will be 12.4 billion miles from the Sun and Voyager 2 will be 10.5 billion miles from the Sun.
Eventually, the Voyagers will pass other stars:
- Voyager 1, in 40,000 years, will float by within 1.6 light years (9.3 trillion miles) of a star known as AC+79 3888 in the constellation Camelopardalis.
- Voyager 2, in 296,000 years, will sail within 4.3 light years (25 trillion miles) of Sirius, which today is the brightest star in Earth's sky.
Project management. Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology, manages the Voyager Interstellar Mission for NASA's Office of Space Science, Washington, DC.
- Voyager Project Jet Propulsion Laboratory
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| Beyond: | Pioneers | Voyagers | |||
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