Only 10 years after the Space Age began, two young mathematicians discovered a way to have a space probe visit all of the planets in the outer solar system.
As you read this, Voyager 1 is hurtling out of the solar system at a speed of more than 37,000 mph. Its long voyage, which has lasted more than 35 years, began in an entirely different era in space travel.
The Voyager 1 space probe was launched in September 1977, just 20 years after the beginning of the Space Age. The aerospace industry had already made tremendous progress, peaking with the moon missions. But while the lunar trips were highly anticipated, no one at that time expected that we could launch two space probes that would visit all of the planets of the outer solar system — much less that they would one day leave our cosmic neighborhood.
The Voyager missions were primarily the result of two young mathematicians groundbreaking calculations. In the early 1960s, Michael Minovitch and Gary Flandro started to look into the fuel-saving opportunities of using the outer planets gravitational fields to give passing space probes an extra push.
Today, the technique, called a gravity assist, is widely used, but back then it was unheard of. At first the focus was on missions in the inner solar system, but in 1965, Flandro went one step further, aiming for the outer planets. He calculated that a space probe could pass by the four gas giants in a single mission. First it would swing by Jupiter, whose gravity would direct it toward Saturn. The latter’s gravity would push the probe toward Uranus, which in turn would push it toward Neptune.
But the mission required the planets to be aligned precisely, so the project could only be carried out if the probe were launched between 1976 and 1979. The necessary planetary alignment wouldn’t repeat for another 176 years, meaning that the next opportunity wouldn’t come until 2153.
NASA agreed, but Congress trimmed the project repeatedly. The original goal of having four probes pass by all of the outer planets (including Pluto) ultimately became a four-year mission in which two space probes would visit the two closest outer planets, Jupiter and Saturn.
LIFTOFF 1977 – The two Voyager probes left Cape Canaveral in August and September 1977.The probes passed by Mars after three months, but they were in for a long journey across the unknown asteroid belt.
MARCH 1979 – First stop: Jupiter – The visit to Jupiter was worth the wait. Several months before the flyby, astronomers could clearly observe the movements of the clouds and the eternal storms raging in Jupiter’s dense atmosphere. Though astronomers learned many new things about Jupiter, its moon lo held the biggest surprise.
Voyager 1 had only two days to observe Jupiter’s four largest moons at close range. It found that lo is covered by large sulfur volcanoes that are erupting, which give it a distinctive orange color. This marked the first time that astronomers observed volcanic activity elsewhere in the solar system.
NOV. 12,1980 – Saturn gained thousands of rings – The first close-ups of Saturn’s 43,000-mile-wide rings were overwhelming. Astronomers had expected to see four large rings, but instead found a system consisting of thousands of very narrow rings. The images also revealed dark markings, called spokes, cutting across the rings. Because the innermost ring particles move faster than the outermost ones, this pattern should be wiped out, but it was discovered that the planet’s magnetic field holds them in place.
August 1981 – Technical difficulties – As Voyager 2 flew past Saturn in 1981, a rotating platform holding optical instruments seized up when its lubricant became depleted due to overuse. As a result, the probe could no longer point the instruments at specific targets. To solve the problem, NASA engineers built and tested 86 models of the platform’s gear box. After several years of trial and error, the engineers were able to compensate for the damage, allowing the platform and its instruments to function well enough to continue the flight past Uranus and Neptune.
JAN. 24, 1986 – Tragedy overshadowed Uranus – Voyager’s closest approach to Uranus was overshadowed just four days later by the Challenger disaster, when the shuttle exploded and killed all seven astronauts aboard. The probe captured images of the moon Miranda and the planet’s ring system, and also discovered 10 new moons.
August 1989 – Farewell to the planets – Voyager 2’s final planetary mission, to Neptune, heralded the end of one era for the probe and the start of a new one. On its closest approach, it flew by the planet’s north pole at the relatively close distance of 3,045 miles.
In some ways the images captured by Voyager resembled a view of Earth’s north pole; they showed white clouds casting shadows on a sea of blue. But unlike those on Earth, Neptune’s clouds are comprised of frozen methane, not water vapor, and its blue coloring does not mean that the outermost planet is covered in oceans, but that its upper atmosphere is rich in methane. The probe also flew by Neptune’s moon Triton, whose nitrogen geysers surprised astronomers.
Jan. 1, 1990 – Voyager’s mission out of the solar system is initiated.
Feb. 14.1990 – Voyager 1 produces an image of all of the solar system’s planets.
Feb. 17.1998 – Voyager 1 becomes the most distant man-made object, bypassing Pioneer 10.
Dec. 15,2004 – Voyager 1 passes the termination shock, where the solar wind begins slowing.
Dec. 5, 2007 – Voyager 2 passes the termination shock.
2012-2014 – Voyager 1 crosses the heliopause; astronomers think that the final approach began in spring 2012.
Though Voyager 1’s solar-wind detector no longer functions, measurements from several other instruments indicate that the probe is tantalizingly close to the edge of interstellar space.
The data are still being analyzed, but all indications are that Voyager 7 is now crossing the last vestiges of the heliosphere, in the uncharted territory just before it reaches the heliopause, the point where the solar system ends and interstellar space begins.
In spring 2012, scientists confirmed that the intensity of the cosmic radiation from space began to increase considerably. Since then, this radiation has continued to increase dramatically, meaning that Voyager 1 is so far from the sun that its magnetic field can no longer protect the probe from radiation from beyond the solar system. This was followed in September by a considerable decrease in the intensity of the solar wind. In December, NASA announced that the probe had entered a “magnetic highway” but that it had not yet detected the change in direction of the magnetic field from the sun’s east-west orientation that will mark its entry into interstellar space.
Once free of the solar system, both of the Voyager probes should continue to transmit data as they explore the Milky Way.