One of Mankind's Fastest, Furthest and Most Enduring Spacecraft
Most of us remember the launch of many, many spacecraft over the last five decades. For a while those surrogates for human exploration captured the headlines and sent back pictures of amazing landscapes none of us would ever stand on and scientific data of even more significance. After a while, however, our interests turned elsewhere, and only the scientists and engineers concerned with a particular probe continued to observe its progress.
Amazingly, some of these spacecraft are still sending back information decades after their launch. The longest operating of these is Voyager 2. It was launched on August 20, 1977 and sped away toward Jupiter. Voyager 2 did not have an auspicious beginning. There were computer problems at launch that seemed to doom its stability and ability to point its cameras and its radio transmission antenna. After many resets and other efforts to normalize its behavior, it settled down, operating with its backup systems. For a while there was doubt that it would even be able to return information from its first flyby–that of Jupiter. Given all of this, it seems unlikely, even though true, that Voyager 2 is the longest operating of all of our existing spaceships and is still returning scientifically significant data.
The Voyager Spacecraft
Recently Voyager 2 completed 12,000 days of operation. At this time (mid-2010) it is almost 14 billion km from the Sun (92.74 times the distance from the Earth to the Sun, and has traveled 21 billion km along its curving trajectory. Its present speed is approximately 86,000 kph. At that speed it would make the journey from the Earth to the Moon in about five hours.
The significant scientific importance of Voyager 2 is that it is very close to leaving the Solar System altogether and entering interstellar space. The Sun gives off a massive amount of charged particles and other radiation. These charged particles speed away from the Sun in all directions. Interstellar space is not completely empty. It consists of matter such as hydrogen and other particles emitted by other suns, gas left over from the interstellar clouds created as part of the original Big Bang and dust created by exploding stars. As the Sun orbits the center of the Milky Way Galaxy, carrying the planets with it, the Sun pushes through this very tenuous interstellar medium much as a ship pushes through water. The energy and particles emitted by our Sun and its gravitational effects create a bubble filled with Sun matter that is carried along with the Sun and pushed ahead of it through the interstellar medium. This bubble of Sun matter, called the heliosphere, prevents the particles and dust in the interstellar medium from entering the inner Solar System where the planets, including the Earth, are placidly circling the Sun. Were it not for this bubble of Sun matter the interstellar medium would rain radiation and dust on all of the planets. It certainly would have had a significant impact on the life on Earth.
Voyager 2 nears the heliopause and interstellar space
The heliosphere of Sun matter has boundaries. The energy the Sun creates pushes particles and gas out only so far. Outside of this boundary lies true interstellar space, beyond the influence of our Sun. The boundary is commonly referred to as the heliopause. Voyager 2 is not far from the heliopause. In a year or two it will cross from inside the bubble of Sun matter out into interstellar space. Scientists believe its power source is sufficient to survive the two or three years it will take to make this transition, and Voyager 2 will radio information about the interstellar medium back to Earth, something like 35 or 36 years after its launch. That information will give us our first look at the makeup of that interstellar space, tell us about the properties of our Galaxy and perhaps shed light on Dark Matter and Dark Energy, which constitute most of our universe.
Scientists believe that immediately outside the heliopause what Voyager 2 will find is something they call the “local fluff.” The local fluff is a thin but discernible cloud of gas and dust about 30 light years wide, emitted by several supernovae that exploded millions of years ago and filled the area of the galaxy through which the Sun and our Solar System are now passing. Voyager 2 will tell us what generally is the content of the interstellar medium and the local fluff—our first direct information about the makeup of our Galaxy outside of the sheltered environment of our Solar System.
All of this interstellar science is on top of a very
productive journey by Voyage 2 through the outer Solar System,
which was its primary mission. Voyager 2 encountered Jupiter on
July 9, 1979 and took numerous photographs measurements of Jupiter, its
Great Red Spot, and its many moons.
Voyager 2 discovered that Jupiter's closest moon, Io, has sulfur volcanoes driven by the heat created by Io’s rapid orbit through Jupiter’s intense magnetic field. It also got photographs of Jupiter's moon Europa, showing it has a surface of water ice sheets cracked like the surface of a melting Minnesota pond in the early spring. Voyager 2 measurements showed that Jupiter is almost all gas with only a small solid core; and that the Great Red Spot is in effect a centuries-old Jovian hurricane.
Using Jupiter's gravity to adjust its course, Voyager 2 encountered Saturn on August 26, 1981 and took similar photographs and measurements of Saturn, its moons and ring system.
Saturn's Rings from the "backside"
Not satisfied with these two major encounters, using
Saturn’s gravity well as a slingshot, Voyager 2 headed to Uranus,
the 7th planet from the Sun, and on January 24, 1986 it passed that
planet, discovering that it had a small ring system like Saturn and 10
moons, neither ever before observed, and that its axis of rotation was
tilted 90° to the plane of its orbit around the Sun, an extraordinary
and unique characteristic. It also discovered that Uranus had an almost
featureless cloud top and almost no indication of cloud bands similar
to Jupiter and Saturn even though Uranus, too, is a gas giant. This was
an extraordinary amount of scientific information about a planet so far
away that it was little more than a bright star like object in Earth
Finally, after swinging around Uranus and using its
gravity for a course adjustment, Voyager 2 encountered Neptune on
August 25, 1989, 12 years after launch, observing that it had a Big
Blue Spot, smaller than, but similar in character to Jupiter's Great Red
Spot and more rings, which is turning out to be characteristic of the
gas giant planets. Neptune also had a giant moon--Triton. Although
Triton had been observed from Earth, little of it could be discerned
from the vast distance of Neptune's distant orbit. Triton is the only
moon in the Solar System with a retrograde orbit. It circles Neptune in
the opposite direction of Neptune’s rotation. This means that unlike
Earth’s Moon, Triton is likely a planetoid from the Kuiper Belt,
captured by Neptune’s gravity during an orbit by Triton through the
outer edge of the Solar System. Voyager 2’s observations of
Triton yielded much new data about the millions of planetoids believed
to orbit the Sun in the cold and lonely Kuiper Belt, almost 7 billion
Passing Neptune bent Voyager 2’s orbit far to the south of the ecliptic, the plane in which the Sun’s family of planets revolve. Voyager 2 is now headed 33 degrees south of the ecliptic toward the constellation of Telescopium. Despite the huge amount of information it has given us about the four largest planets in the Solar System, scientists eagerly await even more data from it on the heliosphere and interstellar space.
Because of the chances for failure early in the US’s space exploration program, most space missions involved two almost identical spacecraft launched on similar, but slightly different missions. The Voyager mission entailed two probes—Voyager 1 and Voyager 2.
The second craft was named Voyager 1, even though it was launched after Voyager 2. Voyager 1 was intended to concentrate its attention on Jupiter and Saturn, a safer trip to accomplish compared to the multiple gravity assisted slingshots intended for Voyager 2. Voyager 1 reached its primary target, Jupiter, on March 5, 1979. There it took detailed pictures of the planet and its moons, and because of the timing of the mission and the orbits of the Jovian moons it was able to observe the four largest moons in great detail. The Jupiter flyby sharply bent Voyager 1’s orbit and sped it up dramatically. On November 12, 1980 Voyager 1 reached Saturn where it took dramatic photographs passing south of the ring plane and observing sunlight filtered through the rings. Voyager 1 was not intended to visit Uranus or Neptune. Its encounter with Saturn bent its orbit far to the north of the ecliptic and added to its speed. It is now heading out of the Solar System 35° to the north of the ecliptic in the direction of the constellation Ophiuchus.
Voyager 1 is now the most distant man-made object in existence. It is 17 billion km from the Sun and has traveled approximately 22 billion km along its orbit. It is, however, further away from the heliopause than Voyager 2. In the direction Voyager 2 is traveling the heliopause is closer to the Sun, and Voyager 2 will penetrate it sooner. This is good news since Voyager 2’s power supply and radio transmitter are in better condition than Voyager 1’s.
Voyager 1 and Voyager 2 were preceded in their trips to the outer planets by two less sophisticated spacecraft, Pioneer 10 and Pioneer 11. Pioneer 10 reached Jupiter on December 8, 1973 and took the first dramatic up-close pictures of this gas giant. It gave us our first detailed information from the outer Solar System. Pioneer 10 demonstrated that a spacecraft could successfully cross the asteroid belt. Until then many scientists feared that all those science fiction movies showing manned spacecraft destroyed by asteroid collisions might turn out to be true. In fact, all of the outer Solar System spacecraft have made that journey unaffected.
Pioneer 10 was not intended to fly by Saturn or any of the other outer planets. After its encounter with Jupiter it began to leave the Solar System. It is traveling almost in the plane of the ecliptic and is further from the Sun than Voyager 2 but less distant than Voyager 1. Its speed is a somewhat less than either of them. Unfortunately, Pioneer 10’s power system failed, and its last signal to the Earth was on January 23, 2003. It is heading toward the constellation of Taurus.
Pioneer 10 was followed by Pioneer 11. It was launched on April 5, 1973 and encountered Jupiter on December 2, 1974. This was a very close encounter, just 40,000 kilometers above the cloud tops. This meant that the pictures and scientific data sent back by Pioneer 11 was very valuable. It also meant that Pioneer 11’s orbit could be sharply bent back across the Solar System by Jupiter’s intense gravity so that it encountered Saturn on September 1, 1979 after traveling almost directly back across the inner Solar System. It passed Saturn through the ring plane and provided much new information about the nature, creation and future of Saturn's rings. Pioneer 11 is the most leisurely of these four space probes, traveling more slowly than any of them and having reached only about 75% of their distances from the Sun. Nonetheless, its power supply appears to be in good shape and there is every possibility that it will eventually reach the heliopause and be able to transmit information to Earth from a part of that frontier in almost the opposite direction of Voyager 2.
MESSAGES TO OTHER CIVILIZATIONS
All four of the Pioneer and Voyager spacecraft carried a message that might outlive mankind. It is extraordinarily unlikely that any intelligent being will ever see these messages, but it is likely that they will outlive the Earth. (Mankind may make its own interstellar voyages and so escape the inevitable death of the Sun, and consequently the Earth, billions of years from now.) The Pioneers carried a plaque showing a representation of the spin state of a hydrogen atom, by far the most abundant element in the Universe, figures of a man and a woman to scale against an outline of the spacecraft itself, the position of the Sun in the Galaxy relative to 14 prominent pulsars, and a schematic of the Solar System, showing Pioneer’s path through it.
The Pioneer Plaques
The Voyagers carried a long play record. Not a record like a Beatles LP, but a gold plated copper disk digitally encoded with 115 images of life on earth, greetings in 55 languages, a selection of music and a variety of natural sounds, including the iconic Rock and Roll song, "Johnny B. Goode" by Chuck Barry, and "Dark Was the Night, Cold Was the Ground," an unforgettable Blues song by Blind Willie Johnson. Text messages from US President Carter and Secretary General Waldheim of the UN were also included. The cover of the record carried a diagram showing how to “play” it.
The Voyager Records
As fast as Voyager 1 and 2 and Pioneer 10 and 11 are traveling, they are not the fastest moving of the spacecraft launched by Earth. The fastest moving spacecraft were not sent to the outer planets at all but instead were launched inward to study the Sun. Helios 1 and 2, developed by Germany but launched by the US, were designed to provide us with close up scientific detail about our Sun. Because of their very elliptical orbits (oval shaped) they pass very close to the Sun and then come back out almost to the orbit of Earth. Helios 2’s speed on its first orbit as it rounded the Sun well inside the orbit of Mercury was about 241,000 kph.
Helios 1 and 2 were not the only satellites launched to study the Sun. In 1990 the US launched Ulysses, designed to study the Sun at both solar maximum and solar minimum–those times when the Sun is most and least active. Ulysses was launched by the space shuttle and so reached its observation points in an unusual way. At first it was pointed in the direction of the orbit of Jupiter. It rounded Jupiter and, as with the Voyagers and the Pioneers, Jupiter’s strong gravitational field dramatically bent Ulysses' orbit. This time the orbit was bent so that Ulysses transited over the north pole of the Sun to the opposite side of the Solar System, back down through the plane of the ecliptic and then over the south pole of the Sun, passing over the Sun’s polls at the time of solar maximum and solar minimum, respectively. Ulysses made this journey very fast but not quite as fast as Helios. Ulysses’ top speed was approximately 191,000 kph.
So Helios 2 is the fastest; Voyager 1 the farthest away; and Voyager 2 the longest still functioning. But which of our many spacecraft is the most productive? Certainly Voyager 2’s observations of the outer planets provided us with information in nature and quantity far, far beyond anything available to us to that point in time. The recent missions to Jupiter and Saturn, by spacecraft Galileo and Cassini, respectively, have substantially augmented and refined our understanding of the gas giants, which leads to a far better understanding of the origin of the Solar System and the Earth. But despite the audacity of these far ranging missions, perhaps the biggest leap in our advancement of knowledge of non-Earth-based objects comes from the two Mars rovers–Spirit and Opportunity.
SPIRIT AND OPPORTUNITY
These modest looking spacecraft landed on Mars by bouncing down in what amounted to the Michelin man's suit. Parachutes and retrorockets slowed their descent somewhat, but their final arrival was achieved by simply bouncing along surrounded by a bunch of balloons until they rolled to a stop. After the balloons’ deflation, the rovers opened their solar cells, pushed out their antennas, and stood up on their wheels.
Arrival of the Rovers
Designed to last only three months, both have been in operation for over 6 Earth years. Sadly, Spirit had chronic difficulty with its drive wheels and became stuck in sand. It was designated a fixed observation station and provided useful observations until its solar cells could no longer gain enough power.
Opportunity, an indefatigable explorer, has driven many miles over the Martian surface. It has explored craters, found extensive signs of the existence of water in previous epochs, and has given us a whole new understanding of the formation and geological history of Mars. Opportunity continues to hunt for signs of chemical reactions similar to what we call life. It has already found signs of conditions in which microbial life is found on Earth. In many ways the Mars rovers have been the most productive of our spacecraft. If one of them finds the fossils of microbial life, the title of Most Productive should certainly be awarded to it.