30 Years Ago -- Voyager 2's Historic Neptune Flyby
Wrapped in teal and cobalt colored bands of clouds, Neptune looks like a blue-hued sibling to Jupiter and Saturn -- the blue indicating the presence of methane. A massive, slate-colored storm was dubbed the "Great Dark Spot," similar to Jupiter's Great Red Spot. Six new moons and four rings were also discovered. This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the Voyager 2 narrow angle camera. The images were taken at a range of 4.4 million miles from the planet, 4 days and 20 hours before closest approach. The picture shows the Great Dark Spot and its companion bright smudge. On the west limb, the fast moving bright feature called Scooter and the little dark spot are visible. These clouds were seen to persist for as long as Voyager's cameras could resolve them. North of these, a bright cloud band similar to the south polar streak may be seen. (Image Credit: NASA/JPL)
30 Years Ago -- Voyager 2's Historic Neptune Flyby
Thirty years ago, on August 25, 1989, NASA's Voyager 2 spacecraft made a close flyby of Neptune, giving humanity its first close-up look at our Solar System's eighth planet and marking the end of the Voyager mission's Grand Tour of the Solar System's four giant planets - Jupiter, Saturn, Uranus, and Neptune. But that historic first was also a last -- No other spacecraft has visited Neptune since.
Voyager 2 launched on August 20, 1977, about two weeks before its twin spacecraft, Voyager 1. The two spacecraft are today the most distant human-made objects, having recently passed through the Heliosphere – a giant bubble that surrounds the Solar System.
The original goal of the two Voyager spacecraft was to just explore Jupiter and Saturn. However, as part of a mission extension, Voyager 2 also flew by Uranus in 1986 and Neptune in 1989, taking advantage of a once in a 176 year alignment of the planets that enabled it to take a grand tour of the outer planets.
Among its many findings, Voyager 2 discovered Neptune's Great Dark Spot and 450 meter per second (1,000 mph) winds. It also detected geysers erupting from the pinkish-hued nitrogen ice that forms the polar cap of Neptune's moon Triton. Working in concert with Voyager 1, it also helped discover actively erupting volcanoes on Jupiter's moon Io, and waves and kinks in Saturn's icy rings from the tugs of nearby moons.
"The Voyager planetary program really was an opportunity to show the public what science is all about," said Ed Stone, Professor of Physics and Voyager Project Scientist at the California Institute of Technology in Pasadena since 1975. "Every day we learned something new."
"Voyager 2's initial mission was a four-year journey to Saturn, but it is still returning data 33 years later," said Ed Stone in 2010. "It has already given us remarkable views of Uranus and Neptune, planets we had never seen close-up before. We will know soon what it will take for it to continue its epic journey of discovery."
Wrapped in teal and cobalt colored bands of clouds, the planet that Voyager 2 revealed looked like a blue-hued sibling to Jupiter and Saturn -- the blue indicating the presence of methane. A massive, slate-colored storm was dubbed the "Great Dark Spot," similar to Jupiter's Great Red Spot. Six new moons and four rings were discovered.
During the encounter, the engineering team carefully changed the probe's direction and speed so that it could do a close flyby of the planet's largest moon, Triton. The flyby showed evidence of geologically young surfaces and active geysers spewing material skyward. This indicated that Triton was not simply a solid ball of ice, even though it had the lowest surface temperature of any natural body observed by Voyager: minus 391 degrees Fahrenheit (minus 235 degrees Celsius).
The conclusion of the Neptune flyby marked not an end, but a new beginning -- the beginning of the Voyager Interstellar Mission, which continues today, 42 years after launch. Voyager 2 and its twin, Voyager 1 (which had also flown by Jupiter and Saturn), continue to send back dispatches from the outer reaches of our Solar System. At the time of the Neptune encounter, Voyager 2 was about 2.9 billion miles (4.7 billion kilometers) from Earth. Today it is 11 billion miles (18 billion kilometers) from us. The faster-moving Voyager 1 is 13 billion miles (21 billion kilometers) from Earth.
By the time Voyager 2 reached Neptune, the Voyager mission team had completed five planetary encounters. But the big blue planet still posed unique challenges.
At about 30 times farther from the Sun than is the Earth, the icy giant receives only about 0.001 times the amount of sunlight that the Earth does. In such low light, Voyager 2's camera required longer exposures to get quality images. But because the spacecraft would reach a maximum speed of about 60,000 mph (90,000 kph) relative to Earth, a long exposure time would make the image blurry. (Just imagine trying to take a picture of a roadside sign from the window of a speeding car. That gives you an idea of how difficult this was.)
So the team programmed Voyager 2's thrusters to fire gently during the close approach, rotating the spacecraft to keep the camera focused on its target without interrupting the spacecraft's overall speed and direction.
The probe's great distance also meant that by the time radio signals from Voyager 2 reached Earth, they were weaker than those of other flybys. But the spacecraft had the advantage of time -- The Voyagers communicate with Earth via the Deep Space Network, or DSN, which utilizes radio antennas at sites in Madrid, Spain; Canberra, Australia; and Goldstone, California. During Voyager 2's earlier Uranus encounter in 1986, the three largest DSN antennas were 64 meters (210 feet) wide. To assist with the Neptune encounter, the DSN expanded the dishes to 70 meters (230 feet). They also included nearby non-DSN antennas to collect data, including another 64 meter (210 feet) dish in Parkes, Australia, and multiple 25 meter (82 feet) antennas at the Very Large Array in New Mexico.
The effort ensured that engineers could hear Voyager loud and clear. It also increased how much data could be sent back to Earth in a given period, enabling the spacecraft to send back more pictures from the flyby.
In the week leading up to that August 1989 close encounter, the atmosphere was electric at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, which manages the Voyager mission. As images taken by Voyager 2 during its Neptune approach made the four hour journey to Earth, Voyager team members would crowd around computer monitors around the Lab to see.
"One of the things that made the Voyager planetary encounters different from missions today is that there was no internet that would have allowed the whole team and the whole world to see the pictures at the same time," Stone said. "The images were available in real time at a limited number of locations."
But the team was committed to giving the public updates as quickly as possible, so from August 21 to August 29, they would share their discoveries with the world during daily press conferences. On August 24, a program called "Voyager All Night" broadcast regular updates from the probe's closest encounter with the planet, which took place at 4 AM GMT (9 PM in California on August 24).
The next morning, Vice President Dan Quayle visited the Lab to commend the Voyager team. That night, Chuck Berry, whose song "Johnny B. Goode" was included on the Golden Record that flew with both Voyagers, played at JPL's celebration of the feat.
Of course, the Voyagers' achievements extend far beyond that historic week three decades ago. Both probes have now entered interstellar space after exiting the Heliosphere, a protective bubble around the planets created by the high speed flow of particles and magnetic fields spewed outward by our Sun.
There, they are reporting back to Earth on the "interstellar space weather" and conditions from this region, which is filled with the debris from stars that exploded elsewhere in our galaxy. They have taken humanity's first tenuous step into the cosmic ocean where no other operating probes have flown.
Voyager data also complement other missions, including NASA's Interstellar Boundary Explorer (IBEX), which is remotely sensing that boundary where particles from our Sun collide with material from the rest of the galaxy. And NASA is preparing the Interstellar Mapping and Acceleration Probe (IMAP), due to launch in 2024, to capitalize on Voyager observations.
The Voyagers send their findings back to DSN antennas with small 13 watt transmitters. (To put this in perspective, 13 watts is just about enough power to run a refrigerator light bulb).
"Every day they travel somewhere that human probes have never been before," said Stone. "Forty two years after launch, and they're still exploring."
Voyager 1 – First to enter the final frontier
In 2012, Voyager 1 spacecraft sensors began to send back readings that the venerable deep-space explorer had encountered a region in space where the intensity of charged particles from beyond our Solar System markedly increased. Voyager scientists looking at this rapid rise drew closer to the inevitable but historic conclusion: Humanity's first emissary to interstellar space was on the edge of our Solar System.
"The laws of physics say that someday Voyager will become the first human-made object to enter interstellar space, but we still do not know exactly when that someday will be," said Ed Stone at the time, Voyager project scientist at the California Institute of Technology in Pasadena. "The latest data indicate that we are clearly in a new region where things are changing more quickly. It is very exciting. We are approaching the Solar System's frontier."
The "frontier" he referred to is the edge of the Heliosphere, a great magnetic bubble that surrounds the Sun and planets. The heliosphere is the Sun's own magnetic field inflated to gargantuan proportions by the Solar wind. Inside lies the Solar System – Our "Home." Outside lies interstellar space, where no spacecraft had gone before.
The data making the 16 hour and 38 minute, 11.1 billion mile (17.8 billion kilometer), journey from Voyager 1 to antennas of NASA's Deep Space Network on Earth detail the number of charged particles measured by the two High Energy telescopes aboard the spacecraft. These energetic particles were generated when stars in our cosmic neighborhood went supernova.
"From January 2009 to January 2012, there had been a gradual increase of about 25 percent in the amount of galactic cosmic rays Voyager was encountering," said Stone. "More recently, we have seen very rapid escalation in that part of the energy spectrum. Beginning on May 7, 2012, the cosmic ray hits have increased five percent in a week and nine percent in a month."
This marked increase was one of a triad of data sets which needed to make significant swings of the needle to indicate a new era in space exploration. The second important measure from the spacecraft's two telescopes was the intensity of energetic particles generated inside the Heliosphere, the bubble of charged particles the sun blows around itself. While there had been a slow decline in the measurements of these energetic particles, at the time they had not dropped off precipitously, which would be expected when Voyager broke through the Solar boundary.
The final data set that Voyager scientists believed would reveal the major change was the measurement in the direction of the magnetic field lines surrounding the spacecraft. While Voyager was still within the Heliosphere, these field lines run East-West. When it passed into interstellar space, the team expected Voyager would find that the magnetic field lines would orient themselvs in a more North-South direction.
"When the Voyagers launched in 1977, the space age was all of 20 years old," said Stone. "Many of us on the team dreamed of reaching interstellar space, but we really had no way of knowing how long a journey it would be -- or if these two vehicles that we invested so much time and energy in would operate long enough to reach it."
At the time, the two Voyager spacecraft were traveling through a turbulent area known as the Heliosheath. The Heliosheath is the outer shell of a bubble around our solar system created by the solar wind, a stream of ions blowing radially outward from the sun at a million miles per hour. The wind must turn as it approaches the outer edge of the bubble where it makes contact with the interstellar wind, which originates in the region between stars and blows by our solar bubble.
In June 2010, when Voyager 1 was about 11 billion miles away from the sun, data from the Low Energy Charged Particle instrument began to show that the net outward flow of the solar wind was zero.
"Because the direction of the solar wind has changed and its radial speed has dropped to zero, we have to change the orientation of Voyager 1 so the Low Energy Charged Particle instrument can act like a kind of weather vane to see which way the wind is now blowing," said Edward Stone at the time. "Knowing the strength and direction of the wind is critical to understanding the shape of our solar bubble and estimating how much farther it is to the edge of interstellar space."
Voyager engineers performed a test roll and hold on Feb. 2, 2011 for two hours and 15 minutes. When data from Voyager 1 was received on Earth some 16 hours later, the mission team verified the test was successful and the spacecraft had no problem in reorienting itself and locking back onto its guide star, Alpha Centauri.
The Low Energy Charged Particle instrument science team confirmed that the spacecraft had acquired the kind of information it needed, and mission planners gave Voyager 1 the green light to do more rolls and longer holds. There will be five more of these maneuvers over the next seven days, with the longest hold lasting three hours 50 minutes. The Voyager team plans to execute a series of weekly rolls for this purpose every three months.
"We do whatever we can to make sure the scientists get exactly the kinds of data they need, because only the Voyager spacecraft are still active in this exotic region of space," said Jefferson Hall, Voyager mission operations manager at JPL. "We were delighted to see Voyager still has the capability to acquire unique science data in an area that won't likely be traveled by other spacecraft for decades to come."
At one point, budget cuts threatened the program
Back in 2006, when budget cuts threatened the program, The Planetary Society, an influential non-profit space organization, and its members fought successfully with help from NASA scientists to save the Voyagers and maintain communications with these remarkable spacecraft. Were it not for the Society's stand, it may well be that this latest chapter in the Voyager saga would never have been written.
Up until the summer of 2007 Voyager 2, despite being over 7 billion miles from the Sun, was still traveling inside an enormous magnetic bubble around the Sun known as the Heliosphere. The outer envelope of the Heliosphere, where the solar wind collides head-on with the interstellar medium, is known as the Termination Shock.
Significantly, Voyager 2 reached this landmark at a point substantially closer to the Sun than Voyager 1 had been when it crossed the Termination Shock. Back in 2004, the Sun had been near its solar maximum, a peak of activity characterized by strong solar winds that increased the pressure in the Heliosphere and pushed the termination shock outwards. In fact, scientists believe that for a full two years, as Voyager 1 was streaking away from the Sun, the Termination Shock was also moving ahead of it, staying just out of reach of the spacecraft. Finally the Solar activity waned, causing the Heliosphere to slow down its expansion and eventually to contract back towards the Sun. This allowed Voyager 1 to catch up with the Termination Shock, and finally pass through it at a distance of 94.1 Astronomical Units (AU) from the Sun. (Each AU representing the average distance of the Earth and Sun).
Voyager 2, in contrast, was approaching the outer Heliosphere at a time of declining solar activity, and scientists therefore expected that it would cross the Termination Shock much closer to the Sun than its twin. And so it proved to be. The first sign of the approaching crossing came on August 1, 2007 in the form of electron plasma oscillations detected by the Voyager 2's instruments when the spacecraft was 83.4 AU from the Sun. But according to Don Gurnett and Bill Kurth of the University of Iowa, even with this forewarning scientist were surprised when the crossing came a mere 30 days later, when the spacecraft was 83.7 AU from the Sun.
The crossing itself, Gurnett and Kurth explained, is not a single event because the Termination Shock itself keeps shifting its location. As a result Voyager 2 made the passage not once but at least five times, back and forth over the span of two days, before it was finally clear of that turbulent place. The five crossings were marked by sharp spikes registered on the plasma wave instrument and the magnetometer, and on two of the occasions by intense bursts of broadband electric field noise.
One of the most surprising results to come out of Voyager 2's crossing of the Termination Shock relates to the energy levels of the particles at the termination shock. Since the solar wind slows down drastically from a million miles an hour to less and a thousand, scientists expected the excess energy to appear as heat, raising the temperature of the solar wind plasma to around 1 million degrees Kelvin. But Voyager 2's measurements showed that the temperature reached only around 100,000 degrees Kelvin, raising the question of what happened to around 70% of the energy released by the solar wind particles.
The answer came from an unexpected source. Back in 2006 NASA launched twin spacecraft named STEREO A and B into orbits around the Sun to obtain three dimensional pictures of the surface of the Sun and measure Solar magnetic fields and ion fluxes. But for four months, between June and October 2007, STEREO's sensors detected something else: a stream of highly energized neutral atoms flowing from a spot in the outer reaches of the Solar System.
A bit of astronomical sleuthing revealed the nature of this unexpected discovery. The super-energized particles of the Solar wind that arrive at the Termination Shock collide with the cold atoms of the interstellar medium, relinquishing both their charge and their energy. This explains the fate of the missing 70% of the total energy released by the Solar wind -- It goes towards energizing the cold particles of the interstellar medium which form the region beyond the termination shock known as the Heliosheath. Meanwhile the Solar wind atoms, shorn of their energy and no longer inhibited by their charge, flow back towards the Sun where they are captured by STEREO's sensors. Mystery solved.
"This is the first mapping of energetic neutral particles from the edge of the Heliosphere" said Robert P. Lin of the University of California. "You can't get a global picture of this region through normal telescopes" Lin said, and hence the importance of STEREO's discovery. In fact, Lin said, the unexpected detection "heralds a new kind of astronomy using neutral atoms," which can be used where more traditional methods fail.
Today, Voyager 2 is about 18.2 billion kilometers, or 11.3 billion miles, from Earth. Voyager 1 is about 22.0 billion kilometers (13.7 billion miles) away from Earth. Judging by their unmatched track record, it seems likely that Voyager 2 and its twin, Voyager 1, will continue to be humanity's eyes and ears into the realm of the stars for years to come. As the Space Age hits the 62-year mark, there is little doubt -- The Voyagers are going the distance.
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