There was a time over a generation ago when people still talked about men walking on the Moon like it was yesterday, where the space shuttle was still something of the future and memories of the Vietnam War still burned painfully in the minds of many. It was five years after Gene Cernan and company blasted off the Moon to end that last manned mission to our nearest neighbor. It was four years before the space shuttle would fly for the first time. Jimmy Carter was President of the United States and on a muggy September morning in 1977, NASA launched Voyager 1, a 1,590 lb spacecraft designed to explore the outer planets of the Solar System and destined to make history. On September 12, exactly 36 years and 1 week after it left Earth, NASA confirmed that Voyager 1 has become the first manmade object to enter interstellar space.
Being humankind’s first robotic ambassador to the stars wasn’t part of Voyager’s original mission. During the late 1960’s and early 70’s, NASA sought missions for exploring the gas giant outer planets. This led to the Pioneer 10 and 11 spacecraft (http://www.nasa.gov/centers/ames/missions/archive/pioneer.html), which were the first to visit the outer planets. Pioneer 10 sent its last signal in 2003. During this time, the tantalizing prospect of sending spacecraft on a “Grand Tour” of the outer solar system was pursued. Scientists realized that, by taking advantage of a favorable alignment of the planets and successive gravity-assist flyby missions, it was possible to send a single spacecraft to visit all of the planets.
Out of that effort came the Voyager program – twin spacecraft taking slightly different trajectories to explore Saturn, Jupiter, Uranus and Neptune.
The Voyagers blasted off from Earth 16 days apart in the late summer of 1977, powered by computers thousands of times less powerful as today’s common mobile phones.
Voyager 1 reached the vicinity of Jupiter in January 1979 (compare its 1 ½ year transit time to that of NASA’s current Juno mission) and began photographing the giant planet. Its closest approach occurred on March 5 at a distance of about 217,000 miles from the center of the planet. Most of Voyager’s photographic studies of the Jovian system took place during the two days before and after its closest approach. This was a time of many discoveries as scientists observed things never before seen anywhere else in the solar system.
Voyager 1 discovered dozens of new Jovian moons and the first active volcanoes ever observed off Earth. Voyager 1 also discovered that Jupiter possesses a planetary ring system, albeit much more tenuous than Saturn’s famous rings.
Both Voyagers used the gravity of Jupiter to slingshot them toward Saturn. Voyager 1 reached the ringed planet in November 1980, making its closest approach on November 12. The space shuttle still hadn’t made its first flight.
On the way to Saturn, Voyager 1’s control team decided to steer the spacecraft for a close encounter with Saturn’s moon Titan, around which, a year earlier, the Pioneer 11 spacecraft indicated had a thick atmosphere. Because of this, Voyager 1 would not complete its Grand Tour, although Voyager 2 subsequently visited the other two planets and made amazing discoveries of its own). Instead, Voyager 1 was hurled on a trajectory that would lead it to becoming the first spacecraft to cross the boundary between the solar system and interstellar space. Voyager 2 will not reach this region for perhaps several more years.
On February 14, 1990, Voyager 1 took the first ever “family portrait” of the Solar System as seen from outside, which includes the famous image known as “Pale Blue Dot”.
On February 17, 1998, Voyager 1 reached a distance of 69 AU from the Sun and overtook Pioneer 10 as the most distant man made object from Earth. It is currently the most distant functioning space probe to receive commands and transmit information to Earth. Travelling at about 17 kilometers per second (11 mi/s) it has the fastest heliocentric recession speed of any man made object.
In 2005, NASA announced that Voyager 1 had entered the heliosheath, the region where the solar wind begins to slow down dramatically, eventually reaching zero outward velocity as the pressure of the interstellar medium overcomes that of the Sun. The spacecraft passed the point where the outward flow of reaches zero in June of 2010.
On March 8, 2011, Voyager 1 was commanded to change its orientation to measure the sideways motion of the solar wind at that location in space. A test roll done in February had confirmed the spacecraft’s ability to maneuver and reorient itself. The course of the spacecraft was not changed. It rotated 70 degrees counterclockwise with respect to Earth to detect the solar wind. This was the first time the spacecraft had done any major maneuvering since the family portrait photograph of the planets was taken in 1990. After the first roll the spacecraft had no problem in reorienting itself with Alpha Centauri, Voyager 1’s guide star, and it resumed sending transmissions back to Earth.
For the last few years, scientists have been watching data coming from back from Voyager’s plasma wave experiments, looking for signs that it has crossed the heliopause – the boundary between the solar system and the interstellar medium. On June 14, 2012, data indicated that Voyager had reached the beginning of this region of space and on August 25, Voyager 1 crossed the heliopause and entered interstellar space. However, the process of meticulously analyzing the data and ruling out other possibilities took nearly a year. Finally, on September 12, NASA confirmed once and for all that Voyager 1 has become the first visitor from Earth to travel interstellar space – and to do so while still functioning.
“We have been cautious because we’re dealing with one of the most important milestones in the history of exploration,” said Voyager Project Scientist Ed Stone of the California Institute of Technology in Pasadena. “Only now do we have the data — and the analysis — we needed.”
Voyager 1 is exploring an even more unfamiliar place than our Earth’s sea floors — a place more than 11 billion miles (17 billion kilometers) away from our sun. It has been sending back so much unexpected data that the science team has been grappling with the question of how to explain all the information. None of the handful of models the Voyager team uses as blueprints have accounted for the observations about the transition between our heliosphere and the interstellar medium in detail. The team has known it might take months, or longer, to understand the data fully and draw their conclusions.
“No one has been to interstellar space before, and it’s like traveling with guidebooks that are incomplete,” said Stone. “Still, uncertainty is part of exploration. We wouldn’t go exploring if we knew exactly what we’d find.”
NASA’s Voyager 1 spacecraft officially is the first human-made object to venture into interstellar space. The 36-year-old probe is about 12 billion miles (19 billion kilometers) from our sun.
New and unexpected data indicate Voyager 1 has been traveling for about one year through plasma, or ionized gas, present in the space between stars. Voyager is in a transitional region immediately outside the solar bubble, where some effects from our sun are still evident. A report on the analysis of this new data, an effort led by Don Gurnett and the plasma wave science team at the University of Iowa, Iowa City, is published in Thursday’s edition of the journal Science.
“Now that we have new, key data, we believe this is mankind’s historic leap into interstellar space,” said Ed Stone, Voyager project scientist based at the California Institute of Technology, Pasadena. “The Voyager team needed time to analyze those observations and make sense of them. But we can now answer the question we’ve all been asking — ‘Are we there yet?’ Yes, we are.”
Voyager Reaches Interstellar Space
Messages to Voyager Welcome to Interstellar Space
They to making this determination was detection of the interstellar plasma using one of the spacecraft’s still-functioning instruments. To understand the detective work involved in proving that Voyager had indeed entered the interstellar plasma, one has to go back thirty years, when Voyager 1 was still inside the orbit of Uranus.
Voyager 1 does not have a working plasma sensor, so scientists needed a different way to measure the spacecraft’s plasma environment to make a definitive determination of its location. Voyager’s plasma wave experiment provided the answer.
For the first eight years of exploring the heliosheath, which is the outer layer of the heliosphere, Voyager’s plasma wave instrument had heard nothing. But the plasma wave science team, led by Don Gurnett and Bill Kurth at the University of Iowa, Iowa City, had observed bursts of radio waves in 1983 to 1984 and again in 1992 to 1993. They deduced these bursts were produced by the interstellar plasma when a large outburst of solar material would plow into it and cause it to oscillate. It took about 400 days for such solar outbursts to reach interstellar space, leading to an estimated distance of 117 to 177 AU (117 to 177 times the distance from the sun to the Earth) to the heliopause. They knew, though, that they would be able to observe plasma oscillations directly once Voyager 1 was surrounded by interstellar plasma.
Voyager 1 first detected the increased pressure of interstellar space on the heliosphere, the bubble of charged particles surrounding the sun that reaches far beyond the outer planets, in 2004. Scientists then ramped up their search for evidence of the spacecraft’s interstellar arrival, knowing the data analysis and interpretation could take months or years.
A coronal mass ejection, or a massive burst of solar wind and magnetic fields, that erupted from the sun in March 2012 provided scientists the data they needed. When this unexpected gift from the sun eventually arrived at Voyager 1’s location 13 months later, in April 2013, the plasma around the spacecraft began to vibrate like a violin string. On April 9, Voyager 1’s plasma wave instrument detected the movement. The pitch of the oscillations helped scientists determine the density of the plasma. The particular oscillations meant the spacecraft was bathed in plasma more than 40 times denser than what they had encountered in the outer layer of the heliosphere. Density of this sort is to be expected in interstellar space.
The plasma wave science team reviewed its data and found an earlier, fainter set of oscillations in October and November 2012. Through extrapolation of measured plasma densities from both events, the team determined Voyager 1 first entered interstellar space in August 2012.
“We literally jumped out of our seats when we saw these oscillations in our data — they showed us the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble,” Gurnett said. “Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma.”
The new plasma data suggested a timeframe consistent with abrupt, durable changes in the density of energetic particles that were first detected on Aug. 25, 2012. The Voyager team generally accepts this date as the date of interstellar arrival. The charged particle and plasma changes were what would have been expected during a crossing of the heliopause.
“The team’s hard work to build durable spacecraft and carefully manage the Voyager spacecraft’s limited resources paid off in another first for NASA and humanity,” said Suzanne Dodd, Voyager project manager, based at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “We expect the fields and particles science instruments on Voyager will continue to send back data through at least 2020. We can’t wait to see what the Voyager instruments show us next about deep space.”
Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977. Both spacecraft flew by Jupiter and Saturn. Voyager 2 also flew by Uranus and Neptune. Voyager 2, launched before Voyager 1, is the longest continuously operated spacecraft. It is about 9.5 billion miles (15 billion kilometers) away from our sun.
Voyager mission controllers still talk to or receive data from Voyager 1 and Voyager 2 every day, though the emitted signals are currently very dim, at about 23 watts — the power of a refrigerator light bulb. By the time the signals get to Earth, they are a fraction of a billion-billionth of a watt. Data from Voyager 1’s instruments are transmitted to Earth typically at 160 bits per second, and captured by 34- and 70-meter NASA Deep Space Network stations. Traveling at the speed of light, a signal from Voyager 1 takes about 17 hours to travel to Earth. After the data are transmitted to JPL and processed by the science teams, Voyager data are made publicly available.
“Voyager has boldly gone where no probe has gone before, marking one of the most significant technological achievements in the annals of the history of science, and adding a new chapter in human scientific dreams and endeavors,” said John Grunsfeld, NASA’s associate administrator for science in Washington. “Perhaps some future deep space explorers will catch up with Voyager, our first interstellar envoy, and reflect on how this intrepid spacecraft helped enable their journey.”
NASA / JPL Voyager Program Historical Documentary Film
And Then There Was Voyager – A Retrospective
Scientists do not know when Voyager 1 will reach the undisturbed part of interstellar space where there is no influence from our sun. They also are not certain when Voyager 2 is expected to cross into interstellar space, but they believe it is not very far behind.
JPL built and operates the twin Voyager spacecraft. The Voyagers Interstellar Mission is a part of NASA’s Heliophysics System Observatory, sponsored by the Heliophysics Division of NASA’s Science Mission Directorate in Washington. NASA’s Deep Space Network, managed by JPL, is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The network also supports selected Earth-orbiting missions.
The cost of the Voyager 1 and Voyager 2 missions — including launch, mission operations and the spacecraft’s nuclear batteries, which were provided by the Department of Energy — is about $988 million through September.
For a sound file of the oscillations detected by Voyager in interstellar space, animations and other information, visit: http://www.nasa.gov/voyager.
For an image of the radio signal from Voyager 1 on Feb. 21 by the National Radio Astronomy Observatory’s Very Long Baseline Array, which links telescopes from Hawaii to St. Croix, visit: http://www.nrao.edu.
(By: Matthew Travis and NASA / JPL)