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12.09.2013
NASA's Voyager first spacecraft to exit solar system
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Never before has a human-built spacecraft traveled so far. NASA's Voyager 1 probe has left the solar system and is wandering the galaxy, US scientists said Thursday.
The spacecraft, which looks like a combination of a satellite dish and an old television set with rabbit ear antennas, was launched in 1977 on a mission to explore planets in our solar system.
Against all odds, Voyager kept on moving and now is about 12 billion miles (19 billion kilometers) from our Sun in a cold, dark part of space that is between the stars, said Ed Stone, Voyager project scientist.
"We are indeed in interstellar space for the first time," said Stone, who is based at the California Institute of Technology in Pasadena.
"We got there. This is something we all hoped when we started on this 40 years ago," Stone said. "None of us knew anything could last as long as the two Voyager spacecraft."
The twin spacecraft, Voyager 1 and Voyager 2, were sent off 36 years ago on a primary mission to explore Jupiter and Saturn.
They discovered new details about the nature of Saturn's rings and found volcanoes on Jupiter's moon Io.
Voyager 2 traveled on to Uranus and Neptune, before the duo's mission was extended to explore the outer limits of the Sun's influence.
The precise position of Voyager has been fiercely debated in the past year, because scientists have not known exactly what it would look like when the spacecraft crossed the boundary of the solar system -- and the tool on board that was meant to detect the change broke long ago.
However, US space agency scientists now agree that Voyager is officially outside the protective bubble known as the heliosphere that extends beyond all the planets in our solar system.
Their findings -- which describe the conditions that show Voyager actually left the solar system in August 2012 -- are published in the US journal Science.
NASA said Voyager 1 "is in a transitional region immediately outside the solar bubble, where some effects from our Sun are still evident."
Voyager 1 -- with Voyager 2 a few years behind in its travels -- sent back data to scientists on Earth on August 25 last year, showing an abrupt drop in energetic charged particles, or cosmic rays, that are produced inside the heliosphere.
Scientists expected that the direction of the magnetic field in space would reverse at the barrier known as the heliopause.
The Voyager 1 magnetometer did not show this change, leading scientists to be extra cautious about declaring whether or not the spacecraft had left the solar system.
However, an analysis of data from Voyager's plasma wave science instrument between April 9 and May 22 this year showed the spacecraft was in a region with an electron density of about 0.08 per cubic centimeter.
Astrophysicists have projected that the density of electrons in interstellar space would be between 0.05 and 0.22 per cubic centimeter, placing Voyager squarely in that range.
"This historic step is even more exciting because it marks the beginning of a new era of exploration for Voyager, the exploration of the space between the stars," said Stone.
While the Voyager team has reached a consensus, not all are convinced.
"I don't think it's a certainty Voyager is outside now," space physicist David McComas of the Southwest Research Institute in San Antonio, Texas told Science magazine.
"It may well have crossed," he said. "But without a magnetic field direction change, I don't know what to make of it."
The spacecraft is expected to keep cruising, though the radioisotope thermo-electric generators that power it are beginning to run down.
Voyager's instruments will have to shut down permanently in 2025, Science reported. However, experts say the spacecraft may keep traveling indefinitely.
NASA said the total cost of the twin Voyager missions has been $988 million dollars, including launch, mission operations and the spacecraft's nuclear batteries.
"Even though it took 36 years, it's just an amazing thing to me," said co-author Bill Kurth, of the University of Iowa.
"I think the Voyager mission is a much grander voyage of humankind than anyone had dreamed."
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PASADENA, Calif. -- 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 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.
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. 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.”
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.
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Whether and when NASA's Voyager 1 spacecraft, humankind's most distant object, broke through to interstellar space, the space between stars, has been a thorny issue. For the last year, claims have surfaced every few months that Voyager 1 has "left our solar system." Why has the Voyager team held off from saying the craft reached interstellar space until now?
"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."
Basically, the team needed more data on plasma, which is ionized gas, the densest and slowest moving of charged particles in space. (The glow of neon in a storefront sign is an example of plasma.) Plasma is the most important marker that distinguishes whether Voyager 1 is inside the solar bubble, known as the heliosphere, which is inflated by plasma that streams outward from our sun, or in interstellar space and surrounded by material ejected by the explosion of nearby giant stars millions of years ago. Adding to the challenge: they didn't know how they'd be able to detect it.
"We looked for the signs predicted by the models that use the best available data, but until now we had no measurements of the plasma from Voyager 1," said Stone.
Scientific debates can take years, even decades to settle, especially when more data are needed. It took decades, for instance, for scientists to understand the idea of plate tectonics, the theory that explains the shape of Earth's continents and the structure of its sea floors. First introduced in the 1910s, continental drift and related ideas were controversial for years. A mature theory of plate tectonics didn't emerge until the 1950s and 1960s. Only after scientists gathered data showing that sea floors slowly spread out from mid-ocean ridges did they finally start accepting the theory. Most active geophysicists accepted plate tectonics by the late 1960s, though some never did.
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."
The two Voyager spacecraft were launched in 1977 and, between them, had visited Jupiter, Saturn, Uranus and Neptune by 1989. Voyager 1's plasma instrument, which measures the density, temperature and speed of plasma, stopped working in 1980, right after its last planetary flyby. When Voyager 1 detected the pressure of interstellar space on our heliosphere in 2004, the science team didn't have the instrument that would provide the most direct measurements of plasma. Instead, they focused on the direction of the magnetic field as a proxy for source of the plasma. Since solar plasma carries the magnetic field lines emanating from the sun and interstellar plasma carries interstellar magnetic field lines, the directions of the solar and interstellar magnetic fields were expected to differ.
Most models told the Voyager science team to expect an abrupt change in the magnetic field direction as Voyager switched from the solar magnetic field lines inside our solar bubble to those in interstellar space. The models also said to expect the levels of charged particles originating from inside the heliosphere to drop and the levels of galactic cosmic rays, which originate outside the heliosphere, to jump.
In May 2012, the number of galactic cosmic rays made its first significant jump, while some of the inside particles made their first significant dip. The pace of change quickened dramatically on July 28, 2012. After five days, the intensities returned to what they had been. This was the first taste of a new region, and at the time Voyager scientists thought the spacecraft might have briefly touched the edge of interstellar space.
By Aug. 25, when, as we now know, Voyager 1 entered this new region for good, all the lower-energy particles from inside zipped away. Some inside particles dropped by more than a factor of 1,000 compared to 2004. The levels of galactic cosmic rays jumped to the highest of the entire mission. These would be the expected changes if Voyager 1 had crossed the heliopause, which is the boundary between the heliosphere and interstellar space. However, subsequent analysis of the magnetic field data revealed that even though the magnetic field strength jumped by 60 percent at the boundary, the direction changed less than 2 degrees. This suggested that Voyager 1 had not left the solar magnetic field and had only entered a new region, still inside our solar bubble, that had been depleted of inside particles.
Then, in April 2013, scientists got another piece of the puzzle by chance. 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.
Then on April 9, 2013, it happened: Voyager 1's plasma wave instrument picked up local plasma oscillations. Scientists think they probably stemmed from a burst of solar activity from a year before, a burst that has become known as the St. Patrick's Day Solar Storms. The oscillations increased in pitch through May 22 and indicated that Voyager was moving into an increasingly dense region of plasma. This plasma had the signatures of interstellar plasma, with a density more than 40 times that observed by Voyager 2 in the heliosheath.
Gurnett and Kurth began going through the recent data and found a fainter, lower-frequency set of oscillations from Oct. 23 to Nov. 27, 2012. When they extrapolated back, they deduced that Voyager had first encountered this dense interstellar plasma in August 2012, consistent with the sharp boundaries in the charged particle and magnetic field data on August 25.
Stone called three meetings of the Voyager team. They had to decide how to define the boundary between our solar bubble and interstellar space and how to interpret all the data Voyager 1 had been sending back. There was general agreement Voyager 1 was seeing interstellar plasma, based on the results from Gurnett and Kurth, but the sun still had influence. One persisting sign of solar influence, for example, was the detection of outside particles hitting Voyager from some directions more than others. In interstellar space, these particles would be expected to hit Voyager uniformly from all directions.
"Now that we had actual measurements of the plasma environment – by way of an unexpected outburst from the sun – we had to reconsider why there was still solar influence on the magnetic field and plasma in interstellar space," Stone said.
"The path to interstellar space has been a lot more complicated than we imagined."
Stone discussed with the Voyager science group whether they thought Voyager 1 had crossed the heliopause. What should they call the region were Voyager 1 is?
"In the end, there was general agreement that Voyager 1 was indeed outside in interstellar space," Stone said. "But that location comes with some disclaimers – we're in a mixed, transitional region of interstellar space. We don't know when we'll reach interstellar space free from the influence of our solar bubble."
So, would the team say Voyager 1 has left the solar system? Not exactly – and that's part of the confusion. Since the 1960s, most scientists have defined our solar system as going out to the Oort Cloud, where the comets that swing by our sun on long timescales originate. That area is where the gravity of other stars begins to dominate that of the sun. It will take about 300 years for Voyager 1 to reach the inner edge of the Oort Cloud and possibly about 30,000 years to fly beyond it. Informally, of course, "solar system" typically means the planetary neighborhood around our sun. Because of this ambiguity, the Voyager team has lately favored talking about interstellar space, which is specifically the space between each star's realm of plasma influence.
"What we can say is Voyager 1 is bathed in matter from other stars," Stone said. "What we can't say is what exact discoveries await Voyager's continued journey. No one was able to predict all of the details that Voyager 1 has seen. So we expect more surprises."
Voyager 1, which is working with a finite power supply, has enough electrical power to keep operating the fields and particles science instruments through at least 2020, which will mark 43 years of continual operation. At that point, mission managers will have to start turning off these instruments one by one to conserve power, with the last one turning off around 2025.
Voyager 1 will continue sending engineering data for a few more years after the last science instrument is turned off, but after that it will be sailing on as a silent ambassador. In about 40,000 years, it will be closer to the star AC +79 3888 than our own sun. (AC +79 3888 is traveling toward us faster than we are traveling towards it, so while Alpha Centauri is the next closest star now, it won't be in 40,000 years.) And for the rest of time, Voyager 1 will continue orbiting around the heart of the Milky Way galaxy, with our sun but a tiny point of light among many.
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Reaching interstellar space isn't the first time Voyager 1 has been among stars. The spacecraft transcended into both film and television. In the first "Star Trek" feature film, a V'Ger spacecraft is revealed to be Voyager 6, a fictional Earth space probe modeled after Voyager. In a "Saturday Night Live" segment, Steve Martin's character predicts that an upcoming cover of Time Magazine will show the words "Send More Chuck Berry," in reference to Voyager and its Golden Record.
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In 1990, Voyager-1 looked back and took a picture of Earth - a "pale blue dot"
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Quelle: NASA
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Update: 13.09.2013
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Voyager 1 im Februar 2013 von der Erde mit der NRAO VLBA und GBT Teleskopen erfasst.
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Voyager 1 Spotted from Earth with NRAO's VLBA and GBT Telescopes
Earlier this year, the National Science Foundation's Very Long Baseline Array telescope turned its gaze to NASA's famed Voyager 1 and captured an image of this iconic spacecraft's faint radio signal. The Green Bank Telescope also detected Voyager's signal, picking it out from the background radio noise in less than one second.
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Astronomers using the National Science Foundation's (NSF) Very Long Baseline Array (VLBA) and Green Bank Telescope (GBT) spotted the faint radio glow from NASA's famed Voyager 1 spacecraft -- the most distant man-made object.
According to NASA's Jet Propulsion Laboratory (JPL), the VLBA imaged the signal from Voyager 1's main transmitter after the spacecraft had already passed beyond the edge of the heliosphere, the bubble of charged particles from the Sun that surrounds our Solar System.
Using NASA's Deep Space Network, JPL continually tracks Voyager and calculates its position on the sky, which is known as the ephemeris. Since the VLBA has the highest resolution, or ability to see fine detail, of any full-time astronomical instrument, NRAO astronomers believed they could locate Voyager's ephemeris position with unprecedented precision. This is unrelated to Voyager's distance from the Sun or position relative to the heliosphere.
The initial observations, which were made on February 21, placed Voyager very near, but not precisely at its predicted location. The difference was a few tenths of an arcsecond. An arcsecond is the apparent size of a penny as seen from 2.5 miles (4 kilometers) away. The second observations on June 1 produced similar results.
"It is possible that these observations are at the milliarcsecond [one-thousandth of an arcsecond] level, or better," said NRAO scientist Walter Brisken, who led the observations with the VLBA. At 11.5 billion miles -- Voyager's approximate distance at the time of the initial observations -- one milliarcsecond would be roughly 50 miles across.
Voyager's main transmitter shines at a feeble 22 watts, which is comparable to a car-mounted police radio or -- in visible light -- a refrigerator light bulb. Though incredibly weak by the standards of modern wireless communications, Voyager's signal is astoundingly bright when compared to most natural objects studied by radio telescopes.
"The ability to pinpoint the location of Voyager and other spacecraft is critical as we explore the inner Solar System and beyond," said Brisken. "The NRAO's VLBA has the capability to do this vital task with unprecedented precision."
Voyager 1, which was launched in 1977, is now headed away from the Sun at a speed of about 38,000 miles per hour.
In a remarkably sensitive complementary observation, the NRAO's Green Bank Telescope (GBT), which is the world's largest fully steerable radio telescope, easily detected Voyager's signal, picking it out from the background radio noise in less than one second.
"Voyager is the first man-made object to penetrate the interstellar medium, and we really want to be able to receive the data from this new frontier," said NRAO scientist Toney Minter, who oversaw the Green Bank observations. "This information will provide many clues about how the interstellar medium behaves and how the Sun interacts with it."
"NRAO's instruments have the capability to provide the most accurate position information of distant spacecraft like Voyager," said NRAO Director Tony Beasley. "The remarkable sensitivity of GBT and VLBA's sharp vision are essential for discovery but also have unique capabilities that have enabled us to make this contact with one of humanity's most ambitious missions of exploration."
The VLBA is a system of radio antennas located across the United States from Hawaii to St. Croix. The antennas work together as a single telescope nearly 5,000 miles across, giving the VLBA its ability to see fine details. Only seven of the VLBA's full complement of 10 antennas were used to make these observations.
The 100-meter GBT is located in the National Radio Quiet Zone and the West Virginia Radio Astronomy Zone, which protect the incredibly sensitive telescope from unwanted radio interference. The GBT observations were made by NRAO scientists Toney Minter and Frank Ghigo, and Green Bank Director Karen O'Neil.
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Caption 1: Artist's impression of Voyager 1's position on the sky when observed by the Very Long Baseline Array (VLBA) on February 21, 2013, at which point -- according to NASA's Jet Propulsion Laboratory -- Voyager was already outside of our Solar System. The actual image from the data (enlarged section) is 0.5 arcseconds across. The radio signal as shown is a mere 1 milliarcsecond across.
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Image Caption 2: NRAO's Very Long Baseline Array (VLBA) telescope catches a glimpse of the signal from Voyager 1's transmitter as seen from more than 11 billion miles away. The slightly oblong shape of the image is a result of the VLBA antenna configuration.
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Image Caption 3: Astronomers using the NRAO's Green Bank Telescope (GBT) detected Voyager 1's signal. The signal has a specific polarization, or radio-wave orientation, that helped confirm its identity.
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Quelle: NRAO
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VOYAGER-1 PWS:
ELECTRON PLASMA OSCILLATIONS BEYOND THE HELIOPAUSE
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http://www.youtube.com/watch?v=aNB4FaNh0wQ
This animation combines two ways of displaying the Voyager Plasma Wave Science (PWS) observations of electron plasma oscillations which provide the basis for concluding that the spacecraft is now in interstellar space. The graphic is called a spectrogram that shows the amplitude of waves (in which reds are the most intense and blues the least intense) as a function of frequency (vertical axis) and time (horizontal axis). In many respects, this spectrogram is like a voice print which shows the evolution of the spectrum of sounds as a function of time. The sound track reproduces the amplitude and frequency of the plasma waves observed. The vertical white bar that moves across the spectrogram links the sound track to the graphic.
The frequency range shown from about 1.75 kiloHertz to 3.5 kiloHertz is a portion of the actual frequency range detected by PWS and is well within the audio frequency range. Importantly, the frequency is directly related to the number of electrons per unit volume in the vicinity of Voyager and corresponds to about 1 electron per 10 cubic centimeters or a cube about 1 inch on a side. The time scale for this presentation represents 225 days or a bit more than 7 months, while it only takes about 12 seconds to play the audio file. Hence, the time compression is about 1.6 million to one. It should be noted that this compression was done in such a way as to not change the frequencies.
In this animation, there are two events of interest. In the October-November 2012 time frame there is a tone near 2.1 kHz which gradually increases in frequency. Again, in the April-May 2013 time frame there is another event, somewhat more intense and at a higher frequency near 2.6 kHz. We conclude that these two events indicate an ongoing trend to higher frequencies. The second graphic frame which appears in the animation includes a dashed line showing this increase in frequency and suggests that the density of electrons is continually increasing over this time interval as Voyager moves outwards from the heliopause (which was crossed on 25 August 2012).
Quelle: University of Iowa
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Update: 14.09.2013
In Situ Observations of Interstellar Plasma With Voyager 1
Launched over 35 years ago, Voyagers 1 and 2 are on an epic journey outward from the Sun to reach the boundary between the solar plasma and the much cooler interstellar medium. The boundary, called the heliopause, is expected to be marked by a large increase in plasma density, from about 0.002 cm−3 in the outer heliosphere, to about 0.1 cm−3 in the interstellar medium. On 9 April 2013, the Voyager 1 plasma wave instrument began detecting locally generated electron plasma oscillations at a frequency of about 2.6 kHz. This oscillation frequency corresponds to an electron density of about 0.08 cm−3, very close to the value expected in the interstellar medium. These and other observations provide strong evidence that Voyager 1 has crossed the heliopause into the nearby interstellar plasma.
Quelle:AAAS
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Update: 6.10.2013
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Voyager's view
Spacecraft’s journey to interstellar space helps put the solar system in perspective
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It’s finally official: Voyager 1 has become the first human-made object to enter interstellar space, mission scientists report September 12 in Science. On August 25, 2012, the scientists say, Voyager 1 exited a giant invisible bubble called the heliosphere that is inflated by a torrent of subatomic particles spewing from the sun. Now the probe is surrounded almost exclusively by particles produced by other stars. But whether it’s correct to say that the probe has left the solar system depends on how you define the solar system. “From my perspective, Voyager is nowhere near the edge of the solar system,” says planetary scientist Hal Levison of the Southwest Research Institute in Boulder, Colo. The sun continues to exert gravitational dominance out to hundreds of times the distance of Voyager 1 from the sun, where trillions of icy pebbles, boulders and comets orbit. In the last 36 years, Voyager has traveled an impressive 25.4 billion kilometers, but it still has a long way to go to unambiguously depart the solar system.
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OORT CLOUD
Distance from the sun: 5,000–100,000 AU
The sun, planets and Voyager probes sit inside the tiny yellow dot at right, within a giant sphere called the Oort cloud. This reservoir of trillions of ice chunks extends 100,000 astronomical units out, tethered to the sun by gravity. Astronomers believe these objects got thrown out of the inner solar system as the planets took shape 4.5 billion years ago. Occasionally these castaways pass near Earth: The comet ISON, which may light up the night sky this November, started out in the Oort cloud. The Voyagers would have to travel another 30,000 years before clearing this broadest definition of the solar system.
Distance from the sun: 5,000–100,000 AU
The sun, planets and Voyager probes sit inside the tiny yellow dot at right, within a giant sphere called the Oort cloud. This reservoir of trillions of ice chunks extends 100,000 astronomical units out, tethered to the sun by gravity. Astronomers believe these objects got thrown out of the inner solar system as the planets took shape 4.5 billion years ago. Occasionally these castaways pass near Earth: The comet ISON, which may light up the night sky this November, started out in the Oort cloud. The Voyagers would have to travel another 30,000 years before clearing this broadest definition of the solar system.
VOYAGER 1
Current distance from the sun: 126 AU
1 astronomical unit = 150 million
kilometers (Earth-sun distance)
Voyager 1 is now surrounded by a relatively thick fog of subatomic particles produced in the far reaches of the galaxy. Some particles originated in supernova explosions; others got blasted out of black holes. By 2016 astronomers expect the probe’s sibling spacecraft to pop through the solar bubble. Unlike Voyager 1, Voyager 2 carries a working instrument to measure the temperature and density of the interstellar medium. Both probes have enough plutonium power to communicate with Earth until about 2025.
HELIOPAUSE
Distance from the sun: about 122 AU
Until recently, Voyager 1 was traveling within the heliosphere, bathed in a thin mist of particles from the solar wind. Voyager 1 passed through the boundary between the heliosphere and interstellar space, called the heliopause, last August. But the border crossing was not cut-and-dried: Astronomers expected the magnetic field to change direction in interstellar space along with the particle population, yet the field has barely budged. Theorists are struggling to understand why.
TERMINATION SHOCK
Distance from the sun: about 90 AU
The solar wind gradually slows as it cruises past the planets. About 13 billion kilometers from where that wind originates, it slows down to about 350,000 kilometers per hour and generates a shock wave analogous to the one produced when a jet crosses the sound barrier. Voyager 1 reached this shock wave, known as the termination shock, in 2004. Beyond it, the solar wind wanes as the gateway to interstellar space approaches.
SOLAR WIND
The sun unleashes a continuous stream of subatomic particles at more than 1.5 million kilometers per hour. This solar wind permeates a radius of billions of kilometers in all directions and inflates the heliosphere. For some astrophysicists, the solar system is defined by the presence of the solar wind.
KUIPER BELT
Distance from the sun: 30–100 AU
For much of the past quarter century, Voyager 1 has been traversing this disk of icy objects (including Pluto) that were not incorporated into planets when the solar system formed.
TAIL
The sun, planets and entire heliosphere orbit the center of the galaxy at a brisk 83,000 kilometers per hour. In July NASA’s Interstellar Boundary Explorer satellite discovered that the sun drags behind it a cometlike tail of subatomic particles (not shown) that may stretch 10 times as far from the sun as Voyager 1’s current position. The finding shows that the solar bubble is shaped more like an elongated bullet than a sphere. Fortunately Voyager 1 trekked toward the leading edge of the bubble, where the distance to interstellar space is comparatively short.
THE PLANETS
Neptune’s distance from sun: 30 AU
The notion of the solar system as the sun plus eight planets (or nine, depending on your age) largely gets abandoned after grade school. Voyager 1 passed Neptune’s orbit in May 1987 and has since logged 14.2 billion kilometers.
Quelle: ScienceNews
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