Voyager: Beyond the Great Beyond
Hurtling through the frigid expanse beyond the planets, NASA’s aging spacecraft Voyager 1 has reached the edge of the Solar System, where it has encountered a massive shock wave, according to two paper that will be published in Nature by a team of scientists. Voyager‘s journey through this turbulent zone will give scientists the first direct measurements of our solar system’s unexplored final frontier, the heliosheath. Scientists are debating whether this passage has already begun.
|Earth as seen by the departing Voyager spacecraft: a tiny, pale blue dot.
"Voyager 1 has seen striking signs of the region deep in space where a giant shock wave forms, as the wind from the Sun abruptly slows and presses outward against the interstellar wind. The observations surprised and puzzled us, so there is much to be discovered as it begins exploring this new region at the outer edge of the solar system," said Dr. Edward Stone, Voyager project scientist at the California Institute of Technology in Pasadena.
Let the Debates Begin
The first paper, by Dr. Stamatios Krimigis of the Johns Hopkins University Applied Physics Laboratory, Laurel, Md., and his team, supports the claim Voyager 1 passed beyond the termination shock. "We have used an indirect technique to show the solar wind slowed down from about 700,000 miles per hour to much less than 100,000 mph. We used this same technique when the instrument measuring the solar wind speed was still working. The agreement between the two measurements was better than 20 percent in most cases," Krimigis said.
The second paper, by Dr. Frank McDonald of the University of Maryland, College Park, and his team, disputes the claim. A third paper, published October 30 in Geophysical Research Letters by Dr. Leonard Burlaga of Goddard Space Flight Center, Greenbelt, Md., and collaborators, states Voyager 1 did not pass beyond the termination shock. "The analysis of the Voyager 1 magnetic field observations in late 2002 indicate that it did not enter a new region of the distant heliosphere by having crossed the termination shock. Rather, the magnetic field data had the characteristics to be expected based upon many years of previous observations, although the intensity of energetic particles observed is unusually high," Burlaga said.
The controversy would be resolved if Voyager could measure the speed of the solar wind, because the solar wind slows abruptly at the termination shock. However, the instrument that measured solar wind speed no longer functions on the spacecraft.
Beyond the Great Beyond
More than eight billion miles from Earth, the farthest an operating spacecraft has ever journeyed, Voyager 1 is providing scientists with data from a fascinating yet little understood region of space – the frontier where the Sun’s influence begins to wane, and the tenuous vastness of interstellar space takes over.
It is here that charged particles streaming out from the Sun – called the solar wind – bump into the ionized gas and dust that is spread thinly between stars, causing a shock wave in the process. The termination shock is where the solar wind, a thin stream of electrically charged gas blown constantly from the Sun, is slowed by pressure from gas between the stars. At the termination shock, the solar wind slows abruptly from its average speed of about 700,000 to 1,500,000 miles per hour.
Estimating the location of the termination shock is hard, because we don’t know the precise conditions in interstellar space. We do know speed and pressure of the solar wind changes, which cause the termination shock to expand, contract and ripple.
"When the Voyager missions were launched in 1977, we never thought the instruments we developed more than three decades ago would one day probe the edge of the Solar System," said Louis Lanzerotti of Bell Labs, an expert on how the solar wind affects terrestrial communications. Lanzerotti has been involved with NASA’s Voyager missions since their inception in 1972. Launched on September 5, 1977, Voyager 1 explored the giant planets Jupiter and Saturn before being tossed out toward deep space by Saturn’s gravity. It is approaching, and may have temporarily entered, the region beyond termination shock. At more than 13 billion kilometers (approximately eight billion miles) from the Sun, Voyager 1 is the most distant object from Earth built by humanity.
"Voyager 1 and 2 provided us with unparalleled views of the planets. I find it very exciting that Voyager 1 will soon begin to explore the vastness of the interstellar medium," he said.
Astronomers now know that the space between stars, once thought to be completely empty, is filled with a dilute plasma of gas and dust termed the interstellar medium. The solar wind blows out a giant bubble called the heliosphere within the interstellar medium, and the boundary between the heliosphere and the interstellar medium is a place where a lot of interesting physical phenomena take place.
Much like a shock wave precedes a supersonic airplane, astronomers think that a "termination shock" occurs near the edge of the solar system. This is a region where the speed of the solar wind drops dramatically as the wind brakes as it mixes with the interstellar medium, and where the density of ionized particles increases many times. It is also a region where some ions from the interstellar medium, relics of previous generations of stars, manage to diffuse through the edge of the Solar System and are accelerated tremendously.
Using an instrument on Voyager 1 called the low energy charged particle detector, Lanzerotti and his colleagues found evidence of all three effects. They saw a hundred-fold increase in the number of charged particles detected during a six-month period starting in August 2002; they deduced that the speed of the solar wind had dropped by a factor of seven; and they detected ions that came from beyond the Solar System.
"When we saw all that, we were pretty sure that we had reached the termination shock," Lanzerotti said. Lanzerotti is a distinguished research professor in physics and at the Center for Solar Research, New Jersey Institute of Technology.
From about August 1, 2002 to February 5, 2003, scientists noticed unusual readings from the two energetic particle instruments on Voyager 1, indicating it had entered a region of the solar system unlike any previously encountered. This led some to claim Voyager 1 may have entered a transitory feature of the termination shock.
|V-ger, Spock investigates ancient probe. The alien from the original 1983 film Star Trek, The Motion Picture. V-ger, as it called itself (pronounced vee-jer) was an ancient mechanical space probe that was on a mission to explore and discover and report back its findings. Captain Kirk discovers that the probe was called Voyager 6, which had been launched by humans on Earth in the late twentieth century. Apparently, the ‘oya’ that fits between the ‘V’ and the ‘ger’ had been badly tarnished and was unreadable.
As the Krimigis team analyzed its data from Voyager 1, it noticed something strange. It seemed that after six months, in early 2003, the termination shock region moved outwards, possibly as a result of increased solar activity. Scientists postulate that the heliosphere is a dynamic entity, which expands and contracts with the Sun’s 11-year activity cycle.
"Solar eruptions must have caused the solar wind to pick up speed," said Lanzerotti. "That forced the heliosphere to expand outwards, and the termination shock must have moved outwards as well. Voyager 1 will probably encounter it again."
The Krimigis team expects to get confirming data from a similar detector on Voyager 2, which is expected to follow Voyager 1 to the edge of the Solar System. The Voyager probes, among NASA’s most successful spacecraft, are expected to provide data until approximately 2020, when they will exhaust their power supply from nuclear isotopes and drift off into space. Still operating in remote, cold and dark conditions 26 years later, the Voyagers owe their longevity to these generators, which produce electricity from the heat generated by the natural decay of plutonium dioxide.
"Meanwhile, they will continue to provide a treasure trove of wonderful, sometimes unanticipated data about the far reaches of the Solar System and push the frontiers of our knowledge," Lanzerotti said. Data of how the solar wind behaves will provide scientists with knowledge useful not only to astronomy but to terrestrial concerns such as how solar emissions affect wireless telephone calls, satellite communications and electric power grids.
When initially launched, the Voyager missions to Jupiter and Saturn were considered mission-capable for around 5-year lives. But after 25 years, there is a good chance that another 25 years for their tour is on-course. Voyager project manager Ed Massey recently concluded: "I don’t think anybody expected we would still be here 25 years later though we probably could go another 25 years."
Assessing their key radio-isotope generators that power the on-board battery, Massey evaluates: "We don’t run out of electrical power until about 2020", or at least for Voyager I, around 43 years towards its lifetime of some communication with its originating star, Sol, and its home planet, the Earth.
Voyagers 1 and 2 were built by JPL, which continues to operate both spacecraft 26 years after their launch. The spacecraft are controlled and their data returned through NASA’s Deep Space Network, a global spacecraft tracking system also operated by JPL. The Voyager Project Manager is Ed Massey of JPL.