For the second time in history, ahuman-made object has reached the space between the stars. NASA's Voyager 2probe now has exited the heliosphere - the protective bubble of particles andmagnetic fields created by the Sun.
Members of NASA's Voyager team willdiscuss the findings at a news conference at 11 a.m. EST (8 a.m. PST) today atthe meeting of the American Geophysical Union (AGU) in Washington. The news conferencewill stream live on the agency's website.
Comparingdata from different instruments aboard the trailblazing spacecraft, missionscientists determined the probe crossed the outer edge of the heliosphere onNov. 5. This boundary, called the heliopause, is where the tenuous, hot solarwind meets the cold, dense interstellar medium. Its twin, Voyager1, crossed this boundary in 2012, but Voyager 2 carries a workinginstrument that will provide first-of-its-kind observations of the nature ofthis gateway into interstellar space.
Voyager 2 nowis slightly more than 11 billion miles (18 billion kilometers) from Earth.Mission operators still can communicate with Voyager 2 as it enters this newphase of its journey, but information - moving at the speed of light - takesabout 16.5 hours to travel from the spacecraft to Earth. By comparison, lighttraveling from the Sun takes about eight minutes to reach Earth.
Artist's concept of Voyager 2 with 9 facts listed around it. Image Credit: NASA
Larger view
The mostcompelling evidence of Voyager 2's exit from the heliosphere came from its onboardPlasma Science Experiment (PLS),an instrument that stopped working on Voyager 1 in 1980, long before that probecrossed the heliopause. Until recently, the space surrounding Voyager 2 wasfilled predominantly with plasma flowing out from our Sun. This outflow, calledthe solar wind, creates a bubble - the heliosphere - that envelopes the planetsin our solar system. The PLS uses the electrical current of the plasma todetect the speed, density, temperature, pressure and flux of the solar wind. ThePLS aboard Voyager 2 observed a steep decline in the speed of the solar windparticles on Nov. 5. Since that date, the plasma instrument has observed nosolar wind flow in the environment around Voyager 2, which makes missionscientists confident the probe has left the heliosphere.
At the end of 2018, the cosmic ray subsystem aboard NASA's Voyager 2 spacecraft provided evidence that Voyager 2 had left the heliosphere. There were steep drops in the rate of heliospheric particles that hit the instrument's radiation detector, and significant increases in the rate of cosmic rays. Image Credit: NASA/JPL-Caltech/GSFC
Larger view
"Workingon Voyager makes me feel like an explorer, because everything we're seeing isnew," said John Richardson, principal investigator for the PLS instrument and aprincipal research scientist at the Massachusetts Institute of Technology inCambridge. "Even though Voyager 1 crossed the heliopause in 2012, it did so ata different place and a different time, and without the PLS data. So we'restill seeing things that no one has seen before."
In additionto the plasma data, Voyager's science team members have seen evidence from threeother onboard instruments - the cosmic ray subsystem, the low energy chargedparticle instrument and the magnetometer - that is consistent with theconclusion that Voyager 2 has crossed the heliopause. Voyager's team membersare eager to continue to study the data from these other onboard instruments toget a clearer picture of the environment through which Voyager 2 is traveling.
"There isstill a lot to learn about the region of interstellar space immediately beyondthe heliopause," said Ed Stone, Voyager project scientist based at Caltech inPasadena, California.
Together, thetwo Voyagers provide a detailed glimpse of how our heliosphere interacts withthe constant interstellar wind flowing from beyond. Their observationscomplement data from NASA's Interstellar Boundary Explorer (IBEX),a mission that is remotely sensing that boundary. NASA also is preparing an additionalmission - the upcoming Interstellar Mapping and Acceleration Probe (IMAP),due to launch in 2024 - to capitalize on the Voyagers' observations.
"Voyager has avery special place for us in our heliophysics fleet," said Nicola Fox, directorof the Heliophysics Division at NASA Headquarters. "Our studies start at theSun and extend out to everything the solar wind touches. To have the Voyagerssending back information about the edge of the Sun's influence gives us anunprecedented glimpse of truly uncharted territory."
While theprobes have left the heliosphere, Voyager 1 and Voyager 2 have not yet left thesolar system, and won't be leaving anytime soon. The boundary of the solarsystem is considered to be beyond the outer edge of the OortCloud, a collection of small objects that are still under theinfluence of the Sun's gravity. The width of the OortCloud is not known precisely, but it is estimated to begin at about 1,000astronomical units (AU) from the Sun and to extend to about 100,000 AU. One AUis the distance from the Sun to Earth. It will take about 300 years for Voyager2 to reach the inner edge of the Oort Cloud and possibly 30,000 years to flybeyond it.
The Voyagerprobes are powered using heat from the decay of radioactive material, containedin a device called a radioisotope thermal generator (RTG).The power output of the RTGs diminishes by about four watts per year, whichmeans that various parts of the Voyagers, including the cameras on bothspacecraft, have been turned off over time to manage power.
"I think we'reall happy and relieved that the Voyager probes have both operated long enoughto make it past this milestone," said Suzanne Dodd, Voyager project manager atNASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "This is whatwe've all been waiting for. Now we're looking forward to what we'll be able tolearn from having both probes outside the heliopause."
Voyager 2launched in 1977, 16 days before Voyager 1, and both have traveled well beyondtheir original destinations. The spacecraft were built to last five years andconduct close-up studies of Jupiter and Saturn. However, as themission continued, additional flybys of the two outermost giant planets, Uranusand Neptune, proved possible. As the spacecraft flew across the solar system,remote-control reprogramming was used to endow the Voyagers with greatercapabilities than they possessed when they left Earth. Their two-planet missionbecame a four-planet mission. Their five-year lifespans have stretched to 41years, making Voyager 2 NASA'slongest running mission.
The Voyager story has impacted not only generations ofcurrent and future scientists and engineers, but also Earth's culture,including film, art and music. Each spacecraft carries a Golden Record of Earth sounds, pictures andmessages. Since the spacecraft could last billions of years, these circulartime capsules could one day be the only traces of human civilization.
Voyager'smission controllers communicate with the probes using NASA's Deep Space Network(DSN),a global system for communicating with interplanetary spacecraft. The DSNconsists of three clusters of antennas inGoldstone, California; Madrid, Spain; and Canberra, Australia.
The VoyagerInterstellar Mission is a part of NASA's Heliophysics System Observatory,sponsored by the Heliophysics Division of NASA's Science Mission Directorate inWashington. JPL built and operates the twin Voyager spacecraft. NASA's DSN,managed by JPL, is an international network of antennas that supportsinterplanetary spacecraft missions and radio and radar astronomy observationsfor the exploration of the solar system and the universe. The network alsosupports selected Earth-orbiting missions. The Commonwealth Scientific andIndustrial Research Organisation, Australia's national science agency, operatesboth the Canberra Deep Space Communication Complex, part of the DSN, and theParkes Observatory, which NASA has been using to downlink data from Voyager 2since Nov. 8.
For more information aboutthe Voyager mission, visit:
https://voyager.jpl.nasa.gov/
https://www.nasa.gov/voyager
More information about NASA'sHeliophysics missions is available online at:
https://www.nasa.gov/sunearth
