Voyager 1 has left the solar system, scientists say in journal article

Published March 21, 2013 12:50am

The Voyager 1 (V1) spacecraft left the solar system and has gone beyond the reach of the sun's cosmic rays, according to scientists whose research work on the deep space probe was published in preliminary form on the Internet on Wednesday. V1 would be the first man-made spacecraft ever to travel beyond the solar system.

Voyager 1 and its companion Voyager 2 (V2) launched in 1977 on a mission to study planets. V2 was launched first on August 20 and V1 was sent into space on September 5.

In their article for the American Geophysical Union's Geophysical Research Letters (GRL), Bill Webber and Frank McDonald said V1 crossed the boundary of the solar system on August 25, 2012 when it was 121.7 astronomical units (AU) from the sun.

"At the Voyager 1 spacecraft in the outer heliosphere the intensities of both anomalous cosmic rays (ACR) and galactic cosmic rays (GCR) changed suddenly and decisively on August 25th (121.7 AU from the Sun)," the scientists said.

One AU is 150 million kilometers or approximately the distance between the Earth and the sun.

ACRs, according to the Center for Astrophysics and Supercomputing at the Swinburne University of Technology in Melbourne, Australia, "are thought to originate from neutral interstellar atoms that have been ionized by solar ultraviolet (UV) radiation after entering the heliosphere. Once ionized, these charged particles are carried on the solar wind (and associated magnetic field) back out to the shock created where the solar wind meets interstellar space."

GCRs, on the other hand, are believed to "originate in the shocks of supernova remnants" from deep space, the Center for Astrophysics and Supercomputing also said on its Cosmos website.

Webber and McDonald posit that the "suddenness of these intensity changes indicate that V1 has crossed a well-defined boundary for energetic particles at this time possibly related to the heliopause."

Webber acknowledged that the actual location of the spacecraft -- whether in interstellar space or just an unknown region beyond the solar system -- remained a matter of debate.

"It's outside the normal heliosphere, I would say that," said Webber, professor emeritus of astronomy at New Mexico State University in Las Cruces, according to the AGU's web site.

"We're in a new region. And everything we're measuring is different and exciting."

Voyager on a "magnetic highway"

In December, scientists said Voyager had reached what they called a "magnetic highway," where magnetic field lines from the sun connect with magnetic field lines from interstellar space.

"We believe this is the last leg of our journey to interstellar space," Voyager project scientist Edward Stone said at the time. "Our best guess is it's likely just a few months to a couple years away."

"The Voyager team is aware of reports today that NASA's Voyager 1 has left the solar system," said Edward Stone, Voyager project scientist based at the California Institute of Technology, Pasadena, California.

In a statement on Wednesday, Stone said more evidence is needed to indicate Voyager has left the solar system.

"It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space," Stone said.

"A change in the direction of the magnetic field is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed," he said.

Riding winds of outer space

According to the United States' National Aeronautics and Space Administration (NASA), the heliopause "is the outermost boundary of the solar wind, where the interstellar medium restricts the outward flow of the solar wind and confines it within a magnetic bubble called the heliosphere. The solar wind is made up of electrically charged atomic particles, composed primarily of ionized hydrogen, that stream outward from the Sun."

NASA also said on its webpages devoted to the Voyager Mission that "by studying the radio emissions, scientists now theorize the heliopause exists some 90 to 120 astronomical units (AU) from the Sun."

For months, experts have been closely watching for hints that Voyager 1 has left the solar system and most have estimated it would likely happen in the next year or two.

NASA has described Voyager 1 -- now 11 billion miles (18 billion kilometers) away from the Sun -- and its companion Voyager 2 as "the two most distant active representatives of humanity and its desire to explore."

NASA said Voyagers 1 and 2 are "identical spacecraft...equipped with instruments to conduct 10 different experiments. The instruments include television cameras, infrared and ultraviolet sensors, magnetometers, plasma detectors, and cosmic-ray and charged-particle sensors."

Voyager scientist went ahead to the 'great beyond'

Dr. McDonald died on August 31, 2012 six days after Voyager 1 left the solar system.

The AGU GRL journal paper pays tribute to McDonald. "This article was conceived by our Voyager colleague, Frank McDonald, who is no longer with us," Webber wrote.

Webber also said, "Frank, we have been working together for over 55 years to reach the goal of actually observing the interstellar spectra of cosmic rays, possibly now achieved almost on the day of your passing. You wanted so badly to be able to finish this article that you had already started. Together we did it. Bon Voyage!" — with reports from Agence France Presse and Reuters/Earl Victor Rosero, GMA News

Tags: voyager1, spaceexploration