By Elizabeth Landau
At the edge of the heliosphere, you wouldn’t know by looking whether you left the cradle of humanity behind and floated out into interstellar space. You would just see unfathomably empty space, no matter which side of the invisible line you were on.
But scientists now have strong evidence that NASA’s Voyager 1 probe has crossed this important border, making history as the first human-made object to leave the heliosphere, the magnetic boundary separating the solar system’s sun, planets and solar wind from the rest of the galaxy.
“In leaving the heliosphere and setting sail on the cosmic seas between the stars, Voyager has joined other historic journeys of exploration: The first circumnavigation of the Earth, the first steps on the Moon,” said Ed Stone, chief scientist on the Voyager mission. “That’s the kind of event this is, as we leave behind our solar bubble.”
A new study in the journal Science suggests that the probe entered the interstellar medium around August 25, 2012. You may have heard other reports that Voyager 1 has made the historic crossing before, but Thursday was the first time NASA announced it.
The twin spacecraft Voyager 1 and 2 were launched in 1977, 16 days apart. As of Thursday, according to NASA’s real-time odometer, Voyager 1 is 18.8 billion kilometers (11.7 billion miles) from Earth. Its sibling, Voyager 2, is 15.3 billion (9.5 billion) kilometers from our planet.
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Voyager 1 is being hailed as the first probe to leave the solar system. But under a stricter definition of “solar system,” which includes the distant comets that orbit the sun, we’d have to wait another 30,000 years for it to get that far, Stone said.
Another milestone for long after we’re gone: The probe will fly near a star in about 40,000 years, Stone said.
How do we know?
Voyager, currently traveling at more than 38,000 miles per hour, never sent a postcard saying “Greetings from interstellar space!” So whether it has made the historic crossing or not is a matter of controversy.
“The spacecraft itself really doesn’t know,” Stone said. “It’s only instruments that can tell us whether we’re inside or outside.”
Further complicating matters, the device aboard Voyager 1 that measures plasma — a state of matter with charged particles — broke in 1980.
To get around that, scientists detected waves in the plasma around the spacecraft and used that information to calculate density. Vibrations in the plasma came from a large coronal mass ejection from the sun in 2012, resulting in what Stone called a “solar wind tsunami.” These vibrations reached the area around Voyager this spring.
Measurements taken between April 9 and May 22 of this year show that Voyager 1 was, at that time, located in an area with an electron density of about 0.08 per cubic centimeter.
In the interstellar medium, the density of electrons is thought to be between 0.05 and 0.22 per cubic centimeter. The particles of interstellar plasma were created by the explosions of giant stars, and carry the magnetic field of the galaxy, scientists said.
Last year, between October 23 and November 27, researchers calculate that Voyager 1 was in an area with an electron density of 0.06 per cubic centimeter. That’s still within the interstellar space range, and it means that over time the spacecraft passed through plasma with increasing electron density.
The study, led by University of Iowa physicist Donald Gurnett, suggests that the plasma density is about 30 times higher in the interstellar medium than in the heliosphere, which is close to what scientists thought based on other kinds of measurements. The boundary is called the heliopause.
By Elizabeth Landau