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Voyager 1 becomes first human-made object to leave solar system

Voyager 1 and 2 were launched in 1977, 16 days apart

CNN News Wire | 9/12/2013, 1:23 p.m.
This artist's concept depicts NASA's Voyager 1 spacecraft entering interstellar space, or the space between stars. Interstellar space is dominated by the plasma, or ionized gas, that was ejected by the death of nearby giant stars millions of years ago. The environment inside our solar bubble is dominated by the plasma exhausted by our sun, known as the solar wind. The interstellar plasma is shown with an orange glow similar to the color seen in visible-light images from NASA's Hubble Space Telescope that show stars in the Orion nebula traveling through interstellar space. NASA/JPL-Caltech

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.

“It’s still taking data and telling us what it’s actually like out there,” said Marc Swisdak, associate research scientist in the Institute for Research in Electronics and Applied Physics at the University of Maryland. Swisdak was not involved in the new study, but has worked with Voyager data.

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.

Technically, if you include the distant comets that orbit the sun, then Voyager 1 has not left “the solar system,” Stone said. For that, we’ll have to wait another 30,000 years.

How do we know?

Voyager 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 the 1980s, Swisdak said.

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.

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.