Voyager I May Soon Become the First Spacecraft to Reach Interstellar Space

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After more than 33 years of travel, the two Voyager spacecraft will soon reach interstellar space, which is the space between stars.

In a major milestone, NASA has reported that the Voyager I spacecraft may be close to becoming the first human creation ever to cross the invisible boundary separating our solar system from interstellar space. A surge of new data shows that the probe has been encountering a huge uptick in galactic cosmic rays — a key factor that would signify that the Voyager has, indeed, left the building.

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To be absolutely sure, scientists are relying on a couple of other key criteria. In addition to the uptick of certain particles, the presence of another particle would have to decrease completely. And while that number is not at zero, there has been a slow decline, which physicists expect to change dramatically when the spacecraft crosses the barrier. The magnetic field surrounding the spacecraft would also completely change, from east-west to north-south. The problem is that no one is sure if there will be a definitive moment when the Voyager crosses the threshold.

“The laws of physics say that someday Voyager will become the first human-made object to enter interstellar space, but we still do not know exactly when that someday will be,” said Ed Stone, a physicist and professor at the California Institute of Technology on the project.

Whenever Voyager crosses that threshold, it would certainly go down as a pivotal moment in space history, akin to when Sputnik became the first artificial satellite to be launched into Earth’s orbit in 1956.

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Traveling at a speed of about 38,000 miles per hour, the spacecraft is estimated to be almost 11 billion miles away from the Earth. As a result, it takes scientists nearly 17 hours to receive data from the probe. The Voyager was originally launched  back in 1977.

The significance of the new developments was not lost on Stone. “The latest data indicate that we are clearly in a new region where things are changing more quickly. It is very exciting. We are approaching the solar system’s frontier.”

Erica Ho is a contributor at TIME and the editor of Map Happy. Find her on Twitter at @ericamho and Google+. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.