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Going… Going… Gone! Voyager I Enters Interstellar Space

September 12, 2013

About one year ago, I wrote a blog commemorating the 35th anniversary of the launch of Voyager I, the second of the twin Voyager spacecraft to be launched to deep space; Voyager II launched 16 days earlier. These amazing spacecraft — the fastest ever made — have been rocketing toward the end of the solar system at roughly 1 million miles per day for more than 35 years, and scientists have been eagerly awaiting signals of their departure from the edge of the solar system where solar wind and plasma have dominant influence (called the heliopause). Voyagers’ main mission was to explore the outer planets, which they did with great success, taking some of the first high-resolution images of the planets and their moons, and inspiring Carl Sagan to use Voyager I to take a photo of Earth from a distance of 3.7 billion miles, leading to his profound speech, “Pale Blue Dot.”

Now, scientists have announced with certainty that Voyager I has left the heliopause, the hypothesized boundary between the solar system and interstellar space (sometimes referred to as the interstellar medium). This is the first human-made object to enter the unchartered waters beyond the heliopause.  To be clear, Voyager is both in interstellar space and in a region that is still governed by the Sun’s gravity — technically this is both interstellar space, and the solar system.

Artist's concept of the Voyager location. The scale bar is in astronomical units (AU), with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the sun to the Earth, which is about 93 million miles or 150 million kilometers.  Neptune, the most distant planet from the sun, is about 30 AU. Informally, the term "solar system" is often used to mean the space out to the last planet. Scientific consensus, however, says the solar system goes out to the Oort Cloud, the source of the comets that swing by our sun on long time scales. Beyond the outer edge of the Oort Cloud, the gravity of other stars begins to dominate that of the Sun (Credit: NASA).

Artist’s concept of the Voyager location. The scale bar is in astronomical units (AU), with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the sun to the Earth, which is about 93 million miles or 150 million kilometers. Neptune, the most distant planet from the sun, is about 30 AU. Scientific consensus says the solar system goes out to the Oort Cloud, the source of the comets that swing by our Sun on long time scales. Beyond the outer edge of the Oort Cloud, the gravity of other stars begins to dominate that of the Sun (Credit: NASA).

Voyager I is now approximately 12 billion miles (19 billion km) from the Sun. Data analyzed by a research team at the University of Iowa revealed that for about one year, Voyager I has been traveling through ionized gas called plasma that is present in the space between stars. These data are the definitive clues that this spacecraft has finally made the historic leap to interstellar space (August 25, 2012 is thought to be the actual departure date). Free from influences of solar wind and plasma, Voyager I will now come in contact with interstellar cosmic rays, enabling scientists a first look at these high-energy particles originating from distant galaxies. Now that this threshold is crossed, scientists will also be able to explore the interaction between the solar wind and interstellar medium.

This video describes how scientists determined Voyager’s transition beyond the solar system boundaries of main solar influence:

In reviewing detections on Voyager from March 2012 and April 2013, scientists found plasma densities that were over 40 times denser than what would be expected in the outer layers of the heliosphere — the outer boundary of the solar system still under influence by the Sun. Such a high plasma density indicated that Voyager I is in fact beyond this boundary, in interstellar space, but still in a region that has some solar influence. Scientists are not sure when Voyager I will reach the undisturbed part of interstellar space where there is no influence from our Sun, or when Voyager II will reach interstellar space.

Signals emitted from Voyager I and II are at a very dim 23 watts, about the same power as a refrigerator light bulb. Traveling at the speed of light, signals from Voyager take about 17 hours to reach Earth, and arrive at a fraction of a billion-billionth of a watt.

Space travel enthusiasts — some of film and television fame — bid farewell to Voyager, highlighting the adventure and human spirit carried on these remarkable spacecraft:

What happens next? The Voyager timeline is truly unique compared to human lifetime-scaled calendars. After leaving the heliopause, the spacecraft will have enough electrical power and thruster fuel to operate until the year 2020. At this time, Voyager I and II will be 12.4 and 10.4 billion miles from the Sun, respectively. It will take Voyager 1,300 years to reach the inner edge of the Oort Cloud — a hypothesized spherical cloud of icy comets that occasionally make their way toward the Sun — and about 30,000 years to fly beyond it. In about 40,000 years, Voyager I will drift within 1.6 light-years (9.3 trillion miles) of the star AC+79 3888 in the constellation of Camelopardalis, heading toward the constellation Ophiuchus.  In about the same period of time, Voyager II will pass 1.7 light-years (9.7 trillion miles) from the star Ross 248. In approximately 296,000 years, it will pass 4.3 light-years (25 trillion miles) from Sirius, the brightest star in the sky.

Annotated artist's conception of the Milky Way (Credit: NASA)

Annotated artist’s conception of the Milky Way (Credit: NASA)

Armed with primitive technology by today’s standards (each with an 8-track tape recorder and one-240,000th the memory of a low-end iPhone), the little Voyagers are now the truest of the space adventurers, destined to wander our Milky Way for, quite possibly, all eternity, extending our reach through space and time. Perhaps, one day in the distant future, a far-off civilization may cross Voyagers’ path, the first interstellar envoy of humankind.

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