Just as it helps to have an experienced Sherpa to assist a climber up the slopes of Mount Everest, the Hubble Space Telescope is, from afar, keeping watch on the mysterious interstellar medium as the twin Voyager spacecraft fly deeper into the unknown.
NASA's Voyager Program — which consists of two spacecraft, Voyager 1 and Voyager 2 — was launched in 1977 and both probes continue to relay data back to Earth to this day. Although their power is dwindling (both are powered by decaying plutonium fuel) and many of their instruments have been switched off, the pair are the most distant human-made spacecraft ever to be launched and are still carrying out rudimentary studies of the environment that surrounds them. (NASA's Pioneer 10 and 11 spacecraft are also flying through interstellar space, but neither are operational.)
Both Voyagers are now traveling through interstellar space having left our sun's region of influence, called the heliosphere. These trailblazers are free from the solar system's magnetic boundary and powering though a region of space that we've never experienced.
As the interstellar expanse is so mysterious, and we have two operational probes flying through this new frontier, it helps to have a powerful space telescope scanning the environment around the spacecraft and take a peek in front of their direction of travel to see what surprises lie in wait.
"This is a great opportunity to compare data from in situ measurements of the space environment by the Voyager spacecraft and telescopic measurements by Hubble," said Seth Redfield of Wesleyan University in Middletown, Conn., in a statement. "The Voyagers are sampling tiny regions as they plow through space at roughly 38,000 miles per hour. But we have no idea if these small areas are typical or rare. The Hubble observations give us a broader view because the telescope is looking along a longer and wider path. So Hubble gives context to what each Voyager is passing through."
The positions of Voyager 1 and 2 and the locations of stars used by Hubble to carry out spectroscopic analysis of the interstellar medium that lies ahead of the twin probes (NASA/ESA)
In early results presented this week by Redfield and his team at the 229th meeting of the American Astronomical Society (AAS) in Grapevine, Texas, Hubble has shown that far from being an empty place, the space between the stars is actually a complex "interstellar ecology," featuring clouds of hydrogen enriched with other elements. And we are in a very privileged position to have two operational (albeit on very limited power) spacecraft in that interstellar ecology, sampling the "ground truth" while Hubble gives astronomers an overview of the interstellar clouds the probes are embedded inside. It is also showing us what lies ahead for both spacecraft, long after they lose power and die, resigned to becoming lifeless interstellar emissaries for the human race.
"Ideally, synthesizing these insights with in situ measurements from Voyager would provide an unprecedented overview of the local interstellar environment," said Hubble team member Julia Zachary, also of Wesleyan University.
Having toured the solar system since launch, neither spacecraft were aimed at any interstellar target in particular. Voyager 1 is currently 13 billion miles from Earth, an incredible distance when you consider that it takes radio signals, traveling at the speed of light, over 38 hours to be transmitted from the probe to Earth. In other words, Voyager 1 is an incredible 38 light-hours away (as a comparison, the moon is 1.3 light-seconds away!). In 40,000 years time, Voyager 1 will pass within 1.6 light-years of the nearby star Gliese 445. Voyager 2, which is traveling in a different direction, is 10.5 billion miles from Earth (over 31 light-hours away) and will pass within 1.7 light-years from the star Ross 248 in 40,000 years time.
Before we lose contact with our interstellar Voyagers, the pair will make direct measurements of the magnetic environment and interstellar gases, including energetic particles known as cosmic rays. These measurements are not possible from deep inside the heliosphere, filled with solar wind particles, overpowered by the sun's magnetism. While it is still operating, Hubble will scan along the Voyager spacecraft's future trajectory using its imaging spectrograph to see how starlight is absorbed by interstellar gases. From these measurements, we will gain astronomical measurements of exactly what material the spacecraft are traveling through, even after they stop transmitting.
Already, Hubble has complemented measurements made by the Voyager Program, revealing the "clumpiness" of the interstellar gases surrounding the sun's heliosphere. This, and measurements of the interstellar magnetic field will reveal how our sun moves through interstellar space, also giving us a glimpse of how other stars interact with their interstellar surroundings.
"I'm really intrigued by the interaction between stars and the interstellar environment," Redfield added. "These kinds of interactions are happening around most stars, and it is a dynamic process."
Voyager 1 and 2 completed their primary missions decades ago, and yet they are still making new discoveries in their 40th year in space. But considering they're only just dipping their toes into the interstellar expanse after four decades underscores how isolated we are in our corner of the galaxy and if we're to stand any chance of visiting a nearby star (the closest, Proxima Centauri, is over 4 light-years away) we'll need to see some stunning advances in propulsion technology.
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