As NASA's Voyager 1 and 2 unmanned deep space probes head out of the Solar System, they are in territory that is the very definition of "uncharted." To help fill in the map, the Hubble Space Telescope is charting their path in order to learn more about the mysterious regions of deep space.

Voyager 1 and 2 were launched in 1977 from Cape Canaveral, Florida, and carried out flyby missions to Jupiter and Saturn, with Voyager 2 squeezing in visits to Uranus and Neptune. Since then, their parabolic trajectory has sent them on a path out of the Solar System.

Voyager 1 is currently 13 billion miles (21 billion km) from Earth and according to NASA will pass within 1.6 light years of the star Gliese 445 in the constellation Camelopardalis in 40,000 years. Meanwhile, Voyager 2 is 10.5 billion miles (17 billion km) from Earth and is expected in 40,000 years to pass within 1.7 light years of the star Ross 248 in the constellation of Andromeda.

The two probes are expected to exhaust their nuclear power plants within 10 years, after which they will continue on their trajectories. In the meantime, the data about interstellar material, magnetic fields and cosmic rays they are sending back from the edge of the Solar System, correlated with Hubble's observations, will go a long way toward telling astronomers whether the regions the Voyagers are passing through are rare or typical, as well as the nature of the interstellar medium through which the Sun travels.

Paths of Voyager 1 and 2(Credit: NASA)

The reason for bringing Hubble into the picture is that the two Voyagers are only able to study the immediate vicinity along their line of travel, so Hubble is expanding things by using its Imaging Spectrograph to study the spectra of starlight shining along the sight lines of the spacecrafts' routes for several light years. By looking at how material in the flight path absorbs light from the stars, scientists can deduce their chemical composition.

The latest spectrographic analysis indicates that the probes are passing through multiple clouds of hydrogen and trace elements. For the next 2,000 years or so, the Voyagers will continue through the interstellar cloud surrounding the Solar System, then they will travel for 90,000 years through a second cloud before passing through a third. Along the way, their flight paths show a slight variation in the chemicals present, and that the Sun is passing through an area of clumpier material (or as clumpy as a near total vacuum can get) that may affect the boundary between the Solar System and interstellar space proper.

"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," says team leader Seth Redfield of Wesleyan University. "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 research was presented at the winter meeting of the American Astronomical Society in Grapevine, Texas.

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