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NASA's Voyager spacecraft (artists rendition).

Star Stuff: Come along and ride on a fantastic voyage

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Sep 7, 2017

Star Stuff is a weekly column by rocket scientist & astrophysicist Summer Ash highlighting some amazing things happening every day on and off the planet, especially great science done by and/or for women.  She harnesses her science communication powers to smash the patriarchy and advocate for equity and inclusion across all time and space. Throwdowns with pseudoscience may occur.

Forty years ago this week, a team of NASA scientists and engineers were cautiously optimistic that their robotic emissaries would make it to the outer solar system. The idea that they might still be going today likely never crossed their minds.

This week marks the 40th anniversary of the launches of NASA’s Voyager spacecraft, 1 and 2. The Voyager program was designed to take advantage of a planetary alignment that only happens once every 175 years. This alignment enabled a grand tour of the gas giants of the outer solar system, each planet giving the spacecraft a gravitational assist to reach the next. Both Voyagers did flybys of Jupiter and Saturn, but Voyager 2 went on to visit Uranus and Neptune (and is still the only spacecraft to have done so). After completing their primary missions, each spacecraft then began the secondary Voyager Interstellar Mission (VIM) to explore beyond our solar system and to search for the outer limit of the Sun's magnetic field (aka the heliopause).

Trajectories of Voyager 1 and Voyager 2 spacecraft.

But the Voyagers weren’t the first artificial objects to travel that far. Pioneers 10 and 11 blazed the trail for them. Launched in 1972 and 1973, respectively, the Pioneer probes were literally pioneering the way to through the asteroid belt and into the outer solar system.

On July 15, 1972, Pioneer 10 was the first spacecraft ever to enter and travel beyond the asteroid belt, demonstrating that it could be done. Then on September 1, 1979, Pioneer 11 became the first spacecraft to visit Saturn, passing a mere 13,000 miles above its atmosphere. Since both Voyagers were slated to flyby Saturn, NASA was eager to know if particles from Saturn’s rings would pose a danger. While Pioneer 11 survived unscathed, it did have a near miss (less than 2,500 miles) with one of Saturn’s smaller moons that was only discovered in the data the day before.

Just weeks after Return of the Jedi hit the theaters and audiences were getting lost in a galaxy far, far away in the spring of 1983, Pioneer 10 became the first human-made object to journey into the unknown, past all the major planets in the solar system. Thanks to Newton’s first law of motion, the kick Jupiter gave Pioneer 10 a decade earlier set the probe on its trajectory for life. For the foreseeable future of everyone reading this, it’ll be sailing toward the bull’s eye, Aldebaran, in the constellation Taurus while its sister probe heads in the opposite direction, headed for the constellation Aquila.

Given that the Pioneers launched five-ish years before the Voyagers, you’d be forgiven for thinking that they will forever lead the interstellar spacecraft race. However, both Voyagers were launched with higher velocities, not to mention the gravitational assists they received from their planetary targets. Voyager 1 is the fastest of them all, overtaking its twin shortly after launch, Pioneer 11 in the late 1980s and Pioneer 10 in 1998. It is now just under 13 billion miles away (or just under 140 astronomical units [AU], 140x the distance of Earth from the Sun). Voyager 2 is following Voyager 1’s lead. It overtook Pioneer 11 in 1997 and is now over 10.5 billion miles away (~115 AU). Voyager 2 is on track to pass Pioneer 10 in 2021 (stay tuned!).

You can follow along with both spacecraft at NASA’s Voyager website, which tracks their distances in real time. While both Pioneers are now radio silent, Pioneer 10 was in contact up until 2003 and both Voyagers are still sending back data. NASA has an array of large radio antennas set up around the world to stay in touch with each and every metallic ambassador we’ve put out there. The Deep Space Network (DSN) is distributed across three main sites near Madrid, Spain, Barstow, California, and Canberra, Australia. Any minute of any given day, you can drop in on the DSN to see which spacecraft is saying hello and which spacecraft is receiving its next instructions.

At their current distances, signals from Voyager 2 take just under 16 light-hours to reach Earth, while Voyager 1’s beeps and boops now take over 19 light-hours. Neither spacecraft is under our control anymore, but the instruments they have on board to monitor cosmic rays are still functioning and they can still transmit that data back to NASA. The instrument records the frequency of charged particles, and it can distinguish between charged particles from our Sun versus those from interstellar space. Voyager 1’s instrument is now dominated by galactic cosmic rays since it passed the heliopause, but Voyager 2’s readings are still dominated by cosmic rays from the Sun.

Some people misinterpret these results to mean that Voyager 1 has left the solar system, but I would have my astronomer card revoked if I let you believe that. The heliopause is the extent of our Sun’s magnetic influence, but its gravitational influence has a much farther reach. By that definition, the edge of our solar system is the Oort Cloud, a vast, spherical, cometary reservoir lurking out at 500 AU. Even at its accelerating pace, Voyager 1 won’t reach this boundary for another 300 years and won’t come out the other side for another 30,000 years after that.

Estimated location of Voyager spacecraft as of 2013. 

But this is far from a downer to me. Regardless of whether I will be here to “see” it or not, both Voyagers and both Pioneers will leave the solar system. And we humans on our tiny pale blue dot will have made it happen. That's phenomenal.

Not only that, but they’ll have company. NASA’s New Horizons mission launched in 2006 and flew by Pluto in 2015. When it launched it was the fastest human-made object ever to leave Earth orbit (at over 36,000 mph). Currently on target for a 2019 rendezvous with the melodically named Kuiper Belt Object (486958) 2014 MU69, it too will eventually overtake the Pioneer probes in the next 110 years (though not either Voyager). And I haven’t even mentioned the rocket boosters for four out of five of these spacecraft that are also on solar system escape trajectories. These boosters are following the paths they were originally launched on, but barring any unforeseen gravitational perturbations, they will take their leave of us too (thanks Newton!).

Whenever things here on terra firma get me down, I remember these cosmic wayfarers and all they’ve seen and all they have yet to see. Fantastic voyages indeed.