Scientists working on the Voyager program have received further data which suggests the probe is close to crossing into interstellar space.

This seems like an ideal time to do a quick review of Voyager’s history.

In The Beginning

Image: NASA –
(Slightly, ahem, amended)

The two Voyager spacecraft launched in 1977.  Voyager 2 actually launched before Voyager 1, but was on a slower trajectory.  Voyager 1 reached its target first.

The original targets were Jupiter and Saturn, with the unusual planetary alignment at the time meaning that they could use the gravity of Jupiter to send them on towards Saturn.  Voyager 2 was then sent on towards Uranus and Neptune.

Voyager 1 could have been sent to Pluto, but it was decided that a closer look at Titan was more important, so it made a little detour and took some pretty pictures instead.

You can go here to see pretty pictures.

The two Voyager probes discovered a multitude of things between them, most of which are not independently interesting.  If you’re into astrophysics and want to know more, go here.

One snippet that did interest me was this one:

The Voyagers found aurora-like ultraviolet emissions of hydrogen at mid-latitudes in the atmosphere, and auroras at polar latitudes (above 65 degrees). The high-level auroral activity may lead to formation of complex hydrocarbon molecules that are carried toward the equator. The mid-latitude auroras, which occur only in sunlit regions, remain a puzzle, since bombardment by electrons and ions, known to cause auroras on Earth, occurs primarily at high latitudes.

Complex hydrocarbons and a puzzle, you say?  Now I have this absurd mental image of some aliens hiding under a helium cloud saying “Those pesky Earthlings have sent another spaceship to look at us again.  It’s getting quite annoying, I wish they’d just leave us alone!”

The Science Bit

When most people think of the solar system, they think this:

  • Mercury (tiny, hot),
  • Venus (bad atmosphere, also hot),
  • Earth (yay us!),
  • Mars (not made of mars bars, alas),
  • asteroids,
  • Jupiter (very large, has spots),
  • Saturn (with rings),
  • Uranus (childish jokes),
  • Neptune (maybe blue? or was it green?),
  • Pluto (very cold).

It turns out that Pluto isn’t a planet.  It’s a dwarf planet, and it’s not even the largest one.  That would be Eris, which was originally designated the tenth planet before the discovery of several more objects in the area meant that people had to actually define “planet”.  So now Pluto and Eris are dwarf planets, and millions of adults have to re-learn the solar system.

That’s not the end of the story, though.  The solar system doesn’t end at the last planet (or dwarf planet).

The solar system is divided into the Inner Solar System (from the Sun through to the Asteroid Belt), the Outer Solar System (Jupiter through to Neptune), and the Trans-Neptunian Region.  The trans-Neptunian region contains the Kuiper Belt and Scattered Disc (lots of dwarf planets and a bunch of ice, mainly).  After that comes some mostly empty space, and in that region, somewhere, is the “end” of the Solar System.

This model has recently been proven wrong – there is no bow shock. As far as I can tell, nobody has come up with a better model yet, though. We’ll just have to wait for Voyager to get there!
Image from

But what is the end?  There is no line drawn through space, no defined point where the solar system ends.  The shape of it all is defined by solar winds and solar gravity.  The solar wind travels outwards until it meets the interstellar wind.  The point they meet is the “termination shock”, and after that is a region of slower moving, more turbulent wind, called the heliosheath.  The end of the heliosheath, where the solar winds stop, is the heliopause, and is the beginning of interstellar space.

The termination shock is roughly 94AU from the sun – two to three times as far as Pluto, and the heliopause is further away again.  To get to the end of the Solar System is a very long way indeed.

Where Are The Voyagers Now?

Since the 1990s, Voyager 1 has been the furthest man-made object from Earth, and it looks likely that this will remain the case for quite some time.  It crossed Termination Shock in December 2004, and is currently in the heliosheath, 18 billion km from Earth.  Just pause for a moment and consider that number.  Eighteen billion kilometres.  Boggles the mind, doesn’t it?

When it passes the heliopause then Voyager 1 will officially be in Interstellar Space.

Scientists are expecting to see some changes in the readings Voyager sends back at that point.  The BBC reports that:

In the last three years, Voyager has seen a steady increase in the number of cosmic rays entering its two high-energy telescopes, but in the past month the counts have jumped markedly.

The cosmic ray count is one of three indicators Nasa is using to determine when the probe has moved to interstellar space.

The second is a change in the intensity of the energetic particles Voyager detects around it coming from our Sun.

The number of these hits is declining, but not dramatically so, which should happen when Voyager leaves the region of space dominated by our star.

A third indicator will be a change in the direction of the magnetic field lines. These are expected to undergo a major reorientation when Voyager breaks into interstellar space.

Voyager 2 is taking a slightly different path.  In December 2007 it sent back data that suggested the Solar System is not symmetrical.  It has also reached the termination shock, around 10 billion miles from where Voyager 1 crossed it.

Onwards to the Future!

The Voyager probes are powered by nuclear reactors, and these should continue producing power for 10 to 15 years.  During this time the amount of electricity generated will slowly decrease, and more and more of the instruments will have to be turned off.  After the last one is gone, Voyager will continue on alone, a silent tribute to the marvels of human ingenuity.