There is a frontier in space where the realm of the Sun ends, where the Solar System gives way to interstellar space. This is not an arbitrary line like the ones we draw on Earth through political negotiation; it is a real place. It is where the weakening flow of the solar wind is finally brought to a standstill by the dust, gas and magnetic fields lying between the stars.
We have accomplished a lot since the beginning of the space age in October, 1957. However, most of those achievements lie within the orbit of the Moon, less than 400,000 km from the Earth. With a couple of exceptions, all of our other space triumphs lie within the Solar System, within the orbit of Pluto, roughly 5.5 billion kilometres away. However, one spacecraft has crossed that frontier into interstellar space, and another is about to.
In 1972 a spacecraft called Pioneer 10 was launched into the outer Solar System. Its mission was to explore the outer planets, but like many spacecraft, it achieved much more. In 1973 it crossed through the asteroid belt between Mars and Jupiter. In November 1973 it flew past Jupiter and continued on beyond the planets. As the spacecraft got further from Earth, its signals weakened. They were further reduced by the gradual running down of its nuclear power source. On 23 January, 2003, after a 30-year mission, its signals were lost. At the time it was about 12 billion kilometres away. It has probably crossed the frontier by now, but silently.
In 1997 two spacecraft, Voyagers 1 and 2, were launched. Their mission was to visit the outer planets. Voyager 2 was on a more leisurely orbit, so Voyager 1 has raced ahead. The Voyagers succeeded in their intended mission marvellously well, and are still operating. Voyager 1 is now about 18 billion kilometres away and still sending back data, and what it is sending is fascinating. It is telling us that the solar wind is weakening rapidly, and the amount of cosmic radiation, high-energy particles from interstellar space, which are largely blocked by the solar magnetic field, is increasing rapidly. It is close to the frontier. We will know exactly when Voyager leaves the Solar System when the Sun’s magnetic field gives way to that of the Milky Way. We are really looking forward to the data coming back to us after that crossing.
Radio astronomy has taught us, and continues to teach us about the material between the stars, how it contains the remains of extinct stars and planets, material dating back to the beginning of the universe, and how new stars and planets form from it. However, we have never had a direct look it, and never on such a small scale. Telescopes don’t show you magnetic fields, just their consequences. Voyager will see them directly, by being there.
There is a sequence in one of the Star Trek movies where a Klingon spaceship happens upon Pioneer 10, somewhere out there between the stars. One Klingon says to the other, “I bet you can’t hit that”. Hopefully, real aliens will be kinder.
Mercury is low in the west after sunset. Mars and Saturn dominate the southern sky. The Moon will be at first quarter on the 26th, and full on the 3rd.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.
Ken Tapping, 26th June, 2012