This is about recent results from the Voyager spacecraft. Voyager 1 was launched way back in September 1977. Its first and primary task was to fly by Jupiter and then Saturn, gathering information about the planets and their moons. It then set off towards Alpha Centauri on the second part of its mission, to examine the heliosphere, and it still has just enough power in its radioactive generators to last a few more years and send back useful data.
First, a reminder of distances:-
- From the Earth to the Sun is defined as 1 Astronomical Unit (1AU)
- The outermost planet, Neptune, is about 30 AU away, and Voyager passed that in 1988
- Voyager is now 127 AU away, just about at the edge of the solar system
- Alpha Centauri is about 280,000 AU’s away (4.4 light years, nearly 6 x 1012 miles)
- Voyager is travelling at about 17km/sec (325 million miles/year), and, it won’t be entirely free of the sun’s gravitational embrace until it passes the Oort field at about 10,000 AU, which will take it thousands of years.
There are 3 factors in deciding whether Voyager 1 has crossed the border from the sun’s realm into interstellar space.
- is it still within the heliosphere ? – formed by the sun’s magnetic field and it’s flow of charged particles? according to the instruments, it saw a drop in the number of solar wind particles in 2012
- is it still affected by the sun’s magnetic field ? the Voyager’s compass is still the same, showing it is inside the sun’s magnetic field
- is it affected by the sun’s gravitational field ? – as above, it will take a long time still to leave this behind.
So only one of these is showing that it has left our system
However, in April 2013, NASA reported that the gas around it had shaken violently, as a giant eruption of gas from the sun 400 days earlier finally reached Voyager. The strength of the reaction showed that it was in an area of space with far more dense material than inside the heliosphere (which is composed of particles rather than atoms). So, they tend to think that it really IS now in interstellar space.
So, what do we now know about interstellar space ? It is 40 times denser than the gases inside the heliosphere, and there is a “wall of atoms” piling up at the impenetrable boundary of the heliosphere. Again, this is something that protects the solar system in general (and us in particular) from most of the high-speed atoms and high-energy cosmic rays which might cause damage. Research is continuing into these, though cosmic rays are also thought to be one of the driving forces behind evolution, as the most vigorous ones can cause mutations, and if there were no mutations, there would be no evolution.
Other experiments show that interstellar space is not a featureless void. It contains clouds of gas and dust which are being blown about by the force of ancient supernova explosions. Our solar system lies in a cloud about 40 light-years across, called the Local Interstellar Cloud, which contains about 1 atom per cubic centimetre – far denser than other areas. It seems there is also a “ribbon” – a prominent band across the sky caused by an external magnetic field. Comparison with earlier result also shows that the gas beyond the solar system has changed over the last 40 years – there are “gusts of wind” in outer space.