About a month ago I read a news story that described how the Voyager 1 space probe had just reached the edge of the solar system. Here is the graph that caught my eye:
When I saw this graph and understood what it meant, I felt a chill run down my spine. This simple graph communicated something to me that closed the vast distance between me and the Voyager space probe. I was quickly pulled into that odd mental space where you find yourself wrestling with the true immensity of the universe, and our tiny place in it.
So what does this graph actually show?
The sun is an incredibly hot ball of nuclear fusion. The sun is so hot that it sends a constant stream of particles out into space in all directions, called the solar wind. Scientists were interested in finding out how quickly the solar wind dies out as you travel farther away from the sun, so Voyager was equipped with an instrument that counts how many of these particles strike its surface every second. We know the solar wind grows weaker as you get farther from the sun, but does it ever die out completely? It does! That’s what this graph shows, quite clearly. This matches most of our predictions about what we would find at the edge of the solar system.
Entering the Milky Way
I grew up with the simplistic notion that the edge of the solar system must be defined by the path of Pluto, the farthest planet from the sun. To understand what the edge of the solar system really is, you have to understand why this graph drops off so suddenly. Basically, for 40 years the Voyager space probe has been getting bombarded by particles from the sun. In August, for the first time, Voyager was free of this bombardment. What is the boundary that these solar particles can’t cross, that Voyager did cross? It’s like the boundary between a fairly protected bay and the open ocean. There are plants and animals that thrive in the relatively protected waters of a calm bay, many of which could not survive in the rougher open ocean. Voyager has crossed out of the region that is dominated by the sun, and has entered a region that is more influenced by the rest of the Milky Way. For example, there is another graph that shows a corresponding increase in the bombardment of Voyager by galactic particles. Voyager is not free of bombardment, it is just in a region of space where solar particles are no longer the strongest particles around.
A little perspective
When I fully understood the significance of this graph, I thought about what I was doing those last weeks of Augst when Voyager 1 was crossing the boundary at the edge of the solar system. I am a high school teacher, so I was getting started with the school year. I was meeting with colleagues to get ready for the year, and meeting students during our orientation activities. While we were doing these everyday Earth activities, going about our regular lives, Voyager was crossing a boundary we had dreamt of since the first humans looked consciously up at the night sky. It trips me out to think of the Voyager probe just heading straight out into space for almost 40 years, and suddenly crossing that boundary.
Hell yes, science!