A couple of years ago (in 2013) NASA announced that Voyager 1 had “left the solar system“, by which they meant that it had crossed the Heliopause. (For more on this topic, see this recent post by Bob Trembley.)
A recent question I received asked when the New Horizons mission to Pluto would be likely to reach the Heliopause itself. The answer to that one is, of course, “it depends…” But an article in the Smithsonian notes that it is following in roughly the same longitude of space as Voyager 2, which passed the orbit of Pluto (but not Pluto itself) in 1989, and it hasn’t reached the heliopause yet. New Horizons has power to keep it transmitting well into the 2030s, but it may well take that long to reach this boundary.
The bigger question, of course, is… what is the Heliopause? My PhD thesis advisor at the University of Arizona, Randy Jokipii, had a wonderful way to illustrate what that boundary meant.
Think of it this way… you know that a static gas has a pressure, which you can think of as the momentum transferred when the individual molecules in the gas, whizzing about randomly at a wide variety of speeds, bounce off the walls of their container, or the sensor in your pressure gauge, or your face as you are sitting in the air. The more atoms in a given space, or the faster they move on average (i.e. the higher their temperature), the more pressure you will feel.
But if the gas is moving (or you are moving through the gas), your pressure gauge or your face will feel a “ram pressure” since the speed of the wind (or you on the bicycle) adds its speed to the average speed of the random motions of the molecules.
Now, the Sun is pouring out ions of hydrogen and other species in a wind called the Solar Wind; this wind eventually expands away from the sun at roughly a constant speed, but as it fill more and more of space, it gets less and less dense. So its ram pressure drops and it moves away from the Sun. Meanwhile, it is expanding into the interstellar medium, the space between the stars. In our galaxy, the space between the stars is pretty empty, but not perfectly so; on average, there’s about one atom per cubic centimeter out there. (Cold regions are a million times denser; hot regions are ten thousand times less dense.) Thin as this is, it does still represent a real static pressure.
The Heliopause, then, is the boundary where the ram pressure of the expanding solar wind (which, remember, is getting weaker and weaker as you go away from the Sun) is finally balanced by the static pressure of the interstellar medium. Since both the ISM and the Solar Wind fluctuate with time, this boundary can move back and forth.
And here’s where you can see it in your sink. Find a sink with a nice flat bottom, and turn on a thin stream of water, and let it hit the bottom. You can see a very shallow region where the water flows outwards; since it is flowing into an ever growing circle, its flow speed and thus its ram pressure drops until the ram pressure exactly matches the static pressure due to the weight of the water sitting in the sink itself. You can see the boundary in the photo; and by changing the amount of water coming out of the tap, you can adjust the position of the boundary. The boundary is almost circular; not perfect, because the sink bottom isn’t perfect…