Ralph McNutt thinks he may have heard some tiny pieces of the sun ricochet off the walls of the solar system -- composed of the sun, the planets and the thin soup of protons called the solar wind. That noise, Dr. McNutt says, is the first direct evidence that the walls are there.
The physicist with Johns Hopkins University's Applied Physics Laboratory is part of a team of scientists analyzing data being gathered by the Voyager 1 and Voyager 2 spacecraft. Both flew past the outer planets a decade ago and are headed into the vast cold space among the stars.
In April and May 1992, powerful solar flares erupted on the surface of the sun. By August, the Voyagers began picking up powerful radio signals. Dr. McNutt and his colleagues think the signals were triggered when the solar wind, traveling at about 1 million mph, slammed into what is called the heliopause -- the edge of the heliosphere, the bullet-shaped bubble of gas surrounding the solar system.
By measuring the time between the solar flares and the resulting radio signals and estimating the speed of the solar wind, Dr. McNutt has come up with a rough estimate for the size of the heliosphere, the first based on the new data.
Q: You've been hitchhiking electronically aboard both Voyagers since they were launched in 1977. Have you enjoyed it?
A: It's been the most exciting, thrilling thing that I could imagine. There's been nothing like going through the various planetary encounters. [Voyager 1 encountered Jupiter and Saturn, while Voyager 2 swept past Jupiter, Saturn, Uranus and Neptune.]
I've been lucky enough to be present at four of them, between the two spacecraft, out at the Jet Propulsion Laboratory [in Pasadena, Calif.].
These are exciting things. It's almost instant science.
For example, take Uranus and Neptune. Probably 90 percent of what the human race knows about those two planets comes from the Voyager encounters. You're really making an incredible contribution to the overall knowledge of mankind.
Of course, there's the idea that we may actually, with a spacecraft -- an extension of ourselves -- reach out to the void between the stars for the first time.
That, I think, is one of the most exciting things to still look forward to with the Voyager mission. Once we get into interstellar space there may be more discoveries to be made.
Q: What is the heliopause?
A: The heliopause is sort of a boundary in space, although it's probably not a static boundary. We think it has ripples and that it moves around. But it's the boundary between solar system material and true interstellar material -- that is, the material that actually fills in the void between the stars.
The sun has an outer atmosphere that's continually expanding from it called the solar wind. It's rather tenuous. It has an average density of something like about eight protons per cubic centimeter at the distance of the earth from the sun. Some helium and some minor ions are mixed in, but mostly it's protons.
It's blowing quite rapidly. At some point it has to run into interstellar material.
Q: How far away is this boundary?
A: My current best guess is between about 82 and 130 astronomical units (AUs) from the sun. An astronomical unit is the distance from the sun to the earth -- about 93 million miles.
[That would put the solar system's edge somewhere between 6.5 billion and 12.1 billion miles out.]
Q: Is the heliopause farther than you expected?
A: Suffice it to say there's been a great deal of uncertainty in the distance. Probably the first person to think about this was Prof. Leverett Davis Jr., of CalTech, back in 1955. He was talking about 100 AUs. At one point people were estimating that the boundary might be as close as between the orbits of Mars and Jupiter. There have been estimates as high as 200 AUs. People have talked about it perhaps being even larger.
Q: You estimate the distance to within about 4 billion miles. Could you narrow that down a bit?
A: (Laughing.) Not at the moment.
Basically, the way one gets the estimates is to try to identify the velocity of the solar wind. The real question is, what is the appropriate velocity to use? That really needs some more thought and some more refinement.
The distances of 82 and 130 AUs come from what I think are the best guess at the slowest possible velocity for the solar wind and the best guess at the fastest possible velocity.
By going back and looking more closely, I think we can gain some physical insight into what's going on.
If we can do that, then I think we can have a better guess as to what is the correct velocity to use. And if we can do that, then I think we can refine the distance estimate.
Q: When will the Voyager spacecraft hit the edge of the heliosphere?
A: Again, it depends on which of these numbers you use.
Four spacecraft are going to go into interstellar space -- Pioneer 10 and 11, and Voyager 1 and 2.
Voyager 1 is actually traveling faster than the any of the others, and in the late 1990s will take over the distance record from Pioneer 10.
Voyager 1 now is at about 51 AUs out. It's traveling roughly about 3.5 AUs a year. So if it turns out that the heliopause is as close as about 60 AUs, it could be just four years. I think that's the lower limit.
Q: What will Voyager see when it reaches the edge? Will it take pictures?
A: We won't be taking any pictures. The mission is an extended one of doing measurements of cosmic rays, properties of the solar wind -- which includes low-frequency radio emissions -- as well as continuing to use the Voyager's ultraviolet [sensors] to observe various stars.
What we'll see will be a change from hot solar wind to [cooler] interstellar [material.] Right now, one of the big questions is what is really confining the solar wind and setting up the heliopause?
Is it a plasma that is in the very local interstellar [material]? Or is it primarily due to pressure from a magnetic field in the very local interstellar [material]?
Q: The sun's magnetic field?
A: No, we know from various observations that there is a magnetic field between the stars. It's associated with the Milky Way galaxy.