Sun Leaves No Shock Wave in its Wake

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Shown in a Hubble Space Telescope image, the "astrosphere" around the star L.L. Orionis approximates the heliosphere around our solar system.
ESA/NASA

THE GIST

— The sun is traveling at 52,000 mph through interstellar space — slower than previously thought.

— The slower speed means there is not a shock wave beyond our heliosphere.

— NASA's Voyager probes are enroute to interstellar space.

The sun and all that surrounds it — us included — are having a slower ride through space than originally thought, new findings from a NASA satellite show.

Precise measurements of neutrally charged particles such as helium flowing into the solar system from interstellar space show the heliosphere — the region of space under the sun's influence — is moving at 52,000 mph relative to the outside environment.

That's a change from 59,000 mph measured by NASA's now-defunct Ulysses spacecraft.

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That 7,000-mph difference may not sound like much, but the effect of the speed is squared.

"This reduction of 7,000 mph is a reduction of about 25 percent of the pressure pushing on the heliosphere that we thought was there is actually there," David McComas, lead scientist for NASA's Interstellar Boundary Explorer, or IBEX, told Discovery News.

The discovery means there is no shock wave being generated as the sun moves through the area of interstellar space we're in now, McComas added.

Direct proof may come from NASA's Voyager multiple spacecraft, which are heading out of the solar system.

"This research shows that they will never encounter a bow shock, that there's only a wave in front of the heliosphere," McComas said.

There's a big difference between a shock and wave. An airplane traveling just a few miles slower than the speed of sound is just a fast airplane. If it speeds up just a little bit, it becomes supersonic, generating an entirely different phenomenon, which we hear as sonic booms, as it flies through the air.

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"That plane is traveling almost the same speed it was before, but it's passed a very critical speed where the information can't travel ahead of it fast enough to tell the air to get out of the way. You get that very critical point where instead of just sort of pushing the air away, like a boat pushes water away, all of a sudden you have to form this shock where you get very discontinuous deflection of material out from in front of an object.

It's a really big transition in physics between a wave and a shock and it happens at a very precise point in the interaction," McComas said.

The Voyager probes should instead plow through a bow wave, a region in space similar to a wave in front of a ship.

"The interstellar wind is still deflected and flows around the outside of the heliosphere, and so we will see the effects of that flow around the heliosphere, once we cross," Voyager lead scientist Edward Stone told Discovery News.

"It's like the flow of a river around a rock — if you're near the rock, it's displaced, but you get pretty far (in the flow from the rock), you wouldn't even know that the rock was there," Stone said.

The IBEX findings may also have implications for astrobiologists looking for life beyond Earth.

The denser the interstellar cloud, the more compressed a star's heliosphere will be, meaning there's less shielding from galactic cosmic rays.

"It may be that you have to have an astrosphere — a heliosphere — around your star to provide protection. That may be one of the things needed for life," McComas said.

The research is reported in this week's Science