A Leisurely Cruise on the Good Ship Solar System

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NASA / ESA / JPL-Caltech / Goddard / SwRI

Bow shocks exist around other astrospheres, as seen in these images taken by multiple telescopes

Never mind how steady the Earth seems beneath your feet; we're moving in more ways — and at far greater speeds — than we realize. In order to complete a single rotation in just 24 hours, our planet must turn at 1,000 m.p.h. (1,600 km/h) — and that's the least of it. At the same time the Earth is spinning so fast, it's revolving around the sun at a far brisker 67,000 m.p.h. (104,000 km/h), even as the spiral arm in which our little dust-fleck planet sits is orbiting the galactic center at 483,000 m.p.h. (792,000 km/h). And if that's not vertigo-inducing enough, the solar system is also bobbing along through the local interstellar medium at about 59,000 m.p.h. (95,000 km/h).

While all of those speeds have been settled science for a while now, the 59,000-m.p.h. figure has just come in for some revision. New findings published in the journal Science have adjusted the solar system's velocity downward by about 7,000 m.p.h. (11,000 km/h). If you think so small a difference in such a fast-moving cosmos doesn't matter, think again. The change has an impact on our understanding of everything from the shape of the solar system to the amount of cosmic radiation in the vicinity of Earth to the safety of ever trying to send human beings to Mars.

The solar system, as astronomers have long known, is more than just a marble bag of planets, moons, asteroids, comets and other solid objects orbiting the sun. All of those bodies are actually contained within a vast bubble of ionized particles known as the heliosphere. The particles are what make up the so-called solar wind, which flows out from the sun at speeds approaching 1.8 million m.p.h. (2.9 million km/h). The point at which the wind finally slows down and bumps up against the interstellar medium outside the heliosphere is known as the heliopause.

The heliopause ought to be a peaceful spot, and it would be too if the solar system itself weren't moving along at a five-figure clip. As the particles within the heliosphere collide with the particles in the interstellar medium, they create what's called a bow shock, a high-temperature impact zone that causes the forward end of the heliosphere to flatten out. The stern end trails off into a tail.

That widely accepted idea, however, was developed before the sharp eye of the Interstellar Boundary Explorer satellite (IBEX) came on line. Launched in 2008, IBEX circles the Earth in a highly elliptical orbit, studying the distant reaches of the solar system to calculate the quantity of interstellar material that penetrates the heliopause. "IBEX takes a kind of CT scan of the solar system," says space scientist David McComas of the Southwest Research Institute, who headed the new study. "It looks in all directions and creates a 360-degree picture."

That picture showed that there was more material getting through the heliopause than there should be. And since the bow shock serves as a partial barrier to interstellar particles, the only way the amount of in-flowing material could be so high would be if the shock didn't exist after all. That, in turn, suggested that the heliosphere and the solar system within it were moving at a slightly slower speed than was once thought. The most elegant part of that conclusion was how small that velocity change could be and still make a difference.

"Think of it as similar to a sonic boom," says McComas. "An airplane traveling just a mile or two per hour below the speed of sound moves through the air like any other plane. Then it goes a mile per hour above the limit and there's a boom you can hear all the way to the ground."

The fact that the bow shock isn't there is important for a number of reasons. For one thing, it forces a revision of about a quarter of a century of scientific thinking. For another thing, says McComas, "it's cool," which is actually something of an understatement when the very shape of the entire solar system turns out to be different from what we thought it was. The new findings also give us a glimpse into solar-system history. "There have been eras in which we've bobbed up and down and gone faster and slower than we do now," says McComas. "What we get by studying the heliosphere today is a better understanding of how things were different in the distant past and will be different again."

Much more immediately, the absence of a bow shock makes manned interplanetary travel a lot more difficult. Cosmic radiation can be deadly to human beings during a months-long trip to even a nearby destination like Mars. The heliopause successfully filters out about 90% of incoming material, but the 10% that remains can be more than enough to cause serious problems. The new discovery doesn't change the amount of radiation astronauts would encounter, but it helps explain why the level is so high. "This is a very important factor in going to Mars and perhaps a limiting factor," says McComas.

That's a problem we will have to sort out technologically if we ever want to settle other planets, since changes in the speed of the heliosphere may take tens of millions of years or longer to occur. We could soon have a better fix on just how many millions, since IBEX is getting some help in its work, thanks to the venerable Voyager 1 and 2 spacecraft, launched from Earth in 1977 and now approaching the heliopause. If IBEX works like a CT scan, McComas says, the Voyagers work like a biopsy. "They're out there in situ, taking samples," he says. It adds one more level of cool, perhaps, that NASA's oldest working spacecraft are joining hands with one of its newest to unravel mysteries that far, far predate us all.