Beyond the Solar System

Mission “creep” is NASA jargon for the situation when new goals or capabilities (and usually extra associated costs) are added to a mission as it is being developed. The phrase has negative connota­tions, but the Voyager spacecraft wear it as a badge of honor. Their original two-planet mission was designed to last five years. With all the objectives for Jupiter and Saturn achieved, flybys of the two outermost planets—Uranus and Neptune—proved irresistible to mission planners. The five-year mission extended to twelve. As the spacecraft moved into uncharted territory, the time it took for a round-trip signal to be sent from JPL stretched to more than a day, so engineers figured out how to program the Voyagers for remote control, allowing them to be more autonomous. Now the mission has been running long enough that the Voyagers may outlive many of their designers. As the Voyagers recede from Earth, the mod­est power sent back with their high-gain antennas dilutes through an ever-larger volume of space. Currently, the deep-space tracking network detects a minuscule 10-16 Watts (equivalent to a billion billionth of a light bulb) from each spacecraft.22

What lies beyond the Solar System? Uranus is at 30 A. U., or thirty times the Earth-Sun distance. From 30 to 50 A. U. lies a vast ring of debris similar to the Asteroid Belt. Called the Kuiper Belt, it contains about 100,000 rocks larger than 30 miles across, in­cluding some dwarf planets.23 Where the Solar System ends and interstellar space begins is a matter of argument. The Sun sends high-energy charged particles out from its upper atmosphere in all directions. This solar “wind” streaks past us at a speed of a million miles per hour and creates an evacuated region or bubble called the heliosphere. The heliosphere extends past all the planets and ends when the solar wind runs into the rarified gas of hydrogen and helium that permeates the regions between stars in the Milky Way. A shock wave is created as the rapidly moving wind slows to subsonic speeds. The location of this boundary layer is one of the big unanswered questions in astrophysics.

The Voyagers have several ways to diagnose the invisible pro­cesses that take place in the near vacuum of the outer Solar Sys­tem. They can study the strength and orientation of the magnetic field of the Sun, the composition, direction, and energies of solar wind particles, and the strength of the radio emission from be­yond the heliosphere. At the boundary is the place where the solar wind slows suddenly from a speed of 1 million miles per hour and becomes a lot denser and hotter. In December 2004, Voyager 1 crossed this shock.24 A few years later, Voyager 2 followed its twin into the unknown. Recent results have been surprising. The edge of the Solar System isn’t smooth but is filled with chaotic mag­netic bubbles 100 million miles wide. These bubbles are formed when the Sun’s distant magnetic field lines reorganize and form separate structures. Until this observation, it had been expected that the space far beyond the Sun would be smooth and feature­less. In December 2010, Voyager 1 saw the outward speed of the solar wind particles slow to zero. Stagnation of the solar wind was unexpected and may mean that the spacecraft are on the verge of entering a new realm, the vast space between the stars.

Voyager is as ambitious as any project humans have under­taken. To date, the 13,000 work-years devoted to these spacecraft are half as much as all the labor summoned by King Cheops of

Egypt to build the Great Pyramid at Giza, with its slanted passage­way so that the dead pharaoh’s soul could reside with the stars.25 Now, 4,500 years later, we have the means to go there directly as our robotic emissaries glide toward other worlds.