Space scientists at the Hopkins Applied Physics Laboratory have published surprising findings about the edge of our solar system. Data from the Voyager 2 space probe suggest there may be a lot more out there than was once thought.
The Sun constantly emits the solar wind, a stream of charged and highly energetic particles emerging from the outer layers of the star's atmosphere. These fast-moving particles create strong magnetic and electric fields that move away from the Sun in all directions and cross the entire solar system.
The intensity of this radiation ralls off abruptly along the boundary of the solar system, the heliosphere, which is a giant sphere that encloses the Sun and all of the planets. This "termination shock" arises when the solar wind collides with interstellar gas, causing the solar wind to lose a great deal of its energy.
The underlying principle is similar to a car crash. If a speeding car hits a wall, it will obviously come to an abrupt stop - but all of the energy of its motion has to go somewhere. Much of the energy will be disseminated in the pieces of car that litter the crash site. Some of the energy will be transformed into heat, and in fact the wall will heat up after the crash.
This same logic applies at the termination shock. Once the solar wind particles collide with interstellar gas, the wind cools down and the gas particles heat up. Previous observations have shown that the solar wind slows down from 217 miles per second to just 81 miles per second, a value that represents a tremendous amount of energy.
It is at the boundary of the heliosphere where the Voyager 2 satellite is collecting its measurements. Voyager 2 was launched in 1977 and is now at the edge of the solar system. Although built and operated by scientists at NASA Jet Propulsion Laboratory in Pasadena, Calif., scientists at Hopkins and elsewhere run several instruments on the spacecraft.
Stamatios Krimigis and his colleagues from APL were surprised to find that the solar wind cools down much more than they expected. Since interstellar gas can only absorb a certain amount of heat, they hypothesized that the excess energy must have gone elsewhere.
After painstaking analysis of information sent from Voyager 2 last summer, Krimigis and his team concluded that the energy had been picked up by particles that have intruded into our solar system. A light breeze of neutral hydrogen atoms, the most basic atoms in the universe, flows into the solar system at an astronomically slow speed of just 16 miles per second.
Occasionally, these particles collide with the solar wind, which will kick off the hydrogen atom's sole electron to form an ion. The free electrons, which are charged, are caught by the solar wind's electromagnetic fields and directed back to the edge of the heliosphere.
In fact, the Hopkins team has concluded that most of the solar wind's energy loss goes into accelerating these "pick-up ions," as the team has called them. These pick-up ions are especially important in the heliosheath, the much larger area that surrounds the heliosphere and goes out quite far into interstellar space. This area, which is where Voyager 2 is headed, is the next area of interest for the APL group.
