The two Voyager space probes launched by NASA in 1977 completed their inspections of the planets Jupiter, Saturn, Uranus, and Neptune back during the 1980s, yet they have continued to contribute to science since then with their ongoing measurements of conditions in the farthest reaches of the solar system. In particular, their observations, along with those of other spacecraft such as NASA’s Interstellar Boundary Explorer (IBEX), have revolutionized space physicists’ understanding of the huge ionized gas (plasma) structure called the heliosphere. See also: Plasma (physics)
The heliosphere is often described as a massive bubble of ionized particles that extends far beyond the Sun and planets. It consists of gas emanating from the Sun as a fast-moving solar wind, which carries along with it energy from the solar magnetic field. When this material is roughly 100 million times as far from the Sun as Earth is (a distance of 90 astronomical units, or 100 AU), the rapid outward expansion of the plasma is checked by pressure from the diffuse but significant interstellar matter (or medium). The solar magnetic field carried along with the solar wind is the major barrier that blocks powerful cosmic rays originating elsewhere in the galaxy from reaching Earth and the rest of the inner solar system. Changes in solar activity that affect the heliosphere alter the solar modulation of these rays. The size and shape of the heliosphere therefore have a direct bearing on how livable conditions on our world are. See also: Cosmic rays; Interstellar matter; Solar magnetic field; Solar wind
For most of the twentieth century, astronomers and space physicists had a relatively simple model for the heliosphere’s structure. They believed that at 80 million AU or so, a standing shock wave, called the termination shock, marked where the solar wind slowed down drastically from supersonic to subsonic speeds. Beyond this lay a region called the heliosheath, where the solar plasma was more compressed, ending in a heliopause where the solar wind encountered interstellar material, stopped expanding, and drifted away with the passing interstellar medium. On the far side of the heliopause, a bow shock of interstellar material crashing into the heliosphere was assumed to occur. The heliosphere was therefore assumed to be spherically symmetrical with a tail of slow-moving material trailing after it as the Sun moved through the Milky Way.
However, measurements by the Voyager spacecraft during the early twenty-first century have called every part of this model into question. Voyager 1 encountered the termination shock in 2005, at a distance of about 94 million AU from the Sun, whereas Voyager 2 crossed into the termination shock in 2007 at about 76 AU, which indicated that the expanse of the heliosphere north of the ecliptic plane was greater than the expanse south of it. The heliosphere therefore seems to be more bullet-shaped than spherical, and set an angle with respect to the plane of the Milky Way. These observations suggest that the interstellar magnetic field may be strong enough to significantly influence the heliosphere’s structure—an idea that was largely dismissed for decades.
Further studies of Voyager data published in 2011 concluded that heliosheath was also surprisingly complicated: The plasma in the leading edge of the heliosheath seemed to be structured into a froth of gigantic bubbles, some of which are 100 million miles across. The leading explanation for this phenomenon is that portions of the solar magnetic field in that part of the heliosphere are fragmenting and reconnecting into loops that organized the plasma into bubbles.
Another surprise from 2012 was the discovery that the heliosphere is not preceded by a bow shock after all. The relative motion of the Sun through the interstellar medium is apparently slower than had been supposed and is not sufficient to create a standing shock wave. Scientists were also started to learn that Voyager 1 is encountering long, organized streams or “superhighways” of charged particles moving through the heliosphere, possibly where the Sun’s magnetic field and the interstellar magnetic field merge. The observation of these particle streams could be an indication that Voyager 1 is on the very edge of the heliosphere and about to enter interstellar space. See also: Voyager crossing superhighway to solar system exit
When Voyager 1 passes out of the heliosphere, it will be the first object made by humans to leave the solar system. Estimates of when Voyager 1 will pass this historic milestone vary considerably. Some space scientists believe it could happen at almost any moment; others think it more likely that the spacecraft still has two or three years of travel ahead of it. See also: Voyager chasing solar system's edge