NASA’s New Horizons spacecraft measures how thick the interstellar medium is

The only human spacecraft to enter interstellar space are the two Voyager spacecraft, which have traveled for more than 30 years to get there. Once past Pluto’s orbit, where the sun’s influence ends, the spacecraft enters an interstellar region called interstellar space. That interstellar space contains the remains of the Big Bang and other particles from billions of stars in the universe. Data from NASA’s New Horizons spacecraft is used in a recently published study that examined the thickness of a key property of interstellar media, the interstellar medium.

The data show that interstellar media contain 40 percent more hydrogen atoms than some previous studies had expected. These results combine other measurements to give us a new idea of what interstellar space looks like. The magnetic regions around the sun, known as the heminosphere, protect our planets in the solar system from most radiation and interstellar gases as they float around the high magnetic zone.

The sun layer can reject charged particles, but allows about half of the local neutral interstellar gas to pass through because they have a balanced number of protons and electrons. Scientists liken it to running and collecting water in thick fog. As these interstellar gases drift within the sun’s rays, they are hit by sunlight and solar wind particles. This results in the loss of electrons, which become positively charged “picking ions”.

Despite these changes, these particles still show the properties of interstellar media. NASA has made it clear that it has not directly observed interstellar atoms from New Horizons, but they can observe pick-up ions. New Horizons has been in space since 2006, five years after it met Pluto, where it captured the dwarf planet’s first close-up image.

The spacecraft has an instrument called the solar wind around Pluto, or SWAP, that detects pick-up ions and distinguishes them from normal solar winds. The amount of picked ions it detects indicates the thickness of the fog in the interstellar medium we pass through.

NASA's New Horizons spacecraft measures how thick the interstellar medium is