NASA New Horizons Spacecraft Data Measures How Thick Interstellar Medium Is

The only human spacecraft to ever enter interstellar space are the two Voyager spacecraft that traveled more than 30 years to get there. Once well past the orbit of Pluto, where the Sun's influence ends, the spacecraft entered the zone between stars known as interstellar space. That interstellar space contains remains of the Big Bang and other particles from the billions of stars in the universe. NASA's New Horizons spacecraft has provided data used in a study published recently that looks at a key property of the interstellar medium, which is how thick it is.

The data showed that the interstellar medium contains about 40 percent more hydrogen atoms than some prior studies expected. The results bring other measurements together and give us a new idea of what interstellar space is like. The magnetic zone around the sun, known as the heliosphere, protects our planet in the solar system from much of the radiation and interstellar gases as they float around the highly magnetic zone.

The heliosphere can repel charged particles but does allow about half of the local neutral interstellar gases to pass through because they have a balanced number of protons and electrons. Scientists liken it to running through heavy mist and picking up water. When those interstellar gases drift inside the Heliosphere, they are hit with sunlight and solar wind particles. That results in the loss of electrons, and they turn to positively charged "pickup ions."

Despite going through that change, the particles still indicate what the interstellar medium beyond is like. NASA is clear that they don't have direct observations of interstellar atoms from New Horizons, but they can observe the pickup ions. New Horizons has been working in space since 2006 and is five years past its rendezvous with Pluto, where it captured the first close-up images of the dwarf planet.

The spacecraft has an instrument aboard called the Solar Wind Around Pluto or SWAP instrument to detect pickup ions and tell between them and the normal solar wind. The amount of pickup ions it detects indicates the thickness of the fog in the interstellar medium we pass through.