Ultraviolet Background Radiation from beyond Pluto
2018 January 30
2018 May 30
The famous NASA mission New Horizons has now passed well beyond its historic mapping and scientific investigation of the planet Pluto, and is headed farther out into the Kuiper belt—beyond which, New Horizons will venture into the reaches of interstellar space! On its way there, New Horizons will carry out, on 2019 January 1, an investigation of Kuiper belt object (486958) 2014 MU69. Once that solar-system object has been investigated, I am hoping to be able to use the Alice ultraviolet spectrometer that is aboard New Horizons to carry out observations of the cosmic diffuse ultraviolet background radiation from the universe. Alice is ideal for that purpose, first because it has wonderful 9 Å spectral resolution, but, equally importantly, because of the ideal location of the New Horizons spacecraft that carries it, which will, by that time, be more than 43.4 Astronomical Units from the sun. That extreme distance from the sun is crucial for my purposes, because the sun is the source of a very bright general background of hydrogen Lyman alpha radiation that scatters off of the interstellar hydrogen and helium that is flowing through the solar system—and then—unfortunately—that solar Lyman alpha scatters to all wavelengths in the Alice spectrometer! We need to go far enough from the sun so that that scattered Lyman alpha is small compared with the cosmic background, which is composed, to some degree, of the ultraviolet light from hot O and B type stars in our Galactic neighborhood scattering off interstellar dust in our part of the Galaxy—but a second component has a different origin, of unknown provenance: possibly from the radiative decay of the dark matter that forms the bulk of the mass of our Galaxy. Whatever its origin, if that second component of the ultraviolet background should extend below the Lyman limit of 912 Å, it would likely explain the reionization of the universe that followed shortly after the recombination that took place when the universe was only 300,000 years old. Giving us clues to that possibility would be an incredibly valuable byproduct of the wonderful New Horizons voyage to, and beyond, Pluto!