New Horizons NASA's Mission to Pluto and the Kuiper Belt
The Cosmic Optical Background, or COB, is the flux of ordinary light that comes to us from the distant universe. It reflects the total generation of light from all sources outside our own Milky Way galaxy. Much of it should be due to starlight generated by the nearly one trillion galaxies that the universe comprises, as well as light generated by matter falling into black holes in their centers. But New Horizons can attempt to test whether yet more light might be generated by unknown mechanisms, in addition to the flux generated by galaxies. While the light from distant galaxies can be detected by space telescopes, such as the Hubble Space Telescope and James Webb Space Telescope, sunlit dust in the inner solar system makes it very difficult to measure the overall total brightness of light coming to us from the universe.
Far away from the Sun and dusky inner solar system, however, New Horizons is in the perfect position to detect the total flux comprised by the COB.
From New Horizons LORRI instrument images taken for other purposes, and from a single field imaged to test the optimal use of New Horizons to detect the COB, scientists have found intriguing evidence that the universe does provide more light than can be produced by galaxies, alone. During its second extended mission New Horizons will observe several fields scattered around the sky (marked by the green dots in the graphic below) to build on this preliminary result, as well as to validate the use of LORRI for making accurate and precise measurement of the COB.
New Horizons measurements of the universe’s darkness reveal an unexplained glow — showing that while the universe is dark, it is not as dark as scientists once thought. Read more in these features from the Space Telescope Science Institute and The National Science Foundation's NOIRLab.
The Cosmic Ultraviolet Background (or CUVB) is the ultraviolet analog of the COB. With the Alice spectrograph onboard New Horizons, we can attempt to measure the total flux of ultraviolet (UV) light that reaches us from the distant universe. As with the COB, the CUVB is expected to reflect light produced by stars and accreting black holes in distant galaxies, but may also reveal unknown mechanisms that generate UV light in the universe. In detail, galaxies generate UV light from different types of stars and activity than those that generate ordinary light. The flux embodied in the CUVB thus provides a complementary probe of astrophysics of the universe overall. The New Horizons CUVB program is closely tied to the COB effort, and will look at the same fields, but in UV light instead.
Most of the ordinary mass of our Milky Way galaxy (as opposed to its dark matter halo) is in the hydrogen that makes up its stars, nebulae, and the sea of diffuse cold gas that pervades interstellar space. Strong UV light emitted by stars and other forms of activity in our galaxy "excites" this gas and causes it to emit the characteristic spectrum of light unique to hydrogen. By far, the strongest feature in the spectrum is the so-called Lyman-alpha line, which marks a particular frequency of UV light. While many spacecraft have made maps of the full sky as seen in UV, none have been done in the light uniquely generated by hydrogen. A map of the Lyman-alpha line over the full sky will provide an excellent view of star formation and other interesting forms of activity over the full body of the Milky Way.
These New Horizons observations will be done in a scanning mode, where the spacecraft is slowly rotated and stepped around the sky in a way that uses the Alice instrument to scan most of the sky — except the regions visually close to the Sun, which is too bright for Alice to observe safely. Please see additional information about New Horizons Lyman-alpha observations on the heliophysics science page.
This artist’s illustration shows New Horizons in the outer solar system. In the background lies the Sun and a glowing band representing zodiacal light, caused by sunlight reflecting off dust. By traveling beyond the inner solar system and its accompanying light pollution, New Horizons is answering the question: How dark is space? (Credit: STScI/Joe Olmsted)