New Horizons Clearly Sees All The Extragalactic Lights

M87 with fainter galaxies in the background. NASA

New Horizons has opened our eyes to the beauty and complexity of Pluto, and now it has delivered some unique cosmic data that would be next to impossible to obtain from Earth.

The NASA spacecraft, which is currently in hibernation waiting to reach its next target, has captured a very precise measurement – the cosmic optical background – the accumulation of extragalactic visible light produced by every source in the universe. The observations are reported in Nature Communications.

“Determining how much light comes from all the galaxies beyond our own Milky Way galaxy has been a stubborn challenge in observational astrophysics,” lead author Michael Zemcov, assistant professor at the Rochester Institute of Technology, said in a statement.

The main issue in studying the cosmic optical background from the inner Solar System is dust. Comets like 67P/Churyumov-Gerasimenko have over billions of years filled the inner Solar System with minuscule dusty fragments that reflect sunlight, forming the so-called Zodiacal light (studied among others by Queen guitarist Dr Brian May).

The Zodiacal light is too intense near our planet to take a good picture of the cosmic optical background, but beyond the orbit of Pluto, 5 billion kilometers (3 billion miles) away, New Horizons’ Long Range Reconnaissance Imager (LORRI) can observe the combined light of distant galaxies undisturbed.  

“This result shows some of the promises of doing astronomy from the outer solar system,” Zemcov said. “What we’re seeing is that the optical background is completely consistent with the light from galaxies and we don’t see a need for a lot of extra brightness; whereas previous measurements from near the Earth need a lot of extra brightness. The study is proof that this kind of measurement is possible from the outer solar system, and that LORRI is capable of doing it.”

This is not the first use of deep space probes for astrophysical studies. Both Pioneer 10 and 11 were used to look at the cosmic optical background, providing the groundwork for this study. While the missions towards the edge of the Solar System are to look at planets, they could be optimized or tweaked to look at the wider universe in the long months of travels between targets.

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