Free Astronomy Magazine November-December 2024

33 NOVEMBER-DECEMBER 2024 ASTRO PUBLISHING them during a 2021 test COB obser- vation run with New Horizons in which they underestimated the amount of dust-scattered light and overestimated excess light from the universe itself. But this time around, after account- ing for all known sources of light, such as background stars and light scattered by thin clouds of dust within the Milky Way galaxy, the re- searchers found the remaining level of visible light was entirely consis- tent with the intensity of light gen- erated by all galaxies over the past 12.6 billion years. “The simplest interpretation is that the COB is completely due to galax- ies,” Lauer said. “Looking outside the galaxies, we find darkness there and nothing more.” “This newly published work is an im- portant contribution to fundamen- tal cosmology, and really something that could only be done with a far- away spacecraft like New Horizons,” said New Horizons Principal Investi- gator Alan Stern, of the Southwest Research Institute in Boulder, Col- orado. “And it shows that our cur- rent extended mission is making important scientific contributions far beyond the original intent of this planetary mission designed to make the first close spacecraft ex- plorations of Pluto and Kuiper Belt objects.” Launched in January 2006, New Horizons made the historic recon- naissance of Pluto and its moons in July 2015, before giving humankind its first close-up look at a planetary building block and Kuiper Belt ob- ject, Arrokoth, in January 2019. New Horizons is now in its second ex- tended mission, imaging distant Kuiper Belt objects, characterizing the outer heliosphere of the Sun, and making important astrophysical observations from its unmatched vantage point in the farthest re- gions of the solar system. A n artist’s impression of NASA’s New Horizons spacecraft against the back- drop of deep space. More than 5.4 billion miles (7.3 billion kilometers) from Earth, New Horizons is traversing a region of the solar system far enough from the Sun to offer the darkest skies available to any existing telescope – and to provide a unique vantage point from which to measure the overall brightness of the distant universe. The lane of our Milky Way galaxy is in the background. [NASA, APL, SwRI, Serge Brunier (ESO), Marc Postman (STScI), Dan Durda] ground, or COB – provided a way to add up all the light generated by galaxies over the lifetime of the uni- verse before NASA’s Hubble Space Telescope and James Webb Space Telescope could see the faint back- ground galaxies directly. In the Hubble and James Webb tel- escope era, astronomers measure the COB to detect light that might come from sources other than these known galaxies. But measuring the total light output of the universe is extremely difficult from Earth or anywhere in the inner solar system. “People have tried over and over to measure it directly, but in our part of the solar system, there’s just too much sunlight and reflected inter- planetary dust that scatters the light around into a hazy fog that ob- scures the faint light from the dis- tant universe,” said Tod Lauer, a New Horizons co-investigator, as- tronomer from the National Science Foundation NOIRLab in Tucson, Ari- zona, and a co-author of the new paper. “ All attempts to measure the strength of the COB from the inner solar system suffer from large uncer- tainties.” Enter New Horizons, billions of miles along its trek beyond the planets, now deep in the Kuiper Belt and headed toward interstellar space. Late last summer, from a distance 57 times farther from the Sun than Earth, New Horizons scanned the universe with its Long Range Re- connaissance Imager (LORRI), col- lecting two-dozen separate imaging fields. LORRI itself was intentionally shielded from the Sun by the main body of the spacecraft – keeping even the dimmest sunlight from di- rectly entering the sensitive camera – and the target fields were posi- tioned away from the bright disk and core of the Milky Way and nearby bright stars. The New Horizons observers used other data, taken in the far-infrared by the European Space Agency’s Planck mission, of fields with a range in dust density to calibrate the level of those far-infrared emis- sions to the level of ordinary visible light. This allowed them to accu- rately predict and correct for the presence of dust-scattered Milky Way light in the COB images – a technique that was not available to !