Dust between the stars usually factors into our discussions on Centauri Dreams when we’re considering its effect on fast-moving spacecraft. Although it only accounts for 1 percent of the mass in the interstellar medium (the other 99 percent being gas) its particles and ices have to be accounted for when moving at a substantial fraction of the speed of light. As you would expect, regions of star formation are particularly heavy in dust, but we also have to account for its presence if we’re modeling deceleration into a planetary system, where the dust levels will far exceed the levels found along a star probe’s journey.

Clearly, dust distribution is something we need to learn more about when we’re going out from as well as into a planetary system, an effort that extends all the way back to Pioneers 10 and 11, which included instruments to measure interplanetary dust. Voyager 1 and 2 carry dust detecting instruments, and so did Galileo and Cassini, the latter with its Cosmic Dust Analyzer (CDA).

And I’m reminded by a recent news release from the New Horizons team that the Student Dust Counter (SDC) aboard New Horizons is making a significant contribution as it moves ever deeper into the Kuiper Belt. You’ll recall that the SDC played the major role in identifying what may be an extended Kuiper Belt in findings published in January (citation below). Alex Doner (University of Colorado Boulder) is lead author of the paper on that work. He serves as SDC lead:

“New Horizons is making the first direct measurements of interplanetary dust far beyond Neptune and Pluto, so every observation could lead to a discovery. The idea that we might have detected an extended Kuiper Belt — with a whole new population of objects colliding and producing more dust – offers another clue in solving the mysteries of the solar system’s most distant regions.”

Image: Artist’s impression of a collision between two objects in the distant Kuiper Belt. Such collisions are a major source of dust in the belt, along with particles kicked up from Kuiper Belt objects being peppered by microscopic dust impactors from outside of the solar system. Credit: Dan Durda, FIAAA.

We have to account for the variable distribution and composition of dust not only in terms of spacecraft design but also for fine-tuning our astronomical observations. Scattering and absorbing starlight, dust produces what astronomers refer to as extinction, dimming and reddening the light in significant ways. It’s a part of the cosmic optical background, which on the largest scale includes light from extragalactic sources as well as our own Milky Way. This background can tell us about galactic evolution and even dark matter if we know how to adjust for its effects.

Joel Parker (SwRI) is a New Horizons project scientist who notes that even as the craft continues to make observations within the Kuiper Belt (and the search for potential flyby targets continues), its instruments are also returning data with implications for astrophysics at large:

“New Horizons is uniquely positioned to make astrophysical observations that are difficult or impossible to make here on Earth or even from orbit. Many things can obscure observations, but one of the biggest problems is the dust in the inner solar system. It may not be obvious when you look up into a clear night sky, but there is a lot of dust in the inner part of the solar system. There is also a great deal of obscuration at certain ultraviolet wavelengths at closer distances due to the hydrogen that pervades our planetary system, but which is much reduced out in the Kuiper Belt and the outer heliosphere.”

Image: New Horizons mission scientists and external colleagues are taking advantage of the New Horizons spacecraft’s position in the distant Kuiper Belt to make unique astrophysical and heliospheric observations. Alice, the ultraviolet spectrograph on the spacecraft, performed 75 great circle scans of the sky in September 2023, for a total of 150 hours of observations. These data focus on the light from hydrogen atoms in the ultraviolet Lyman-alpha wavelength across the sky as seen from New Horizons’ vantage point in the distant solar system. This map shows the data from the scans overlaid on a smoothed model of the expected Lyman-alpha background. (Credit: NASA/Johns Hopkins APL/SwRI).

We’ve recently talked about hydrogen-alpha transitions, which are a factor in astronomical observations (we saw this in the Project Hephaistos Dyson sphere papers). The Lyman-alpha transitions referred to above produce different emissions as electrons change energy levels within the atom, and are primarily useful for studies of the interstellar and intergalactic medium. So New Horizons is on point for helping us clarify how local dust levels may affect our observations of these distant sources.

Parker puts it this way:

“It’s like driving through a thick fog, and when you go over a hill, it’s clear. Suddenly, you can see things that were obscured. When you’re trying to look for a very faint light far outside our solar system or beyond our galaxy, that obscuration creates a challenge. If we measure how the ‘fog’ changes as we move farther out, we can make better models for our observations from Earth. With more accurate models, we can more easily subtract the effects of light and dust contamination.”

New Horizons records the cosmic ultraviolet background and maps hydrogen distribution as it moves through the outer regions of the heliosphere and eventually through the heliopause and into the local interstellar medium. This is going to be useful in telling astronomers something about the evolution of galaxies by yielding data on star formation rates as we learn how to remove the contaminating signature of interplanetary dust from our observations.

It’s fascinating to see how a single spacecraft can, as have the Voyagers, function as a kind of Swiss army knife with tools useful well beyond a single target. Successors to New Horizons will one day extend these observations as we learn more about dust distribution all the way out to the Oort Cloud.

The paper on a possible extended Kuiper Belt is Doner et al., “New Horizons Venetia Burney Student Dust Counter Observes Higher than Expected Fluxes Approaching 60 au,” The Astrophysical Journal Letters Vol. 961, No. 2 (24 January 2024), L38 (abstract).