NASA's Pluto-Kuiper Belt mission invites public participation in a record-setting astronomical measurement

Have a good-sized telescope with a digital camera? Then you can team up with NASA's New Horizons mission this spring on a really cool – and record-setting -- deep-space experiment.

In April, New Horizons, which by then will be more than 46 times farther from the Sun than Earth, nearing 5 billion miles (8 billion kilometers) from home, will be used to detect "shifts" in the relative positions of nearby stars as compared with the way they appear to observers on Earth.

The technique is called parallax, and it has been used by astronomers for nearly two centuries to measure the distances of faraway stars; see the accompanying sidebar article for more detail.

What's a Parallax? The "parallax effect" is when an object appears to shift in position with respect to more distant objects. This is how our sense of depth perception works: each eye has a slightly different perspective, and the brain uses this to figure out which objects are close and which are far away. You can check this by holding up a finger, blinking with each eye, and noting how your finger it jumps back and forth against more distant background objects. You also see a parallax when you rock from side to side to see around someone blocking your view of a more distant object. In traditional "stellar parallax" measurements, astronomers use Earth's own back-and-forth rocking motion, as it orbits the Sun, to deduce distances to nearby stars. Earth's orbit is about 186 million miles in diameter, so in half a year – the time it takes Earth to go from one side of its orbit to the other – its vantage point to nearby stars will change by that much. The orbit thus serves as a "baseline" for measuring distances. The bigger the baseline, the bigger the parallaxes. As they wondered how far away the stars were, astronomers in the early 1700s predicted that nearer stars should shift in position more than distant stars as Earth moved around its orbit. Because distances to even the nearest stars are almost a half-million times greater than the baseline provided by Earth's orbit, the effect is subtle. It took until 1838 for Friedrich Bessel to obtain the first parallax observations by observing semi-annual shifts in the position of the star 61 Cygni. Accurate stellar parallaxes allow us to survey distances to stars throughout our own Milky Way galaxy, and in doing so anchor our ability to measure distances to other galaxies and determine the overall size of the universe itself! The work to obtain ever more precise parallaxes continues today, with data from the European Space Agency's Gaia mission. As fundamental as stellar parallaxes are to astronomy, however, they are difficult to demonstrate simply because the shifts are typically smaller than the scales on which telescope can easily resolve, so they require exceedingly careful measurement techniques to be accurately detected. An additional complication: all stars have their own random drifts as they orbit around our galaxy, which means that as we wait several months for Earth's movement to provide the parallaxes, the stars are not staying put. The drifts, known as "proper motions," often cause shifts in a star's position larger than its parallax. The solution is to measure the stellar positions over a few years, so that the change in their positions due to Earth's orbit can be recognized and separated from their constant proper motions. This means parallaxes are evident only with careful numerical analysis applied to years of observations. The great distance of New Horizons from Earth provides a baseline that is 23 times larger than that previously used to measure parallaxes, thus the shifts of the stars seen in comparison of Earth and New Horizons images will be visually obvious. And, because New Horizons and Earth-based observers can image the same fields at the same time, proper motions over time are irrelevant – meaning we can obtain parallaxes instantly!

On April 22 and 23, New Horizons will take images of two of the very nearest stars, Proxima Centauri and Wolf 359. When combined with Earth-based images made on the same dates, the result will be a record-setting parallax measurement yielding 3D images of these stars popping out of their background star fields that the New Horizons project will share with the public.

Color images of the Wolf 359 (top) and Proxima Centauri star fields, obtained in late 2019. The large proper motions of both stars (at the center of each image) will cause them to shift by over an arcsecond by April 2020, when NASA's New Horizons spacecraft, nearly five billion miles (8 billion kilometers) from Earth, will image them. A green circle provides a rough estimate of where both stars will appear in the New Horizons images. (Credit: William Keel/University of Alabama/SARA Observatory)

The mission team is coordinating the use of astronomical observatories and a public observing campaign to image the same stars on the same day to demonstrate the "parallax" effect.

"These exciting 3D images, which we'll release in May, will be as if you had eyes as wide as the solar system and could detect the distance of these stars yourself," said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado. "It'll be a truly vivid demonstration of the immense distance New Horizons has traveled, and a cool way to take advantage of the spacecraft's unique vantage point out on the very frontier of our solar system!"

New Horizons' two target stars can be observed by anyone with a camera-equipped, 6-inch or larger telescope. Once New Horizons sends its images to Earth, the mission team will provide them for comparison to images obtained with amateur telescopes. Wolf 359 and Proxima Centauri will appear to shift in position between the Earth-based and space-based images.

In addition, working with New Horizons participating scientist and Queen guitarist Brian May – an astrophysicist himself – the New Horizons team will create and release 3D images showing these two stars.

"For all of history, the fixed stars in the night sky have served as navigation markers," said Tod Lauer, a New Horizons science team member from the National Science Foundation's National Optical-Infrared Astronomy Research Laboratory. "As we voyage out of the solar system and into interstellar space, how the nearer stars shift can serve as a new way to navigate. We will see this for the first time with New Horizons."

Get more details on the New Horizons Parallax program – including background info on the target stars and the best times to take images – at http://pluto.jhuapl.edu/Learn/Get-Involved.php#Parallax-Program.

New Horizons is the first mission to explore Pluto and distant Kuiper Belt. The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, manages the New Horizons mission for NASA's Science Mission Directorate. Alan Stern, of the Southwest Research Institute (SwRI) is the principal investigator and leads the mission; SwRI also leads the science team, payload operations, and encounter science planning, data analyses and archiving. New Horizons is part of the New Frontiers Program managed by NASA's Marshall Space Flight Center in Huntsville, Alabama. APL designed, built and operates the New Horizons spacecraft.

For more information, visit www.nasa.gov/newhorizons and http://pluto.jhuapl.edu.