Check out these sets of images. What you see the phenomenon called “parallax shift”. The first image in each set is the view of a star, either Proxima Centauri or Wolf 359, as seen from Earth. The second view is from a very unique perspective: the New Horizons spacecraft.

New Horizons is currently about 6.6 billion kilometers away from Earth, traveling out through the Kuiper Belt. That distance gives it a very different view of the two stars.

Notice how the scene shifts in each view. That’s the effect of parallax—viewing an object from two different locations. Seeing it like this is reminiscent of Clyde Tombaugh’s view of Pluto through a blink comparator. In that case, Tombaugh took images of Pluto as it orbited the Sun. Over a period of time, the planet itself moved against the backdrop of stars. In the case of the two stars, however, we aren’t getting a view of two stars moving. What’s changing is OUR point of view between Earth and New Horizons.

You can do this same experiment by observing both stars from Earth. However, it would take half a year to see the two different views. That’s how astronomers have done simple parallax measurements for quite some time. But, this time is different. It’s the first time that a spacecraft has been used for this purpose. It has sent back pictures of the sky with stars appearing in different positions than the way they appear to us on Earth.

It’s a record-breaking achievement for a spacecraft that has already done quite a number of really cool things. It passed by dwarf planet Pluto in 2015. Then, it flew by the Kuiper Belt object Arrokoth in early 2019. On its way out, the spacecraft imaged Jupiter and measured dust in the solar system.

New Horizons and Parallax Shift

Caputuring the parallax shift from its current vantage point in the solar system was pretty straightforward for the spacecraft. It focused its long-range cameras in the LORRI instrument on Proxima Centauri and Wolf 359 in mid-April, 2020. It wasn’t the only camera to do so. At the same time, amateur and professional astronomers on Earth also imaged the two stars.

The resulting parallax measurement was something Stern suggested as a possible New Horizons project a while back. The result is a scene he described as an alien view. “It’s unlike what we would see from Earth,” he said. “It has allowed us to do something that has never been accomplished before, to see the nearest stars visibly displaced from the positions we see them on Earth.”

Why Measure Parallax Shift?

Parallax shifts like this one are the basic building block of distance measurements, particularly inside our own galaxy. Until now, the most common way was to look at a star and measure its position against the distant stars. Then, wait until Earth moves along in its orbit for six months, and measure the position of the star again. Its apparent position will have shifted a tiny bit against the distant stars. Using the measurements and some trigonometry, astronomers can get distances to stars within about 300 light-years of Earth.

The New Horizons method took advantage of the spacecraft being very far away from the Sun to image the stars. But, Alan pointed out an added twist: “Those same images and measurements allowed us to calculate exactly where the spacecraft is.”

The experiment on New Horizons is the largest parallax “baseline”—6.4 billion miles long—ever used to calculate distances to stars. Compare that to the two astronomical units that the traditional method uses.

Parallax Shift Provides a Stereo View

This achievement includes an interesting wrinkle: 3D stereoscopic viewing. Three team scientists put their heads together to do something special. They were astronomer Tod Lauer, New Horizons Deputy Project Scientist John Spencer, of SwRI, and astrophysicist, Queen guitarist and stereo imaging enthusiast Brian May. They created images that clearly show the effect of the vast distance between Earth and the two nearby stars. May, who posts and sells stereoscopy images on his own London Stereoscopic Company website, was enthused about the project.

“It could be argued that in astro-stereoscopy—3D images of astronomical objects—NASA’s New Horizons team already leads the field, having delivered astounding stereoscopic images of both Pluto and the remote Kuiper Belt object Arrokoth,” May said. “But the latest New Horizons stereoscopic experiment breaks all records. These photographs of Proxima Centauri and Wolf 359, stars that are well-known to amateur astronomers and science fiction aficionados alike, employ the largest distance between viewpoints ever achieved in 180 years of stereoscopy!”

You can check these images out here and do a little stereo viewing yourself. For those curious about the project, there’s a special “Ask Me Anything” on Reddit on June 12, 2020, at reddit.com/r.askscience. It starts at 1 p.m. EDT. Experts (including Brian May!) will be standing by!

Heralding the Earth-based Parallax Shift Team

To be sure, New Horizons had some pretty impressive help from the ground. That included Earth-based imaging from professional and amateur observers. They came from the Cumbres Observatory, through a remote telescope at Siding Spring Observatory in Australia, and from astronomers John Kielkopf, University of Louisville, and Karen Collins, Harvard and Smithsonian Center for Astrophysics, operating a remote telescope at Mt. Lemmon Observatory in Arizona. It was an extraordinary melding of amateur and professional astronomy along with spacecraft imaging. According to Lauer, the results were spectacular. “The images collected on Earth when New Horizons was observing Proxima Centauri and Wolf 359 really exceeded my expectations.”

Their sheer closeness to the Sun is one of the most important reasons the New Horizons/Earth-based parallax experiment worked so well.

About the Parallax Shifted Stars

Earlier, Brian May mentioned that the two stars, Proxima Centauri and Wolf 359, are dear to astronomers and science-fiction readers alike. Why is that? Well, let’s look at Proxima Centauri. First, it’s in the closest star system to Earth, at a distance of 4.24 light-years from us. It’s thought to have a planet, called Proxima Cen b. And, in the science fiction realm, it’s been in a number of works, including video games. The name “Proxima Centauri” just sounds alien and cool, and I’m sure that’s part of the allure for fiction fans.

Wolf 359 is also a close neighbor of ours, at a distance of 7.9 light-years. It’s a red dwarf, and is a fairly faint, low-mass star. It appears to have two planets, called c and d. Because it, too, is a neighbor, it also features in science fiction books, TVs, movies, and in games. Star Trek fans will recognize it as the site of an important war in Federation history. It took place between the cube-inhabiting Borg species and the Federation fleet in the Battle of Wolf 359.

Will New Horizons do another parallax measurement? According to Stern, probably not soon. “We have to reserve the cameras onboard for other planned observations,” he said. There’s still more to come from the spacecraft, particularly as it transits the Kuiper Belt over the next years.