A NASA parallax experiment involving New Horizons shows the probe now sees some stars in slightly different positions than we do on Earth, revealing just how far the spacecraft is from home.
You can actually replicate this effect at home, kids. Fully extend your arm away from your face and raise a finger (anyone, you choose). You’ll notice that, when alternating between your left and right eyes, your finger will change position. That’s the parallax effect, and it applies just as much to distant stars as it does to your extended finger. The difference, of course, are the scales involved.
In NASA’s case, the length of the fully extended arm is measured in light years, as opposed to a few inches. The two stars used in the recently concluded experiment, Proxima Centauri and Wolf 359, are quite close to Earth as far as these things go. Proxima Centauri is 4.2 light years away and Wolf 359 is 7.8 light years from Earth.
As for the distance between the left and right eyes, so to speak, that’s the distance between Earth and New Horizons, which is now 4.3 billion miles (6.9 billion kilometers) away. New Horizons, as you may remember, visited Pluto in 2015 and the weirdly shaped Arrokoth asteroid in early 2019. The spacecraft is now hurtling towards interstellar space at 31,500 miles per hour (50,700 kilometers per hour).
NASA ran this experiment back in late April, during which time New Horizons, using its long-range telescopic camera, took images of these two nearby stars. Back on Earth, telescopes in Australia and Arizona took images of these stars at the same time.
“The New Horizons experiment provides the largest parallax baseline ever made—over 4 billion miles—and is the first demonstration of an easily observable stellar parallax,” said Tod Lauer, a New Horizons science team member, in a NASA press release.
When paired together, these images showed Proxima Centauri and Wolf 359 at different spots in the night sky, showing the parallax effect in action. The stars in the background were still in their usual spots owing to their more extreme distances to both Earth and New Horizons.
“It’s fair to say that New Horizons is looking at an alien sky, unlike what we see from Earth,” said Alan Stern, the principal investigator for the New Horizons project. “And that has allowed us to do something that had never been accomplished before—to see the nearest stars visibly displaced on the sky from the positions we see them on Earth.”
A cool aspect of this experiment is that these two frames provide a 3D perspective, or stereoscopic view, of Proxima Centauri and Wolf 359. You can download more New Horizons parallax images here, including 3D views.