New Horizons becomes first spacecraft to demonstrate stellar parallax
NASA's New Horizons deep-space probe has, for the first time, returned images to Earth that show stellar parallax, or how the positions of stars shift when seen from two different places. This phenomenon could one day be used for interstellar navigation.
Parallax is a very simple thing to demonstrate. Hold up a finger in front of your eyes at arm's length, close one eye, and then swap to the other. You'll notice that this causes your finger to seem to shift because your eyes are looking from two different locations.
In astronomy, this is a method that's used by astronomers to measure the distance to nearby stars and is at the center of a rather interesting historical controversy.
Most people have heard about Galileo's problems with the Catholic church over his claim that the Earth revolves around the Sun. What most people don't know is that Galileo's claim was just that – a claim. He didn't have any direct evidence that the Earth actually moved. In fact, that was Tycho Brahe's main objections against the Copernican theory. If the Earth moved, you should be able to see the stars shift in the sky due to parallax.
If a star is one parsec, or 3.3 light-years, from Earth, it should shift one arcsecond as the Earth moves from one side of its orbit to the other. However, no star is that close and for centuries instruments were nowhere near accurate enough to measure stellar parallax farther away.
Then, in 1838, the German astronomer Friedrich Bessel measured the parallax for the star 61 Cygni, which turned out to be 11.4 light-years from Earth. And so, Galileo was proven right 196 years after his death.
Today, the unmanned New Horizons probe is far beyond the orbit of Pluto at a distance of over 4.3 billion mi (7 billion km) from the Sun as it heads toward interstellar space, never to return. On April 22 and 23, the spacecraft was ordered to use its long-range telescope to take pictures of the stars Proxima Centauri and Wolf 359, which lie 4.2 light-years and 7.8 light-years away, respectively.
These images were returned to NASA via radio link, taking about 6 hours and 30 minutes to reach their destination, and compared to images taken on Earth at the same time by the Siding Spring Observatory in Australia and the Mt. Lemmon Observatory in Arizona. The two sets of images were compared, showing definite parallax.
According to NASA, this is not only the first time a spacecraft has measured stellar parallax, it is also the longest measuring baseline ever used because the distance to New Horizons is 23 times longer than the baseline of the Earth's orbit. This makes parallax easily observable to the eye instead of precision instruments
"It’s fair to say that New Horizons is looking at an alien sky, unlike what we see from Earth," says Alan Stern, New Horizons principal investigator from Southwest Research Institute (SwRI) in Boulder, Colorado. "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."
Not only is this experiment a record-breaker, NASA says that it could one day have practical applications. Currently, deep -space probes rely on signals from NASA's Deep Space Network for navigation, but as craft move out into interstellar space, this won't be practical. Instead, the onboard navigation systems will have to use new methods or adapt old ones.
One of these will be stellar parallax, which will allow computers to calculate positions by noting the shift of nearby stars. It's a method that mariners have used for millennia by using landmarks ashore and buoys at sea to fix their position by noting changes in angles as the ship sailed in a straight line of known length. It's interesting that one of the most ambitious of future space adventures may one day use a method that the ancient poet Homer would have understood.