Does anyone still know how to read a paper map in the era of GPS?
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I will confess that although I could use such a map for travel, I seldom do so. I love maps, and my home office has a beautiful map of the U.S. hanging on the wall, but GPS is just so easy and convenient.
Finding one’s way on the surface of the Earth is a two-dimensional problem. And even in human crewed low Earth orbit, there is little need for navigation other than to or from the International Space Station. In one direction, the Earth fills your view.
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In the depths of space? The nine Apollo missions to the moon required precise navigation to put the astronauts just 60 miles ahead of the moon’s leading edge so that they could be captured into lunar orbit. NASA tracked them by measuring their velocity as they flew away from or back toward the earth, and used the sun, moon and earth to measure position in other dimensions.
But the moon is our next door neighbor. If we shrink the solar system down to a more manageable scale, we can put the earth 77 feet away from the sun, and Jupiter a little more than 300 feet away from us. On this scale, the moon is a mere 2.4 inches away from the earth. For larger scales, you need a LOT of room.
As uncrewed spacecraft aimed toward ever more distant targets in the solar system, we needed a reference frame for the heavens, similar to latitude and longitude lines on the earth. Tying this to the spinning, orbiting, and shifting earth’s surface is too imprecise for aiming spacecraft. You need fixed points in space.
So…the stars, right? Even in the 1700s, astronomers could see that stars suspended in space do move and do shift their positions over time. I have a star catalog I purchased years ago that is designated “Epoch 1950”, meaning that it shows stellar positions as of that date. Such references aren’t even published in this form any more—they are mostly online. The image below is from my copy, showing an area around the Orion constellation.
Chart VII from Atlas of the Heavens, by Antonin Becvar. Copyright 1962, Publishing House of the Czechoslovak Academy of Sciences
In the 1970s, astronomers began using quasars instead, locating them with radio telescopes. These appear as points of light like stars, but they are actually incredibly bright swirls of gas circling supermassive black holes at the centers of very distant galaxies. Billions of light years away, they hardly move at all during a human lifetime.
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But a radio-based reference frame isn’t especially useful if you are an optical astronomer. Quasars can be precisely located in radio wavelengths, but the earth’s atmosphere makes them “twinkle” in visible light—hard to pin down.
Enter the Gaia spacecraft, a star-mapping spacecraft flying far above our blurring atmosphere. Its data set giving super-precise positions of billions of celestial objects has created a new fixed grid to measure everything else against.
It proved its worth at the end of 2018 as the New Horizons spacecraft neared its encounter with tiny Arrokoth at the edge of the solar system. Images taken as the spacecraft drew nearer were off target. Either the spacecraft’s position, calculated from Earth-based measurements, was off, or that of the intended target, determined by this celestial reference frame, was. A wrong decision would mean that in the one chance to capture images and data as New Horizons flew by, pictures would just show empty space and instruments would be misaimed.
The team trusted the Gaia dataset. Corrections were sent to the spacecraft based on that reference frame. And it worked.
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How precise is this grid? Accuracy of a hundred-millionth of a degree—the width of a basketball on the moon—may seem like overkill. But it can mean the difference between a spectacular image captured from deep in space and a picture of nothing but a dark sky.
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