Incredible Flight to the Stars

Like ancient mariners, astronauts exploring the solar system will navigate by the stars. But when man finally ventures to the stars themselves, says NASA Mathematician and Physicist Saul Moskowitz in a Sky and Telescope article, navigation will become more of a problem. In place of the familiar and steadfast constellations he has learned to rely on, the star traveler will encounter a mystifying and spectacularly changing sky in which stars move, change color, brighten, disappear, and magically cluster together in front of him.

In a study he headed while working at Long Island's Kollsman Instrument Corp., Moskowitz and his co-workers plotted a hypothetical space flight to the star 45 Eridani, barely visible to the naked eye from earth and some 466 light-years away. Picking well-known stars in 45 Eridani's region of the sky, he fed data about their positions, distance and absolute brightness into a computer programmed to generate the appropriate star map for any point along the route of the flight.

Red & Blue. Not surprisingly, as Moskowitz's imaginary ship moved far beyond the solar system, the appearance of the sky began to change. As the ship approached them, the nearer stars began to shift their positions in familiar constellations, eventually disappearing from forward view as the spacecraft passed them. More distant stars remained in relatively fixed positions. In the view from the rear, the sun faded from sight as the craft flew beyond a distance of 30 light-years from the solar system, while 45 Eridani loomed ever larger ahead.

There were other, more remarkable changes in the sky. At the tremendous velocities necessary for interstellar travel, there were noticeable Doppler shifts in the frequencies of light emitted from the stars. As the spacecraft approached stars at high velocity, their light appeared to be shifted toward the blue, or higher frequency end of the spectrum. Moving rapidly away from other stars caused their light to appear more red. Stars viewed through the front window of the spacecraft generally became more blue, those through the rear window more red, as the spacecraft velocity increased.

Relativistic Implosion. Eventually, as the spacecraft approached the velocity of light, some of the stars ahead of it began to blink out; their light had been shifted into higher, ultraviolet frequencies that are invisible to the human eye. Others, like Betelgeuse and Aldebaran, which look red to observers on earth, actually became brighter: their substantial lower frequency infrared output, normally invisible to the human eye, had been shifted into the visible range.

If the celestial navigator on the flight had not already become lost in space, there was a more puzzling effect to confound him: relativistic aberration, the apparent shift of visible stars toward the direction of the spacecraft's motion. Thus, the computer showed, as the craft approached 45 Eridani at ever increasing velocities, other stars in the sky began to converge toward the target star. At 90% of the speed of light, only a few stars remained visible through the rear window. In a nightmarish finale to Moskowitz's flight, the remainder of the visible universe--stars, galaxies, nebulas--seemed to collapse into a single point and simultaneously disappear.

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