That’s a question pondered for centuries: How to permanently leave our world. A cannonball fired upward always returned. With a greater explosive charge and a higher speed, the shell went farther. If aimed sideways, it would start to fall around our planet’s curving surface, extending its travel distance. It wasn’t hard to figure out what speed an object would need to leave permanently. It was 6.9 miles per second. This is Earth’s escape velocity.
Every celestial object’s mass determines how strongly it glues to itself any nearby objects. For the Sun, the escape velocity is a whopping 384 miles per second if you start out just above its surface. But if the Sun should somehow collapse to half its present diameter, pulling its surface twice as close to its center, its surface escape velocity would then be four times greater, or more than 1,500 miles per second.
Our Sun won’t ever do that, but very massive stars do indeed collapse in their old age, when their cores no longer generate enough outward-pushing force. That’s why the most common black holes are massive old stars.
To the public, the most intriguing escape velocity belongs to black holes. Typical ones are stars so massive they’ve collapsed under their own weight until their escape velocity reached the speed of light, which is 186,282.4 miles per second. Since at that fastest possible speed the star’s own light can’t get out, neither can anything else. What TV science shows never seem to mention is that such a perilous one-way vacuum-cleaner-type suction would only apply to objects extremely close to the black hole, which is not much. If our Sun collapsed to be a black hole, you’d have to venture closer than two miles to be in jeopardy. Here on Earth, we’d keep orbiting it as we always have, and wouldn’t be pulled toward it in the slightest, since its overall gravity would remain unchanged.
Another little-known wrinkle is that “escape velocity” only applies to objects traveling without any further boosts to its speed. If a powerful rocket engine could keep firing, a spacecraft could indeed escape from a black hole even though it moved far slower than light-speed.
Yet another factor is that gravity grows weaker with the square of distance. So though you must go 384 miles a second to escape from the Sun if you started off at its surface, you only need to travel at 26 miles a second to escape the Sun’s pull if you started off at Earth’s distance from it. In short, a spacecraft wanting to leave the solar system would have to go around seven miles a second to escape Earth’s clutches, but would need around four times that velocity to keep heading outward toward the stars, unless it was further propelled by engines firing.
And then you face an additional obstacle. Our Milky Way galaxy, composed of 400 billion suns, has its own escape velocity, and it’s nothing trivial. For anything to leave, it would have to travel over 180 miles per second and no star had ever been found possessing such a speed.
Except — in 1988 an astronomer discovered a mathematically possible way. IF it was a member of a binary system, meaning a double star, and IF it passed a precise distance from the supermassive black hole in our galaxy’s center, the black hole could in theory yank at one member of the binary system in such a way that the other member got flung off fast enough to permanently break away from the entire galaxy.
It could happen. But has it? In 2003, astronomers starting searching for just such objects — and found one two years later. It’s a dazzling blue star with the kind of high metal content expected of stars born in the galaxy’s core. Except this star is nowhere near there. Instead, it’s whizzing through the Milky Way’s suburbs at 415 miles per second, as the fastest-moving star in the galaxy.
In just another 80 million years that single escaping star will be an intergalactic wanderer. Any life on planets around it will gaze up into a starless sky, lit only by the fuzzy outline of its long-ago parent galaxy, the Milky Way.
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