How close can you get to a black hole without falling in?

Although I don't recommend it, you can hover your spaceship about 50 miles from a black hole without falling in. 

Naturally, you would need to take some precautions.  The region around a black hole is filled with deadly X-ray light and bits of matter flying about a nearly light speed.  Then there's the strong tidal forces and magnetic fields that could rearrange the atoms in your body.  But if you could somehow shield yourself from this --- oh, and the searing, million-degree heat, too --- then you can zoom up to within about 50 to 70 miles of a stellar black hole.  The gravity would be strong, but you'd be able to maintain a stable orbit.

Scientists at an astronomy meeting in January announced that they saw matter orbiting very close to a stellar black hole, perhaps the closest it could get without falling in.  The orbit was remarkably stable, holding steady for nine years.  This was between 50 and 70 miles from the black hole, they estimated.

A stellar black hole is created by the collapse of a very massive star, probably at least 20 times more massive than the sun.  The core of the star implodes, and all that matter squishes down into a one-dimensional point of infinite density called a singularity.

Black holes have a theoretical border called an event horizon. Gravity is so strong within the event horizon that nothing, not even light, can escape its pull. Outside the event horizon, light can still escape.  For a typical stellar black hole, the event horizon reaches out in all directions about 10 miles from its center.

But you don't want to be at the event horizon.  If you're close, you will eventually fall in.  It is hard to maintain a steady orbit.  It's like the final swirl of water before it does down the drain.  All the momentum is pushing you down, down, down.

From the event horizon and out to about 40 to 60 miles is a region sort of like a no-man's land.  Any gas (or spaceship!) that wanders into this region quickly gets pulled to the black hole within about an orbit or two, which is actually only a few seconds.

At about 50 to 70 miles from the black hole center, though, lies a region called the innermost stable circular orbit.  This is predicted by Einstein's theory of general relativity.  It's like a little groove carved into space.

Scientists led by Jeroen Homan of the Massachusetts Institute of Technology, using NASA's Rossi X-ray Timing Explorer, observed such a groove carved into the fabric of space by a spinning black hole.  The scientists compared observations of this black hole from 1996 and 2005 and found the black hole was still playing the same tune -- that is, they detected X-ray light at the exact same frequency nine years apart.  This implies that gas is orbiting the black hole at the same distance year after year.  A spaceship, in theory, can hang out here too.  The team could not determine if this was the inner most stable orbit, but if not, it was certainly close to it.

Unfortunately this black hole is thousands of light years away.  This means that even if your spaceship could move at light speed, which is theoretically impossible, you'd still have a long, long journey to get there.

This week's question comes from Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.