Why do we hear two sonic booms when the space shuttle lands?

Answer 1

Because you get one from the nose and the other from the tail about half a second apart.

Answer 2

No clue. The shuttle is well below the speed of sound when it lands. Check this out...

"At this point, the pilot deploys the landing gear. As the shuttle's main landing gear touches down, it is dropping at less than 8 kilometers per hour (5 miles per hour) and has a forward speed of about 354 kilometers per hour (220 miles per hour). After touchdown, the pilot deploys a drag chute from a compartment located just below the tail and the commander begins to drop the shuttle's nose gear slowly toward the runway. The drag chute is then jettisoned before the wheels come to a stop to ensure that it falls clear of the shuttle." [See source.]

The speed of sound is about 600 mph or so depending on the temperature and density of the air (the altitude). So there is no sonic boom when the space shuttle lands.

If you mean during the approach to landing, then, yes, there is a sonic boom but if and only if two things are present. First, the shuttle has to be going faster than the speed of sound and, second, there has to be air for the sound to travel in.

Thus, before the shuttle slows down by airspeed braking it's traveling maybe 17,000 mph or so...well above sound speed. But that's in an airless evironment; so there is no sound and there is no sonic boom. But while braking in the atmosphere and before the speed drops below the speed of sound, there will be a sonic boom during the approach, but well before landing. That boom can be heard on the ground, but if and only if the shock wave passes over the listener. And there is no shock wave near the landing because the shuttle is too slow by then.

Finally, there is only one shock wave, on the leading edge of the body producing it...in this case the shuttle. The wave occurs when the air molecules can't get out of the way fast enough; so the wave piles up in front of the leading edge. It's much like the bow wave appears in front of a speed boat going over the water.

If there are indeed two booms, there are two possibilities. One boom could be the direct shock wave and the other could be a reflection of that shock wave. Or the shock wave could have ripples, like bow waves frerquently do. In which case, the first boom could be the first ripple and the second boom could be the next ripple.

I doubt the leading edge of the shuttle's vertical tail would cause a second shock wave, which is what one answer suggests. I doubt this because I think the nose shock wave would shield the vertical tail from the undisturbed air molecules needed to form a second shockwave. As an ex-jet jock in the Navy, I've observed this shielding effect on the aircraft I flew.

But in any case, there is no sonic boom "when the space shuttle lands."