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There are a lot of variables with the speed of sound, a sonic boom is created when all of those variables meet.

the speed of sound, a critical speed known as Mach 1, and is approximately 1,225 km/h (761 mph) at sea level and 20 °C (68 °F).

Sonic boom

During reentry the Space Shuttle is moving at several times the speed of sound

The orbiter's maximum glide ratio/lift-to-drag ratio varies considerably with speed, ranging from 1:1 at hypersonic speeds, 2:1 at supersonic speeds and reaching 4.5:1 at subsonic speeds during approach and landing

Re-entry and landing

There is no sound in space, so presumably there has to be a point when speed and air density combine to create a sonic boom. When does an aerobraking space craft create a sonic boom?

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  • $\begingroup$ Here's a possible reference (Apollo 16, but with not many ground measurements): ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740023384.pdf $\endgroup$
    – Andy
    Commented Jul 7, 2015 at 13:13
  • $\begingroup$ I am not sure that "there is no sound in space" is accurate enough for this question. There is athmospheric drag on the ISS I would expect that sound waves can propagate at those altitudes as well. I guess that the extreme low pressure means that the sound energy in those waves are very low though. So I would expect the space shuttle to generate a sonic boom during already while in orbit. I don't have the math to back this up though. $\endgroup$
    – Taemyr
    Commented Jul 7, 2015 at 13:19
  • $\begingroup$ I searched for a table of mach speed per altitude but couldn't find any data above 122km where mach 1 is 1108km/h. $\endgroup$
    – Antzi
    Commented Jul 7, 2015 at 15:00
  • $\begingroup$ When you say sonic boom, do you mean shock wave? "In space there is no one to hear you boom". $\endgroup$
    – Organic Marble
    Commented Jul 7, 2015 at 15:44
  • $\begingroup$ @OrganicMarble can you have a shock wave if there is insufficient air molecules for a sound? (I am assuming in LEO and lower all the molecules heavier than air don't stay long enough to provide a medium for a shock wave.) $\endgroup$
    – James Jenkins
    Commented Jul 7, 2015 at 15:57

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How many grains of sand does it take to form a heap?

An orbiting spacecraft is flying many times faster than the speed of sound. It's starting in atmosphere too thin to sustain an audible shock wave. As it descends, it's going to be producing a shock cone continuously, but in the very thin atmosphere high up, the amplitude of the shock wave is too faint to hear.

At some point in the descent, the air would be thick enough for you to barely hear it if it were close by -- the tiniest sonic boom.

At some lower point, it would be audible even at a distance.

Low enough, the shock wave is occurring in air dense enough that it's loud enough it could rattle windows.

Because the speed of sound is lower at high altitude -- and doesn't change all that much along the way -- the sonic boom would be continuous all the way down, until the craft went subsonic at relatively low altitude.

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  • $\begingroup$ Related At what altitude would the air be too thin to carry a sound wave? $\endgroup$
    – James Jenkins
    Commented Jul 7, 2015 at 17:23
  • $\begingroup$ Following the trail of questions, the mean free path argument is interesting, but I think that even if the air can't support an acoustic oscillation, whatever ear or microphone the shock wave hits can do so, so it's still, sort of, "sound". $\endgroup$
    – Russell Borogove
    Commented Jul 7, 2015 at 17:39
  • $\begingroup$ There's something inherently hilarious about the phrase "the tiniest sonic boom". +1 for argument from continuity, though. $\endgroup$
    – Nathan Tuggy
    Commented Jul 8, 2015 at 15:26
  • $\begingroup$ @Russell, if the air can't support an acoustic oscillation, there is no way to transport acoustic energy to the ear or microphone. $\endgroup$
    – Hobbes
    Commented Jul 11, 2015 at 11:31
  • $\begingroup$ I don't know much about low-pressure gas behavior, but wouldn't gas molecules still be knocked away from the spacecraft, moving freely, carrying (extremely small) amounts of energy w/o oscillation? $\endgroup$
    – Russell Borogove
    Commented Jul 11, 2015 at 15:48
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According to this page, an SR-71 flying supersonic at 80,000 ft (24.3 km) generates an audible sonic boom, so that gives us a lower bound.
This page explains why you hear two separate booms (video) when the Shuttle comes in for a landing.
And this article states the sonic boom is audible for the last 10 minutes of a Shuttle flight. Coupling that with the Shuttle's landing profile should give us a good answer, but I haven't found that yet.

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I found an answer here: http://www.mach25media.com/history1.html

Thus Mach 25 is the velocity that a spacecraft is flying at the moment it reenters the atmosphere of Earth (entry interface). This interface occurs at approximately 75 miles (400,000 feet) when a vehicle such as the Space Shuttle orbiter has a speed of about 17,200 miles per hour. At that altitude, the speed of sound is generally around 688 miles per hour

That's 121 920m high and 27680km/h, little bit under mach 25.

Boom.

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  • $\begingroup$ I don't think this is correct. Most calculators I found only go to 86000 m, where the temp is around -40C, this calculator shows a speed of sound around 243.010 M/S (543 MPH) it seems that higher and colder = slower speed of sound. $\endgroup$
    – James Jenkins
    Commented Jul 7, 2015 at 16:11
  • $\begingroup$ @JamesJenkins I was surprised too, but this website seems to coroborate: aerospaceweb.org/question/atmosphere/q0112.shtml However this could be consistent with temperatures change: en.wikipedia.org/wiki/… TL;DnR: Temperature does not decrease linearly with altitude. $\endgroup$
    – Antzi
    Commented Jul 7, 2015 at 16:13
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There may be a sonic boom, yet you might be unable to hear it.

When a space craft enters the atmosphere, at some point the surrounding gas is dense enough to behave as a gas, a not mere individual molecules and atoms.
As such a sonic boom is created by the re-entering space craft, the shock waves expand and meet all the various layers and conditions of the atmosphere.

The shock waves will get refracted, reflected and absorbed by the atmosphere in varying degrees depending a the conditions. I guess there is a good chance, that a high-altitude sonic boom is reflected away by the denser atmosphere below.

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As I understand it, a reentering spacecraft wouldn't make a sonic boom in the traditional sense. It is decelerating. A fighter jet, for example, only makes a sonic boom as it accelerates past mach 1. A decelerating spacecraft would create a lot of noise and turbulence, but no boom.

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  • $\begingroup$ That's what I was going to say. A sonic boom happens specifically because the velocity of the ship equals the velocity of the soundwaves. Traveling faster than the speed of sound doesn't create a sustained sonic boom, though presumably slowing down to the speed of and below the speed of sound might. $\endgroup$
    – userLTK
    Commented Jul 11, 2015 at 7:11
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    $\begingroup$ Aircraft etc. make a sonic boom as long as they are above the speed of sound. The Shuttle, for instance definitely makes a sonic boom when coming in to land: youtube.com/watch?v=f6r8wU2tDrc $\endgroup$
    – Hobbes
    Commented Jul 11, 2015 at 8:20
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    $\begingroup$ I'm sorry but you are incorrect. en.wikipedia.org/wiki/Sonic_boom When an aircraft moves faster than the speed of sound, the air that is displaced by the aircraft cannot move out of the way fast enough, so you get a uniform front of air moving at the highest speed it can reach, i.e. the speed of sound. This is what causes the boom. This is why e.g. Concorde was banned from overflying land at supersonic speeds: a flight from New York to LA would have treated everyone on the entire flightpath to a sonic boom, not just the people within earshot as it accelerated past Mach 1. $\endgroup$
    – Hobbes
    Commented Jul 11, 2015 at 10:53
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    $\begingroup$ That's incorrect. A sonic boom is experienced when a shock cone passes over an observer. Accelerating, decelerating, steady state, does not matter. $\endgroup$
    – Organic Marble
    Commented Jul 11, 2015 at 16:32
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    $\begingroup$ @OrganicMarble would that mean an observer travelling with the shock cone would experience a constant sonic boom? Neat, didn't think about a sonic boom only being an instantaneous "boom" because the observer only interacts with the shockwave cone once. Physics really is relative huh. $\endgroup$
    – Magic Octopus Urn
    Commented Sep 10, 2018 at 18:17

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