What are the differences between a sounding rocket and a suborbital flight?

Sounding rockets have long been used for science purposes. Some of them have trajectories elliptic enough to do microgravity research during a few minutes. Sub-orbital is a term which mostly seems to be used when talking about crewed flights. Is the difference simply that sub-orbital flights are optimized for carrying a crew? Are sounding rockets too rough or small for carrying humans?

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    $\begingroup$ Sounding rockets are, so far, more useful. This isn't meant to be flippant, even the name "sounding" tells us that it has a scientific objective. Until we hear of the justifying objective of sub-orbital flight being humanitarian or even merely practical, all the present commercial ventures have is to make a profit from entertaining the rich. $\endgroup$
    – Puffin
    Commented Mar 22, 2016 at 12:49
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    $\begingroup$ @Puffin Old post, I realize. While the statement "Sounding rockets are ... more useful" is subjective, +1 to you (& Absalom below) for alluding to the fact that "sounding" means "to measure" $\endgroup$ Commented Apr 24, 2018 at 17:42

4 Answers 4


If the sounding rockets go past the Kármán line at 100 kilometres (62 mi) above the mean sea-level that's now officially the edge of space since the Outer Space Treaty sort of ratified that, there is no difference, and they're all sub-orbital spaceflight. Meaning, they don't reach required orbital velocity (around 8 km/s) to stay in orbit and their flight is on a ballistic trajectory. If they don't even reach the boundary of outer space, then they don't really qualify as spaceflight and are just missiles.

So while suborbital spaceflight and sounding rockets aren't really directly comparable, and the former defines the speed and achievable trajectory and the latter identifies its purpose and often also size with usually small payloads for short-duration microgravity and/or upper atmosphere / ionosphere environment experiments, most sounding rockets are indeed suborbital. But it's not necessarily the other way around and not all suborbital spaceflights are sounding rockets, they could for example also be manned suborbital spaceflights, like say certain space tourism vehicles or point-to-point suborbital transportation routes.

  • $\begingroup$ The "missile" Wikipedia page you link to defines a missile as a type of weapon. So it's probably not the best link to explain what you mean here, especially since you also say the term "sounding rocket" implies a scientific purpose. $\endgroup$ Commented Aug 7, 2020 at 14:43

Some sounding rockets are sub-orbital. Sub-orbital merely means going into space with less than orbital velocity. Going into space: That's a bit arbitrary, but a person who goes 50 miles or higher (~80km) gets astronaut wings; another definition is the nice round figure of 100 kilometers (~62 miles) in altitude; that's the Kármán line. Yet another is the nice round figure of 400,000 feet (~122 km; ~76 miles); that's "entry interface" for NASA vehicles that reenter the atmosphere.

By whatever definition one uses, some sounding rocket flights are less than sub-orbital. Sounding rockets sent into the stratosphere may top out at much less than 50 miles altitude. A balloon is cheaper than a rocket, but balloons can only go to an altitude of 31 miles / 50 kilometers or so. If your experiment needs to go higher than that you need to use a sounding rocket.

As far as acceleration is concerned, sounding rockets go whoosh! and they're gone. However, their payload capacities are rather small. They aren't intended for carrying massive people and their even more massive environmental control and life support systems.

See the NASA Sounding Rocket Handbook for more info on sounding rockets and their capabilities.

  • $\begingroup$ Very few of the things you describe are inherent to sounding rockets per se, rather that is how they have been to date. I.e. I do not think it is a definitional thing, rather an experiential thing. As you suggest, it is kind of a wishy washy definition. $\endgroup$
    – geoffc
    Commented Mar 11, 2014 at 13:34
  • $\begingroup$ "Sub-orbital merely means going into space with less than orbital velocity" in the plane normal the gravitational gradient $\endgroup$ Commented Aug 8, 2020 at 18:47
  • $\begingroup$ I think it's not "50 miles or higher" but one must be higher than 50 mi to become an astronaut. So it's "above 50 miles". If you reached an apogee of exactly 50 mi you're officially still in the atmosphere. If you reached 50.1 mi you're an astronaut. Or something. $\endgroup$
    – Giovanni
    Commented Aug 10, 2020 at 17:26

A sounding rocket is, technically, a rocket sent up for the purpose of taking measurements, or collecting data. The term 'sounding' is a US naval term for data measurement, and the term 'sounding rocket' began as a differentiator between upper atmospheric data collection missions, orbital insertion missions and human crewed missions. It is not, however, an exclusive term, and a rocket could, potentially, be used for more than one mission type, perhaps both delivering a payload into orbit and also taking measurements of various types while in flight. Sounding rockets tend to be, but are not exclusively, often smaller than orbital insertion missions, and often utilize repurposed military rocket engines. Sounding rocket missions are highly valuable to scientific inquiry, but are not usually 'exciting' to the general public, and therefore frequently gain less attention.


The etymology of “sounding” is informative. https://en.wiktionary.org/wiki/sound#Verb . It comes from Old English “sund”, swimming or sea.

Hence “Long Island Sound” for the body of water or “the whale sounded” for a diving whale.

“Taking soundings” for measuring water depth with a lead line came from releasing the lead to “swim”.

In the case of the OP, “sounding” is closest to the last usage, swimming for the purpose of measurement.

So the difference between sounding rockets and suborbital rockets is purpose, not hardware. Think of a baseball bat. Is it sporting equipment or a lethal weapon? It depends on who is holding it: Babe Ruth or Al Capone.


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