Comment by aw1621107 | Hacker News Reader

Comment by aw1621107 | Hacker News Reader

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The Ars Technica article on this event [0] contains this interesting snippet:

>Probably the closest thing to [an internationally agreed upon boundary for "space"] comes from the World Air Sports Federation, or FAI, which uses 100km (the Karman line) to delineate the boundary of space for the purposes of establishing world records. However, this organization says it is looking at lowering this boundary from 100km to 80km, due to "Recently published analyses (that) present a compelling scientific case for reduction in this altitude."

>Much of the push for a lowering of the boundary has come because of work by Harvard University astrophysicist Jonathan McDowell, who has argued that orbiting objects can survive multiple perigees at altitudes around 80 to 90km and that this altitude range is consistent with the highest physical boundary of the atmosphere, the mesopause.

Does anyone here know more about this? How recent is this work? How controversial is it, if it is at all? This is the first I've heard of work along these lines.

[0]: https://arstechnica.com/science/2018/12/virgin-galactic-just...

I haven't heard of this, but it does sound logical.

The Mesosphere is defined as ending at an altitude of 85km. The next and last layer of the atmosphere is the Thermosphere, which end at 690km, but this is higher than the altitude at which most Shuttle missions orbited, for example.

(The "Exosphere" is above the Mesosphere, extending out to 10,000km, but gas particles in this layer are so far apart they do not behave as a "gas".)

So the most reasonable "boundary" of space would seem to be 85km.

As an aside: The lowest altitude at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 150km.

The "sky" does not turn completely black until 160km.

85km =~ 270,000 feet for those used to thinking in airline terms.

or 85000 meters or 52.8 miles

The defined boundary of space always reminds me of Jeannette Piccard, the first woman in space (as defined in 1934).

https://en.wikipedia.org/wiki/Jeannette_Piccard

It's interesting where we define what is an isn't space, with its own cultural ramifications.

It’s hard to argue that you’ve left the atmosphere, when the craft you’re using is supported by it. Hence the Karman Line.

Tereshkova has a much better claim to being the first woman in space.

You can reach the surface of a lake in a craft that floats on water so you should be able to reach the end of the atmosphere in a craft that floats on it.

The surface of a lake is much more well-defined, though, and has high density. You can't "float" on whatever you define as the "surface" of the atmosphere.

Jeanette Piccard and her husband Jean, another high-altitude ballooning pioneer, are also suggested to be ancestors of one Jean-Luc Picard, captain of the USS Enterprise.

I've always explained the boundary as the point at which, in order to use wings to provide enough lift to keep you in the sky, you have to be travelling at orbital speeds.

As the air gets thinner you have to travel faster in order to displace enough air to keep you aloft, and there comes a point where you're in orbit anyway, so the lift isn't needed.

Different planes have different lifting surfaces, so I have always wondered if the line was set more or less arbitrarily using some 'standard' plane, or if all planes approach the same limit together.

Looks like there are some interesting things to read here!

> The edge of space: Revisiting the Karman Line

> Jonathan C. McDowell

> In this paper I revisit proposed definitions of the boundary between the Earth's atmosphere and outer space, considering orbital and suborbital trajectories used by space vehicles. In particular, I investigate the inner edge of outer space from historical, physical and technological viewpoints and propose 80 kilometers as a more appropriate boundary than the currently popular 100 km Von Kármán line.

https://arxiv.org/abs/1807.07894

Yep, the Karman line is 100km. Only astronauts and X-15 pilots who flew above this heigh were deemed to have flown into space.

Virgin and Branson are very good at marketing and PR, so they probably chose the lower definition to say they had reached space.

The US gave astronaut wings to anyone who flew above 50 miles (about 80 km), not 100 km:

https://en.wikipedia.org/wiki/United_States_Astronaut_Badge#...

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