Minor Leak Being Investigated Aboard the ISS

Josh Fink writes "Space.com is reporting that the International Space Station has a minor atmosphere leak. 'An inspection of a vestibule bridging the station's new Harmony connecting module and NASA's Destiny laboratory indicated a slight air leak of about three pounds (1.3 kilograms) per day ..A close-up inspection of the vestibule seal by the station's three-astronaut Expedition 16 crew using an ultrasonic leak detector found no trace of a leak on Wednesday, [NASA spokesperson Lynette Madison] said. Studies of the station's overall internal pressure also found no signs of decay, she added.' While this is yet another technical issue with the ISS, when will this end? I am all for the space program, but there have been some major issues lately."

Comments

pin sized hole hard to reach

[wizardforce]

While this is yet another technical issue with the ISS, when will this end? I am all for the space program, but there have been some major issues lately."

it's probably a leak about the size of a needle hole hidden somewhere out of reach with a simple leak test. it wouldn't need much, 1.3 kg of air is about 1 cubic meter in size leaking over a day's time. considering there's about 10N/cm^2 force and the force exerted by a moving column of air is mv^2 while the density m is 1.28 kg/m^3 solving for the size of a hole required to vent the gas is about 2mm in diameter if back of the envelope calculations are correct [probably not but you get my point]

Re:pin sized hole hard to reach

[kebes]

Sounds about right. I just ran it through some simple effusion equations (kinetic gas laws). Assuming that the amount of air escaping is 1.3 kg (1.14 m^3), and that the volume of the room it is escaping from is ~200 m^3 (apparently the total final size of the ISS is 1000 m^3), and that the ISS is pressurized to 101.3 kPa (the Wikipedia article says that it is), then we can calculate the time for 1/200 of the air (0.5%) to escape, as a function of the hole diameter. It turns out that a hole of diameter 0.15 mm will lead to that kind of rate of pressure loss (1 m^3 in the first day).

Needles to say, the effusion equations have various assumptions built into them, and I had to make all kinds of assumptions about the values... but at least to within order-of-magnitude, this suggests a pinhole-sized leak.


Details for anyone who cares: The effusion equation can be derived similar to the conventional gas law expressions, by calculating the number of molecules per unit area that impinge on a wall section of a certain size (the hole). (We assume a container in vacuum, so that any molecule that impinges on the hole is lost irreversibly to the outside.) The equation, as you might expect, turn out to be exponential decays (since the derivation incorporates the decreasing internal pressure as air is lost):

N(t)/N_total = exp( -(A/V)*sqrt(k*T/2*pi*m)*t )

or

t = ( -(V/A)*sqrt(2*pi*m/k*T) )*ln(N(t)/N_total)

where:
t, time (until the given loss of atmosphere)
V, volume of container
A, surface area of hole
m, mass of gas molecules
T, temperature (~300 K for room temperature)
k, Boltzmann constant
N(t), # molecules at time t
N_total, total # molecules (initial quantity)

Pound != 0.454 Kilogram on the ISS

[corsec67]

FTA: "a slight air leak of about three pounds (1.3 kilograms) per day".
I hate to break it to this reporter, but on the ISS, a pound is a large number of kilograms, since they are in microgravity. Pound is a unit of weight, and gram is a unit of mass. The conversion between them depends on the gravity that the object is experiencing, which in this case is almost none, so the 1.3 kilograms of air is almost 0 pounds.

Re:Pound != 0.454 Kilogram on the ISS

[turgid]

Pound is a unit of weight, and gram is a unit of mass.

My dad, who is from the Olden Days when people used pounds and inches, and an Engineer, says that there exists a "pound-mass" and a "pound-force" and the reader is expected to have the wit, depending on context, to distinguish between them.

Re:Mod parent up!

[MichaelSmith]

Thats pounds mass. Gas quantities were measured that way all through the Apollo program.