# Are there enough stray gasses in space to justify streamlining a probe?

Space isn't empty - there are tons of things floating around in it. Seem of these things are pockets/molecule of gas - and a space probe is very likely to encounter some of these at some point in its lifespan.

Are there high enough concentrations of gasses to justify "streamlining" spacecraft? Are there any examples?

• – TildalWave Sep 10 '13 at 22:29

Let's just assume the interstellar density of about 10^6 particles/m^3. Let's also assume they are hydrogen. Plugging those numbers in to the drag formula $F_D\, =\, \tfrac12\, \rho\, v^2\, C_D\, A$, and let's assume that 1mN is the first force that is significant enough to merit correcting for. Putting all of that together, assuming a hideous coefficient of drag of 2, and an area of 100 m^2 (For solar panels), the appropriate velocity becomes 2,496,000, or almost 1% of light speed.

Bottom line, Aerodynamics only makes a difference if you are going really fast, much faster than anything we've sent so far.

• When colliding with gas at that speed I don't know if the drag formula applies... – Nick T Mar 3 '14 at 17:09
• 1 mN (millinewton) would be significant for a probe that spends years...maybe over 10 years...journeying through space. Probes usually weight around 1 ton, and assuming a constant 1 mN acted over 10 years makes a dv of 315.6 m/s. It might not sound like much but it's plenty to change the trajectory; the probe would miss its target. Did you mean micronewton instead? However, I agree that 2,496 km/s is pretty far outside the envelope of flight and has no chance of every seeing a probe come near it. – DrZ214 Jul 14 '15 at 22:25
• My numbers assume 1% light speed, which is so far above anything we have now, it's negligible. And it's a v^2 problem, so... – PearsonArtPhoto Jul 15 '15 at 2:14