What Causes Radio Blackouts During Space Capsule Descent?

[Dr Wu]

What causes radio blackouts to occur when, for example, a manned space capsule is descending through the Earth's atmosphere? If the atmosphere itself is in some way responsible for these blackouts (as surely it must be)* then how exactly? NB. Surely it can't be due to the ionosphere. I suggest this because footage of Apollo 13 reveal that its crew were in a state of radio incommunicado with Mission Control during the latter stages of their descent to Earth - that is, until their capsule was seen to emerge from the base of what appears to have been a fairly low cloudbank. Somehow, I can't quite believe the ironosphere extends to such depths. Just an observation.

* I don't recall this sort of thing happening during the actual Moon-landings, for example.

 

[phinds]

I think it's the red-hot plasma created by the friction heat during reentry.

 

[Vanadium 50]

phinds is right. If you Google "radio blackout during reentry", you get this page: https://en.wikipedia.org/?title=Communications_blackout which has a whole section on it, beginning:

The communications blackouts that affect spacecraft re-entering the Earth's atmosphere, which are also known as radio blackouts, ionization blackouts, or reentry blackouts, are caused by an envelope of ionized air around the craft, created by the heat from the compression of the atmosphere by the craft. The ionized air interferes with radio signals. For the Mercury, Gemini, and Apollo spacecraft , such communications blackouts lasted for several minutes.[1] Gemini 2, for example, endured such a blackout for four minutes, beginning at 9 minutes 5 seconds into the flight.[2]

 

[russ_watters]

What causes radio blackouts to occur when, for example, a manned space capsule is descending through the Earth's atmosphere? If the atmosphere itself is in some way responsible for these blackouts (as surely it must be)* then how exactly? NB. Surely it can't be due to the ionosphere. I suggest this because footage of Apollo 13 reveal that its crew were in a state of radio incommunicado with Mission Control during the latter stages of their descent to Earth - that is, until their capsule was seen to emerge from the base of what appears to have been a fairly low cloudbank. Somehow, I can't quite believe the ironosphere extends to such depths. Just an observation.

* I don't recall this sort of thing happening during the actual Moon-landings, for example.

When you say the "footage" of Apolo 13, are you referring to the Hollywood movie? That movie is not a documentary, it is a dramatization and fine details are not necessarily accurate. Here's the full explanation:
http://www.universetoday.com/119921...longer-than-expected-communications-blackout/

During the Apollo era, the radio blackout was a normal part of reentry. It was caused by ionized air surrounding the command module during its superheated reentry through the atmosphere, which interfered with radio waves... [so phinds is correct]

It is difficult to find official NASA documentation about the extended radio blackout time for Apollo 13. In the mission’s Accident Review Board Report, there’s no mention of this anomaly...

Jim Lovell gave the most detailed response – which is the one most often given as a likely explanation — suggesting it perhaps had to do with a shallowing reentry angle problem, with a strange space-like breeze that seemed to be blowing the spacecraft off-course with respect to entry.

“I think the reason why it was longer was the fact we were coming in shallower than we had planned,”

Note also that better communications equipment has eliminated the blackout for modern spacecraft .

 

[Dr Wu]

Russ: rather than the Hollywood movie, I was referring to the actual 'live' footage of the Apollo 13 descent, filmed (if my memory serves me well) onboard a US aircraft carrier somewhere in the Pacific. Yes, this scene is also included in the movie, as I recall. Nevertheless, I should have made the distinction clearer.

So it appears then that the radio blackout problem is now largely eliminated. If so, and wearing my SF hat for the moment, does this mean then that an astronaut onboard a decelerating spacecraft , powered (say) by a fusion drive, would be able to direct a radio (or laser?) signal through the plasma exhaust to a given destination located further astern the spacecraft ? Or is this still a speculation too far?

 

[berkeman]

Note also that better communications equipment has eliminated the blackout for modern spacecraft .

What kind of improvements? I always wondered why they didn't just trail a long wire behind them with an antenna at the far end to get it out of the main ionization envelope, but it sounds like they've done something else to maintain comms. Maybe relaying through a satellite overhead?

 

[davenn]

got some links etc for that Russ ?

I am also curious

Dave

 

[Dotini]

Solar particle events also cause radio blackouts.
https://en.wikipedia.org/wiki/Solar_particle_event

 

[D H]

Note also that better communications equipment has eliminated the blackout for modern spacecraft .

That only applies to the Shuttle, which no longer flies. Russia's Soyuz, SpaceX's Dragon, Boeing's CST-100, and NASA's Orion are (or will be) capsules, and they all suffer (or will suffer) communications blackout during reentry.

What kind of improvements?

Thanks to the Shuttle's shape and reentry profile, parts of the Shuttle's tail could "see" the sky in high frequency communications bands. The TDRSS satellites provided the ability to communicate during what would otherwise be a very long (~25 minutes) communications blackout. Capsules can't do that. All they see during reentry is a hot sheath of plasma that completely surrounds the vehicle.

I always wondered why they didn't just trail a long wire behind them with an antenna at the far end to get it out of the main ionization envelope, but it sounds like they've done something else to maintain comms.

Wire has a nasty tendency to become non-wire when heated to 5000 to 20000 degrees.

 

[Dotini]

Advanced antenna design pondered.
http://www.gizmag.com/communicating-with-hypersonic-vehicles-in-flight/38066/