Reentry communications blackouts
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 friction of the craft against the atmosphere. The ionized air interferes with radio signals. For the Mercury, Gemini, and Apollo spacecraft, such communications blackouts lasted for several minutes. Gemini 2, for example, endured such a blackout for four minutes, beginning at 9 minutes 5 seconds into the flight.
For Apollo missions, the communications blackout was approximately three minutes long. For Apollo 16, for example, pre-advisory data (PAD) for re-entry listed the expected times for re-entry communications blackout to be from 0 minutes 16 seconds after entry interface to 3 minutes 33 seconds after entry interface (a total of 3 minutes 17 seconds). For the Apollo 13 mission, the blackout was much longer than normal because the flight path of the spacecraft was unexpectedly at a much shallower angle than normal. According to the mission log maintained by Gene Kranz, the Apollo 13 re-entry blackout began at 142:39 and ended at 142:45, and was 1 minute 27 seconds longer than had been predicted.
Communications blackouts for re-entry are not solely confined to entry into Earth?s atmosphere. They apply to entry into any atmosphere where such ionization occurs around a craft. The Mars Pathfinder endured a 30 second communications blackout as it entered Mars? atmosphere, for example. The Huygens probe endured a communications blackout as it entered the atmosphere of Titan.
Until the creation of the Tracking and Data Relay Satellite System, the Space Shuttle would, like Gemini, Mercury, Apollo, and others, endure a 30 minute long communications blackout before landing. However, the Shuttle can communicate with a Tracking and Data Relay Satellite during re-entry. This is because the shape of the Shuttle creates a ?hole? in the ionized air envelope, at the tail end of the craft, through which it can communicate upwards to a satellite in orbit and thence to a ground station.
Space weather communications blackouts
Radio blackouts caused by space weather are measured by the National Oceanic and Atmospheric Administration on a scale that goes from 1 (minor) to 5 (extreme).
This section requires expansion.
References
^ a b c d Lucy Rogers (2008). It?s only rocket science: an introduction in plain English. Astronomers? universe: Springer eBooks collection. Springer. pp.159162. ISBN 0387753788.
^ David Shayler (2001). ?Gemini 2?. Gemini: steps to the moon. Springer-Praxis books in astronomy and space sciences. Springer. pp.183. ISBN 1852334053.
^ Neville Warren (2004). Excel HSC physics. Pascal Press. pp.23. ISBN 1741250773.
^ a b W. David Woods (2008). How Apollo Flew to the Moon. Springer Praxis Books in Space Exploration. Springer. pp.354,366. ISBN 0387716750.
^ Joe Pappalardo (2007-05-01). ?Did Ron Howard exaggerate the reentry scene in the movie Apollo 13??. Air & Space. Smithsonian Institution. http://airspacemag.com/need-to-know/NEED-Apollo13.html.
Further reading
Nikki Chandler (2003-03-01). ?SHUTTLE BLACKOUT MYTH PERSISTS?. Urgent Communications. Penton Media, Inc.. http://urgentcomm.com/mag/radio_shuttle_blackout_myth/.
?NOAA Space Weather Scale for Radio Blackouts?. NOAA / Space Weather Prediction Center. 2005-03-01. http://swpc.noaa.gov/NOAAscales/#RadioBlackouts.
See also
sudden ionospheric disturbance
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