Communications blackout

Lore & Background

Communications blackouts during spacecraft re-entry, also known as radio blackouts, ionization blackouts, or reentry blackouts, are caused by an envelope of ionized air around the craft. This ionization is created by the heat from the compression of the atmosphere by the craft, and it interferes with radio signals. For the Mercury, Gemini, and Apollo spacecraft, such blackouts lasted for several minutes. Gemini 2 endured a blackout for four minutes, beginning at 9 minutes 5 seconds into the descent. Apollo missions experienced blackouts of approximately three minutes; for Apollo 16, the expected blackout was from 0 minutes 16 seconds after entry interface to 3 minutes 33 seconds after entry interface. The Apollo 13 mission had a much longer blackout—around 6 minutes—because its flight path was unexpectedly at a much shallower angle than normal, lasting 1 minute 27 seconds longer than predicted.

Communications blackouts are not confined to Earth's atmosphere; they apply to entry into any atmosphere where such ionization occurs. The Mars Pathfinder endured a 30-second blackout as it entered Mars' atmosphere, and the Huygens probe endured a blackout as it entered the atmosphere of Titan. Until the creation of the Tracking and Data Relay Satellite System (TDRSS), the Space Shuttle endured a 30-minute blackout. TDRSS allowed the Shuttle to communicate by relay with a satellite during re-entry through a 'hole' in the ionized air envelope at the tail end of the craft, created by the Shuttle's shape.

On Earth, radio blackouts caused by solar flares are measured by the National Oceanic and Atmospheric Administration on a scale from R1 (minor) to R5 (extreme). Communications can also be lost when the Sun is blocking or behind one station in the same line of sight; such Sun outages periodically interrupt communications with geosynchronous satellites and are a common problem for interplanetary space missions.

Reader's Guide

The significance of communications blackouts lies in their direct impact on the reliability of communications networks, particularly in aerospace and space operations. For spacecraft re-entry, the blackout period is a critical phase during which mission controllers lose contact with the vehicle, relying on pre-programmed systems and predictions. The Apollo 13 incident highlighted how deviations from expected flight paths can extend blackout durations, complicating recovery efforts. The development of TDRSS for the Space Shuttle demonstrated a technological workaround, using a relay satellite to maintain a link through a gap in the ionized envelope. On Earth, solar flare-induced blackouts are categorized by NOAA to help operators anticipate and mitigate disruptions. The phenomenon also affects satellite communications via Sun outages, which are predictable but unavoidable. Overall, understanding and managing communications blackouts is essential for ensuring uptime and safety in both terrestrial and space-based systems.

Did You Know?

• Communications blackouts during spacecraft re-entry are also called radio blackouts, ionization blackouts, or reentry blackouts.
• The Apollo 13 re-entry blackout lasted around 6 minutes, 1 minute 27 seconds longer than predicted, due to a shallower flight path.
• The Space Shuttle endured a 30-minute blackout until the Tracking and Data Relay Satellite System (TDRSS) allowed communication through a hole in the ionized air envelope.
• Solar flare radio blackouts on Earth are measured by NOAA on a scale from R1 (minor) to R5 (extreme).