A distress call was any signal that requested assistance. It was typically sent by an individual or group that encountered a problem they could not handle on their own. The purpose of the signal was to summon those who could help.
A widely known distress signal was "SOS", which dates back to the early 20th century, when broadcast signaling was still a new science. An international convention met to debate and select a "universal" distress call, and "SOS" was selected by most. It did not truly become universal until after the year 1912, when the RMS Titanic, an ancient Earth water-going vessel, sank after striking an iceberg. Her radio operators were forced to send several types of distress signal to ensure the meaning would not be lost.
In the 23rd and 24th century, starships, starbases, and colonies could emit distress signals which were carried in space or through subspace to anything which could receive it. This could include other ships, bases, and colonies.
Because of tense intergalactic relations, distress signals between races may or may not have received a response. In several cases, a Federation ship crossed into, for example, the Neutral Zone, to answer a distress signal. However, extreme care was taken in these situations, as some enemies would sometimes emit false distress signals to lure starships in and attack them. (TNG: "Suddenly Human")
When passing near Mab-Bu VI in 2368, the USS Enterprise-D received a faint distress signal that Data was later able to identify as a Starfleet subspace distress signal standard to Daedalus-class starships. The signal was sent by the crew of the USS Essex, which was lost in the sector over two hundred years earlier. (TNG: "Power Play")
The Federation planetary distress signal was an emergency transmission that was to be used only when a Federation world was under imminent threat. Prior to transmission of a planetary distress signal, the head of council for the Federation planet needed to declare a planet-wide state of emergency.
The planetary distress signal was first known to have been transmitted in 2286, when a massive alien probe of unknown origin arrived in the Sol system, and began transmitting a signal that had a disruptive effect on power systems, and began ionizing the planet's atmosphere. All Earth orbiting space stations and starships were quickly rendered powerless, along with the planet's power generating systems. Within two hours, the president of the Federation Council declared a state of emergency. Massive weather systems caused by the drop in air pressure caused widespread damage across the planet.
Six hours into the crisis, the Federation president transmitted a planetary distress signal. The following is a transcript of the message:
- "This is the President of the United Federation of Planets. Do not approach Earth. The transmissions of an orbiting probe are causing critical damage to this planet. It has almost totally ionized our atmosphere. All power sources have failed. All Earth-orbiting starships are powerless. The probe is vaporizing our oceans. We cannot survive unless a way can be found to respond to the probe. Further communications may not be possible. Save your energy. Save yourselves. Avoid the planet Earth at all costs. Farewell..."
The crisis was relieved by a group of Starfleet officers who were returning from Vulcan. One of the officers analyzed the transmissions of the probe and determined the transmissions were directed to an extinct species of humpback whales. The officers managed to retrieve a pair of the whales and delivered them to Earth. The probe shut down its transmissions and left the system. (Star Trek IV: The Voyage Home)
Another example of a planetary distress call is that which was transmitted by the Scalosians to attract starships to their planet so they could use the crew of the ship to repopulate their planet. The following is a transcript of part of the message:
- "To any and all space travelers passing within range of the planet Scalos, I send you an urgent appeal for help. My comrades and I are the last surviving member of what was once a thriving civilization. Those of us who are left have taken shelter in this area. We have no explanation for what has been happening to us. Our number is now five, we were once a nation of 900,000, This city alone holding a population of 113,477, our planet Scalos on which several enlightened cultures indigenous populations." (TOS: "Wink of an Eye")
In 2368 the USS Enterprise-D received a priority one distress call from the Federation colony on Dulisian IV, which requested an evacuation because of a massive failure of the environmental support systems. The crew later learned that this distress call was a faked one from the Romulans to move the Enterprise-D away from the Vulcan ships. (TNG: "Unification II")
In 2369 the Enterprise-D received another distress call via their long range sensors from a Romulan warbird. Worf informed Commander Riker about this call while he was treated in sickbay. (TNG: "Timescape")
In 2371, Gathorel Labin transmitted a distress call, which was picked up by the USS Voyager. Upon arrival, it was found that Labin only transmitted the distress call because he sensed the crew of Voyager was in distress. (VOY: "Prime Factors")
In 2372, Captain Kasidy Yates sent a priority one distress call to Deep Space 9 when the Xhosa was attacked by a Klingon Bird-of-Prey. The message was interrupted and Captain Sisko manned the USS Defiant to rescue the Xhosa. (DS9: "The Way of the Warrior")