Q:  What is the purpose of Taylor ordering Landon to send a last signal to Earth?

A:    People ask me if it is because the "Icarus" is equipped with some type of space-time communication system that would allow the crew to talk to Earth in the past. 

No.

This is a good question but so few people seem to think it through to its logical end. 

Here is the scene that we are referring to:

Taylor stumbles down the aisle of the rolling ship toward the console and addresses Landon, who is still staring at Stewart's skeleton.

TAYLOR- "Landon! Send a last signal."

LANDON (dazed)- "What signal?"

TAYLOR- "To Earth! That we've landed!"

Landon lurches toward the communications equipment.

Radio propagates at the speed of light throughout the universe, a constant speed, so assuming that the Icarus has crashed on a habitable planet in the Alpha Centauri system then it will take at least 5 years for the signal to reach Earth (and another five years for a rescue mission from Earth to reach the stranded astronauts).  When Taylor gives this order, he has not yet read the clocks in the ship so he is still under the assumption that he and his surviving crew members are still within the local time frame of their mission (i.e. that the real year of their emergency is probably in the late 1970's Earth time).

Once Taylor reads the clocks and discovers that they are not a few years from Earth but rather 2006 years in the future, long after their departure from Earth, then getting a message to Earth becomes meaningless and irrelevant.  Taylor gives the order to send a transmission to Earth because Taylor honestly believes that they have crashed somewhere near Alpha Centauri and that there might be hope yet for a rescue mission to be mounted to retrieve them.  Once he reads the ship's chronometer and discovers just how much time has passed then all hope of a rescue by Earth forces becomes nothing more than wishful thinking and he alone exits the sinking ship with this final knowledge.

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