Conservation

The people of Earth had every means of power at their command, yet they used none of it. Was it due to lack of knowledge and technique; or was there a more subtle, dangerous reason?
The yellow sands of the spaceport stretched, glaring and empty, in every direction. There was no sign of life from the little group of buildings a mile away.
In the control room of the tall, round-nosed starship, technicians labored and officers conferred while the red needles that showed rocket tube temperatures sank slowly toward zero on their dials.
Maybe Earth's depopulated, Tom, suggested John Gray, the executive officer. He ran his fingers through close-cropped red hair and peered through the port with thoughtful gray eyes.
Hardly, John, replied Commander Tom Wallace, frowning. The scout rockets showed some good-sized cities, with smoke.
I was off duty then and haven't had time to read the log, apologized John. What gets me is that they should have a robot-controlled space relay station orbiting outside the atmosphere, and a deserted spaceport. It just doesn't jibe.
That's why we have to be just as careful as though we were landing on an alien planet, said the commander. We don't know what the conditions on Earth are now. How long has it been, John?
Two hundred and fifty-eight years, answered John. Ten years, our time.
Pick three for briefing, John. This is going to be a disappointing homecoming for the crew, but we'll have to send out an exploration party.
The landing ramp slid out from just above the rocket tubes, and the armored car clanked down to the sand. John steered it across the wide expanse of the spaceport toward the group of buildings. Above and behind him, a woman swept the terrain with binoculars from the car's observation turret. In the body of the car, another woman and a man stood by the guns.
The buildings were just as lifeless when they drew near, but there was an ominous atmosphere about them. They were windowless, of heavy concrete. Through slits in their domed roofs, the noses of a dozen cannon angled toward the ship.