The Return Margin

Fifty kilometres above Venus, the arithmetic is posted at six every morning.

One atmosphere of pressure. Nine-tenths of a gravity. It is the most Earth-like place off Earth, and nobody came, because you cannot plant a flag on a cloud.

Rae Oduya is the Margin Officer aboard the aerostat Hesper, and her whole job is a number: how many days the station can feed the living, and how many of the six of them the propellant can lift home. For two hundred and fourteen days the number has been comfortable. Then a lightning transient drops Hesper into the cloud base, where the acid runs three times as thick, and by morning it is not comfortable at all.

The window home opens once every five hundred and eighty-four days. A single seat costs two thousand and sixty-five kilograms of fuel they have to make out of the sky. And the number is falling faster than the machines can explain - which means somebody aboard has been adjusting it.

Rae publishes the figure at six o'clock every morning, without interpretation, because that was the rule she fought for on the ninth day. Soon she will publish one that decides which of her crew goes home.

A closed-room thriller in which the antagonist is a rate, the murder weapon is a calibration constant, and every figure in the book closes.

No faster-than-light travel. No aliens. No rescue. Just the numbers, and what they take out to balance.