Decision to rescue Rocky

The decision to rescue Rocky is the moment Ryland Grace knowingly trades his own life for his friend's, and it is the hinge on which the entire second half of the mission turns. Having just discovered that the Taumoeba-82.5 he bred for nitrogen resistance can also worm its way through xenonite—the material of Rocky's fuel tanks—Grace realizes that the Blip-A is almost certainly dead in space, its Astrophage consumed. He now faces a stark binary: continue home to Earth, where he will be hailed as a hero and live, or turn back to find Rocky, knowing he does not have enough food to survive the round trip. He chooses the latter, launching the four beetles toward Earth with the Taumoeba data and reversing course toward the derelict Blip-A.

The Discovery of the Xenonite Vulnerability

Grace's realization comes in the lab, after he performs a controlled experiment. He sandwiches Astrophage between two xenonite discs and exposes the assembly to Taumoeba-82.5; within hours the Astrophage is gone, while identical samples sealed in plastic remain untouched. The natural Taumoeba cannot penetrate xenonite, but the nitrogen-resistant strain he bred has evolved the ability to hide from nitrogen by working its way into the xenonite's molecular structure. "I bred up a bunch of Taumoeba that evolved to survive…in xenonite breeder tanks," he mutters, slapping his forehead. The implication is immediate and catastrophic: Rocky's ship, the Blip-A, stores its fuel in xenonite compartments, and the same Taumoeba-82.5 that Grace gave him is now eating his fuel from the inside.

The Calculus of Sacrifice

Grace runs the numbers in the control room, floating in zero g. He has enough food—a mix of real meals and coma slurry—to reach Earth, but not enough to go to Erid and back. If he turns the Hail Mary around, finds Rocky, and takes him home to 40 Eridani, he will starve to death shortly after arrival. The alternative is to launch the beetles, each carrying a sealed steel mini-farm of Taumoeba and a USB stick of data, and continue on his original course. "Option 1: Go home a hero and save all of humanity. Option 2: Go to Erid, save an alien species, and starve to death shortly after," he summarizes. He pulls on his hair and sobs into his hands, but the image that stays with him is Rocky's carapace and his little arms always fidgeting with something.

The Launch of the Beetles

Before reversing course, Grace prepares the four beetles—John, Paul, George, and Ringo—for their solo journey to Earth. He installs the steel mini-farms on each, epoxies them to the undercarriage, and adds counterweights to keep the guidance system happy. The beetles carry not only the Taumoeba but also a complete record of his findings, including the data on how to breed nitrogen-resistant predators. He fires them off toward Earth, watching their tiny Petrova-frequency flares dwindle on the Petrovascope. "I know you'd love a beetle to take apart, but I couldn't spare one," he says to the absent Rocky. The beetles will reach Earth in about thirteen years from Earth's perspective; the Hail Mary will not.

The Search and the Rescue

With the beetles gone, Grace turns the Hail Mary around and begins a painstaking search for the Blip-A. He knows Rocky's course and velocity, but not the exact moment the engines died, giving him a margin of error of 20 million kilometers. He devises a crude radar: firing the spin drives in a tight circle and watching for reflections on the Petrovascope. After hours of sweeping through different planes, he catches a flash, triangulates the position, and closes the distance. When he finally matches velocities and sees the Blip-A on radar, he suits up for an EVA, crosses the gap, and bangs on the hull with a wrench. Rocky's voice crackles over the radio, high-pitched with joy: "You are here!" Grace tells him to set up the tunnel; he is taking him to Erid. Rocky protests that Grace will die, but Grace has already made his peace with that. "I made my decision," he says. "This is the only way to save both of our worlds."