Choosing a trade-off between half-life and radioactive mass would be a fun engineering task. Plutonium-238 has a half-life of 87.7 years and would be long deple

Choosing a trade-off between half-life and radioactive mass would be a fun engineering task. Plutonium-238 has a half-life of 87.7 years and would be long depleted before 55 million years are up. Longer term isotopes would need a ton of mass to get you there.