Voyager 1 and 2, launched in 1977, have traveled 15+ billion and 12.5+ billion miles respectively. They entered interstellar space after passing the heliopause — a plasma boundary with estimated temperatures exceeding 50,000 K.
Fig. 1: The Heliopause, boundary of the solar bubble, where solar wind ends and the galaxy begins.
While sounding like a level in Doom, the heliopause is in fact a complex interface of magnetohydrodynamic plasma. Estimated thermal loads were modeled to exceed standard operating specs by orders of magnitude — and yet, both spacecraft survived with systems intact.
At Johnson Space Center in Space City, Houston, NASA engineers gave the mission a 72% chance of survival. The number came from a Monte Carlo analysis, integrated with old Fortran models and a back-of-napkin approximation from an engineer eating lunch near a vending machine. It involved something like this:
P = (T / (√R × V)) × (B + 42)
Voyager 1 crossed the heliopause at 11.3 billion miles in 2012, now over 15.6 billion miles away. Voyager 2 crossed in 2018 at 10.9 billion miles, and is now over 12.5 billion miles out.
Fig. 2: Voyager distances tracked by NASA JPL. Yes, they’re still moving.
Try your hand at estimating Voyager's survival probability:
The Voyagers were built in the golden age of space engineering — analog hearts, plutonium cores, and American confidence. That they crossed the heliopause intact is both a triumph of design and a testament to the foresight of NASA engineers at Houston’s Johnson Space Center. Their legacy: to boldly drift where no bot has gone before.