What is the farthest from Earth that any Earth thing has died?

—Amy from NZ

With Halloween approaching, I guess it's the season for death-related questions.

The farthest from Earth that any human has died is about 167 kilometers,[1]Plus or minus a kilometer. when three cosmonauts on Soyuz 11—Vladislav Volkov, Viktor Patsayev, and Georgi Dobrovolsky—suffered a depressurization accident while returning from Earth. They were moving at about 7,755 meters per second at the time, which is also the highest forward speed at which any human has ever died.

Volkov, Patsayev, and Dobrovolsky are the only humans who have died in space. Every other fatal space accident—and, for that matter, every other human death of any kind—happened within 70 kilometers of the surface.[2]Morbid list from my notes: The crew of Columbia died at just over 60 km, Pyotr Dolgov died at roughly 24 km, James Zwayer died at 23 km, Michael J. Adams died at 20 km, Ying Chin Wang died at 20 km, and Rudolf Anderson died at between 18 and 23 km. Jack Weeks presumably died somewhere between 20 and 0 km the ocean surface.

But humans don't hold this record.

For starters, there are plenty of test animals which have died in space. But, to be honest, I can't bring myself to collect statistics about them. I mean, at least the human pilots who died had all volunteered and understood what was happening to them. So instead, I'm going to skip straight to the organisms that are the real answer to Amy's question: Microbes.

Spacecraft carry bacteria, although we do our best to sterilize them before and during launch. This sterilization is important, because we don't want to contaminate another planet or Moon with Earth bacteria. There are two big reasons for this—one ethical and one practical. The ethical one is that we don't want to accidentally introduce Earth life that disrupts and/or destroys a native ecosystem. The practical one is that if we find life on some other planet, we don't want to have to struggle to figure out whether it was contamination from one of our probes.

But sterilizing spacecraft is hard. NASA has an employee specifically assigned to this task, and she has possibly the best job title of all time: Planetary Protection Officer.[3]Another competitor for this title is Philip M. Breedlove, who has the job title Supreme Allied Commander.

The Planetary Protection Officer is responsible for avoiding spacecraft contamination, although there are occasionally problems.

A 2008 study of lunar missions estimated that spacecraft carried 1.98x10¹¹ viable microorganisms per vehicle. Spacecraft such as the Voyagers and Pioneers, which were ultimately headed for deep space, were also not fully sterilized—the official planetary protection strategy was "try not to hit any planets."

Voyager certainly carries lots of bacterial spores. If we take the number from the 2008 paper as a (very rough) estimate of the number of microbes Voyager might carry, we can try to figure out how many might still be alive.

Some microorganisms can survive for a long time in a vacuum. One study found that the majority of bacteria that spent six years in space survived—though only if a shade protected them from the Sun's UV light. Other studies have agreed that radiation is the main thing to worry about, and the radiation environment inside a spacecraft is complex. The bottom line is that we just don't know for sure how long bacteria can survive in deep space.

But we can still give part of an answer Amy's question. If we assume that 1 in 1,000 bacterial spores on Voyager were of a space-tolerant variety, and 1 in 10 of those is somewhere on the craft where UV light doesn't reach it, then that still leaves on the order of 10 million viable bacterial spores traveling on Voyager.

If they suffer a death rate of 30% per six years, as in one of the studies, then there would still be a million of them alive after 50 years, dying at a rate of 1 every 10 minutes. On the other hand, the author of the 2008 study speculated that microbes could avoid hits from cosmic radiation for extremely long time periods, and other sources have speculated about survival for thousands or even millions of years. But no one really knows.

For our Voyager bacteria, there's a higher death rate at first, for spores in more exposed positions, and a much lower one for the more protected ones. Today, it's quite possible there thousands of bacterial spores still alive on Voyager 1 and 2, lurking quietly in the dead of space. Every few hours, days, or months, one of them degrades enough to no longer be viable.

And each one sets a new record for the most distant Earth thing to die.