Phoenix "Lazarus" mode vs. "safe" mode

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So what is this "Lazarus" mode that Phoenix has? Well, any spacecraft has many modes of operation. All include some sort of "safe mode" to be used when things aren't going so well. In fact, even the instruments have their own safe modes. Phoenix landed in safe mode, and put itself into safe mode a few times later in the mission when things took a turn for the worst. The idea is that the spacecraft uses resources sparingly, while trying to get itself back under Earth control. (Phoenix woke up several times each sol on its own, and only received commands 1-2 times per sol.)

One time Phoenix woke up in safe mode was sol 152. Things were dire, so bad that it was a few sols before any communication to Earth worked. At the end of a 4-sol weekend plan, a dust storm had hit. Power was already depleted, and the spacecraft thought a battery might have failed (it hadn't). After a few more sols of dust storm, Phoenix no longer had the energy to make it through the night. Blackout.

Waking up from blackout, with no access to volatile memory, and no knowledge of anything that had happened since the day before launch, Phoenix needed desparate action to reach Earth. The lander communicates through orbiters that fly over at specific times--only Phoenix did not know when, and did not have the energy to stay awake for long hours. This was never supposed to happen--Phoenix was designed for a 90-sol mission (and was 2 months past warrantee). By about sol 150 (just past), power was expected to be so low that operations would cease--the Sun was low, the air was cold, and Phoenix was a solar powered lander that needed warm parts to act.

As it happened, engineers had prepared for the event that Phoenix wakes up in just such a state, with the expectation that it might happen after a long, cold winter. So, Phoenix woke up and checked: safe mode, yes; low power, yes; clock reset to pre-launch, yes; landing sensor deployed, yes. Action: try to wake up on solar power only, at whatever time of sol that solar power was sufficient. When awake, listen for a beacon on the orbiters for 2 hours. Then, sleep for 19 hours, and try again. (Why--to make the 2 hour window occur at all times of sol.)

What went wrong? Well, mainly, the situation was too bad, and winter was setting in rather than going away. Batteries went to zero each night, so each day was a new start to lazarus mode--wake up for 2 hours as soon as there's energy. The wakeup happened so early, the orbiters could never establish reliable communications.

Summary: Lazarus mode was a specific type of safe mode to potentially regain control of a "dead" spacecraft. It was intended for use after a winter. No significant resources were taken away from the main mission to allow this, since the chances of being revivable after winter were low. In the end, Phoenix simply didn't have the power to operate, and no software "mode" was going to change that. A year before launch, if you had asked the team leaders what the best case ending for Phoenix was, the answer was to run out of power around sol 150. About 2 years later, that's exactly what we got.