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On February 13 2019, NASA announced that Opportunity - a Mars rover launched in 2003 with an intended longevity of 3 months - was likely to be dead.

In the wake of this event, and the outpouring of emotion it triggered, I've seen the following quote widely disseminated on social media:

The only reference to such a quote I've been able to find is in this article:

Last June as the dust storm descended upon Opportunity, the rover’s last message essentially said “my battery is low, and it’s getting very dark.”

I have a feeling that the word "essentially" may be key here - is the writer simply trying to anthropomorphise Opportunity's last contact with NASA, or did Opportunity actually say such a thing? Are the Mars rovers even capable of such advanced communication skills?

From a NASA blog post made the day after contact was lost:

The dust storm that is affecting Opportunity has greatly intensified.

The atmospheric opacity (tau) over the rover has increased to a record 10.8 on Sol 5111 (June 10, 2018). Power levels on the rover have dropped to a record low of ~22 watt hours. As expected, Opportunity has tripped a low-power fault and gone silent. A 72-hour spacecraft emergency was declared on the afternoon of Sol 5111 (June 10, 2018), anticipating the low-power fault.

A high atmospheric opacity indicates that the atmosphere is blocking light, therefore it is dark. A record low battery was also recorded. Both of these are consistent with the claim. Future estimates of atmospheric opacity are far less precise, so we can conclude that the measurement of tau was transmitted from the rover rather than another source.

The last image sent by opportunity was static and blackness. The filename indicates it was taken at 4:24 PM UTC. The blackness at the bottom could indicate that the transmission cut out halfway through.

Opportunity definitely transmitted the information in the claim on its last day. I do not know if it transmitted anything else in a separate transmission afterwards. It most certainly did not transmit a plain English message ready to be memed, but rather a technical status update that NASA's bloggers interpreted for us, and then was reinterpreted into the meme.

As a personal note, if I am on my deathbed and I say something meaningful and wise, and then I follow it up with a mundane comment right before I die, please remember the wise thing as my last words.

Opportunity does not literally speak English, so no such message was expressed in those terms. However, Opportunity did communicate the information that it had a low battery and it was dark outside.

The rover's flight software outputs a variety of data formats (summarized in this paper) ranging from "event reports" to telemetry to binary data produced by the spacecraft's subsystems (including imagery from cameras and other research products). The result of all of that is downlinked to earth and then processed on the ground by different teams into human-readable output. While there is a great deal of public data available for download, I don't believe this applies to the raw transmissions from the rover, but you can see the ACSV format some of the engineering data is processed into. Scientists then process and review that data and write things about it in English, anything from technical reports ("a measured tau of 10.8") to the more poetic "my battery is low, and it’s getting very dark."

Here's the Sol 5110-5114 MER B Downlink Report from mission management conveying information about the last transmissions received from the rover:

Mission Manager

* Tau Value is NOT a Typographical Error *

Overall Assessment
Opportunity is currently in the midst of a severe dust storm though all subsystems are still operating as expected in RAM mode as of the Sol 5111 UHF pass. Solar array energy is approximately 22 W-hrs, with a measured tau of 10.8. This Tau measurement is the highest ever recorded from a ground station on the planet Mars. Dust factor was previously estimated at 3.27 as of Sol 5108.

With analysis from the data from the Sol 5111 downlink, we expected the rover to enter a low power fault mode very soon after that point in time. Since then, there have been no beeps nor fault windows detected during DSN coverage periods. This behavior is fully in line with array energy expectations, as Opportunity will use deep sleep during such times. We are now waiting for the skies above Opportunity to clear enough for the solar energy to support the fault communications windows (which will send signals to Earth). The team has suspended nominal sequence operations, and we are listening every day for Opportunity to talk to us via one of the fault windows. Thermal predictions carry a positive medium to long term message, whether it takes several sols or several weeks for the storm to weaken and let the sun through. We expect no thermal damage to the batteries or computer systems, and every sol is one closer to summer warmth. Obviously, the team is concerned. But, all of the data and our knowledge indicate that Opportunity is likely to be doing more geological experiments during the upcoming Martian summer, helping unwrap the mysteries of Endeavour crater. Until then, our current mission phase is one of patience and readiness.

Opportunity did not drive during this period.

As can be shown in this chart, 22 W-hrs is a very low value for energy collected from the solar array as compared to previous periods. At this time, the scientists considered "a low power fault" to be "inevitable" (note though that "There is no danger to the spacecraft going into a low power fault. The only danger is the temperature getting too cold and damaging the battery or the instruments.") The tau value (atmospheric opacity) is the highest ever recorded, so high that the report includes a prominent note alerting readers that it's not a typo. The rover does not calculate this value itself; it takes a picture of the sun through a solar filter, and scientists process that image on the ground to determine the numerical value. In other words, the spacecraft reported data indicating low energy and darkness.

Opportunity also sent back a final Pancam image at that time. It's the image that was used to calculate that tau value. It's very dark:

Thanks Oppy!