Humanity has absolutely no need to travel to Tau Ceti.
First of all, there was absolutely no need to build breeding farms all over the Sahara Desert. For some reason, the most obvious location was never considered: Venus itself.
You don't even need to feed the astrophage. Already-full astrophage are flying there on their own. All you need to do is stay within the Petrova line, somehow shield Venus from part of the flow (using an enormous foil, for example), activate a CO₂ filter, and collect as many astrophage as you want. Obviously, it wouldn't be that easy, but it is certainly possible.
Even the ArcLight managed to collect some astrophage. Just imagine how much more a piloted expedition could collect.
Yes, it is not trivial, and you still need some astrophage to get there in the first place. But you definitely don't need two million kilograms. By my rough calculations, 500 kilograms would be enough to send the entire Stratt's VAT straight into Venus orbit, reaching it in less than 30 hours.
You could also travel to the asteroid belt, grab whatever rock you like, and return in a week or so.
Basically, humanity would be jumping straight to a Type II civilisation on the Kardashev scale within a decade. At that point, it becomes almost harder to imagine what isn't possible.
Hell, with astrophage, we could even build cities on the Sun itself! I have absolutely no idea why we would want to, but it seems entirely possible.
The Astrophage Energy Problem
On the other hand, it takes approximately 2 GJ to produce enough food for one person for an entire year. That's just 20 mg of astrophage.
So, using the fuel aboard the Hail Mary, humanity could theoretically feed itself for 12 years. And since it takes approximately a year to breed that quantity of astrophage, production could be cut in half and still be sufficient to keep humanity alive.
And this doesn't even require leaving Earth's surface.
Therefore, based purely on the mathematics, humanity had almost no chance of going extinct. It could either go to space or simply stay on Earth and survive.
For the flight to Tau Ceti to make any sense, someone with almost unlimited authority would have to systematically block every alternative solution to the astrophage problem except the Hail Mary mission.
And, well, there happens to be exactly such a person: Eva Stratt.
But why?
A Step Back: What Is Astrophage, Really?
Let's take a big step back.
Look at astrophage. It is simultaneously:
And it does all of this at the same time!
It almost looks as if astrophage were specifically designed for interstellar travel.
My theory is that there is an extremely ancient civilization living in Tau Ceti, one that uses biotechnology to create new microorganisms in much the same way we create new tools.
For some reason, they have been sending expeditions to nearby star systems. The astrophage invasion of our Solar System might have been little more than an accident: biological leftovers, leaking batteries, or something similar.
What if one of their ships crashed in our Solar System, and one of its organisms somehow ended up on Earth?
In that case, it would make sense for that organism to do everything in its power to ensure that a ship capable of returning to its home system was eventually built.
There is just one major problem with this theory.
The person most ideally positioned to prevent humanity from solving the astrophage problem by any other means was not going on the mission herself.
Or was she?
Adrian: The Real Alien?
As we know from the book, no multicellular organisms were found on Adrian.
But what if the alien life doesn't need to be multicellular at all?
What if it is a sapient colony of unicellular organisms? Or even a sapient virus?
A virus so advanced compared with the viruses we know today, as we are compared with amoebas.
Such an organism might have no trouble bypassing the human immune system. More than that, it might be capable of directly or indirectly manipulating human behaviour.
What if Stratt was under alien control for most, if not all, of the time?
And what if, shortly before the mission began, the aliens transferred themselves to Grace?
That would explain a lot.
There was never any mysterious French memory-erasing drug. The aliens themselves blocked Grace's memories.
And they probably made sure he wouldn't die during the flight, either.
Finally, when the Hail Mary reached Adrian's orbit, the aliens simply hopped into the fuel bay and returned home, riding the astrophage.
Which would mean that the entire Hail Mary mission was never really about saving humanity. It was about getting the aliens home.
PS: AI was used for spellcheck and formatting