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In Project Hail Mary, Grace and Rocky only have enough fuel to reach either Earth or Rocky's homeworld Erid, and they choose to go to Erid.

Why do they make this choice when it is stated that the Eridians have many decades before The Phage causes serious problems on their planet, meaning that there would be plenty of time to reach Earth, refuel and then return to Erid? The journey to Erid could be made at higher acceleration if only Rocky went, or a robot like if the Beatles were sent instead.

Given that the two of them know that the situation on Earth is more urgent, doesn't it make more sense for them to go there first?

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  • As it stands, the title looked speculative and open to opinionated answers. I've changed it to focus more on filling in missing information to help the reader understand the choices made in the book.
    – AncientSwordRage ♦
    Commented Feb 21, 2022 at 13:40
  • In making the choice to save Rocky, Grace gave up his chance of returning to Earth - food wise. I think that they could barely make it to Rocky's planet.
    – Phlegon_of_Tralles
    Commented Feb 21, 2022 at 13:51
  • I am reading this now and I think you should change title and put here spoiler tag. Thanks.
    – ne2dmar
    Commented Feb 22, 2022 at 12:51
  • Earth?!? Ugh!!!
    – Paul D. Waite
    Commented Feb 22, 2022 at 13:16

4 Answers 4

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In addition to Pat Dobson's answer, I don't think it would have been easy to just "refuel" the ship. One of the significant challenges that Earth had to overcome prior to the mission was to obtain the fuel. They had to

pave a chunk of the Sahara Desert with black panels. Like ... a quarter of the entire Sahara Desert!

They did this despite

And it would destroy the ecology of Africa and probably Europe.

Even so it took years to breed enough Astropharge. I think it unlikely that on returning to Earth, which would presumably be in severe crisis (remember that Stratt expected a significant part of humanity to have died) that the fuel would have been available to send a mission to Erid.

So if Grace wanted to help Rocky, in addition to Earth, which he did, he did not have any other choice.

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  • That's a good point. They didn't seem to discuss the fact that Rocky is a genius level engineer and could probably help them farm Astrophage more efficiently and with less environmental damage though. If I were Grace that would be a major factor in my decision making, as it would effectively open up interstellar exploration and colonization, so I'd at least ask if it were possible. The Iridians made huge amounts somehow, despite the lack of sunlight on the surface of their planet.
    – user
    Commented Feb 21, 2022 at 15:54
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    The book did mention about how the Iridians made their fuel. It was relatively easy for them because the ambient temperature of their world is above the critical temperature (106F) that astrophage likes.
    – user23087
    Commented Feb 21, 2022 at 15:58
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    Of course I think that the real answer is that Grace wanted to go to Erid more than he wanted to go back to Earth. Once he had a good excuse to do so, that's what he did.
    – user23087
    Commented Feb 21, 2022 at 15:59
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Turning a comment into an expanded answer…

Rocky going to Earth is a gamble, a huge one - Earth is already majorly affected by the astrophage infestation, and would need to enact a major program to grow and implement the Taumeoba solution, as well as build a new interstellar space program to save Rockys world, in addition to attempting to recover from major ecological and societal disasters at the same time.

Sending the Beatles to Earth and the spacecraft to Erid is the best outcome for both parties - Earth expects the Beatles, Erid doesn’t, so cant do it the other way round.

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    I think this answer is strongly supported by the last chapter of the book too. When Rocky bring the news about status changes in Sol, it's welcome news for Grace. He wasn't confident that the solution would be enough to save Earth, so they couldn't rely on going to earth first then Erid.
    – Jontia
    Commented Mar 29, 2022 at 14:26
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In the original planning of the missions, the Erideans had planned on a return trip once they had found a solution.

The humans had planned a one-way 'suicide' mission and had therefore provided Grace with the 'beetles' to return the information to Earth.

Indeed, towards the end of the novel Grace and Rocky parted ways, Grace heading back to Earth and Rocky heading back to Erid.

It was only later that :

Grace soon discovers that the nitrogen-resistant Taumoeba has inadvertently also developed the ability to pass through the material the Blip-A is made from, a strange material called xenonite which is composed of Xenon, a noble gas. It will get into the fuel tanks, consume the Astrophage, and cripple the Blip-A, dooming Rocky's civilization.

So:

Grace sends out the Beetle probes with the mission's findings and live-samples of Taumoeba, then changes course to catch up to and rescue Rocky, and take him back to his planet so he can save his people.

Hence the reason that they both end up going to Erid.

Source : Wikipedia

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    Thanks but that doesn't really explain it. There doesn't seem to be any reason why they can't both return to Earth and then Rocky can go back to Erid. Then nobody has to die. Remember that up until Rocky tells Grace that the Eridians can make him food, his assumption is that helping Rocky is suicide. It doesn't have to be, since Rocky has plenty of food due to the large (now deceased) crew and the expectation that the journey would take much longer due to lack of understanding of relativity.
    – user
    Commented Feb 21, 2022 at 15:47
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    @user why go for the “have to depend on another race wanting to save my race while in the desperate throes of saving themselves” gamble when they already have an option to save both worlds, by sending the Beatles back to Earth and the two ships back to Erid? Much less of a gamble that way.
    – Moo
    Commented Feb 22, 2022 at 5:14
  • @Moo I got the impression that the Beatles were no sure thing either.
    – user
    Commented Feb 22, 2022 at 9:04
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    @user Earth thought that they were good enough to allow a suicide mission…
    – Moo
    Commented Feb 22, 2022 at 10:07
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    @user yes, I agree, but Earth still thought the Beatles were good enough to fulfil the mission requirements given the limitations - to that end, Graces end plan of "hey, I have enough fuel now to return to earth!" is actually the riskier given that the Hail Mary was never intended for a return journey so anything could have gone wrong, resulting in him never reaching earth. Meanwhile, the Earth mission had multiple Beatles for the very specific reason of redundancy so...
    – Moo
    Commented Feb 22, 2022 at 22:49
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The Eridians didn't know about relativity

It's a major plot-point that the Eridians don't know anything about special relativity, which is why Rocky has so much extra fuel, because their calculations were faulty. In the same vein, Grace realized the Eridians didn't have as much time as originally thought due to the time dilation of the trip. There would not be enough time to go to Earth and then back to Erid.

If not for the beetles, after Grace rescues Rocky, they would have faced a choice of which civilization to save (assuming they had enough fuel to turn around again, which is not clear). But the beetles change the equation. They are pre-programmed to head to Earth. Grace did not possess the knowledge to reprogram the beetles to go to Erid. Therefore the course they took was the only available option to save both civilizations.

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    Not to mention that even if they could reprogram the beetles, the Eridians would have no means to understand the data on it. (They don't have computers and even if they did, they'd have to reverse engineer both our binary encodings and the entire English language)
    – DJMcMayhem
    Commented Feb 22, 2022 at 2:41
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    The Eridians were right about the time it would take the ship to make the journey. Time dialation only affects the occupants of the ship and how much time they experience, not the time it takes to travel from the point of view of a "stationary" observer like Erid.
    – user
    Commented Feb 22, 2022 at 9:07
  • @user you're right about time dilation, but relativity can still impact the journey estimate. Under Galilean relativity, if you accelerate at 1g for a year you will be traveling faster than the speed of light (in the home planet's frame). Therefore even in the home frame the journey will take longer than planned. I think Grace's cruise velocity was around 0.25c, but I don't remember if this was ever given for the Eridanians
    – thegreatemu
    Commented Feb 22, 2022 at 18:58

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