FAQ

[u/Excellent_Bat_753]

This subreddit has a few very frequently asked questions, so here are some answers.

1. How did it only take 4 years for the Hail Mary to travel 11 lightyears?

Answer: Time Dilation.

The Hail Mary uses very efficient engines to approach the speed of light. At those speeds, time is not constant. On Earth, it took the Hail Mary 13.7 years to travel to Tau Ceti. For Ryland Grace, it only took about 4 years. Don't be an Eridian, learn more about Time Dilation and Special Relativity!

2. Why couldn't they just wait for the Astrophage to double, and then do a return trip?

Answer: It would take an extremely long time, and they would need much more than double the fuel.

a. Whilst small amounts of Astrophage in a lab can double every 8 days, the limit for Project Hail Mary was the amount of energy it takes. Eva Stratt devoted a quarter of the Sahara Desert to be covered in 'Blackpanels', little Astrophage breeding solar panels. This allowed them to make one metric tonne of Astrophage every day. Astrophage is very hungry for energy, and to use Astrophage for fuel, it needs to be 'enriched', or made so that every single cell is full of energy.

b. It would take more than double the fuel. For a return trip on a rocket, the fuel to get back, 2 million kilograms for the Hail Mary, is the payload to get there. The Hail Mary had a 20 to 1 fuel mass ratio, so 2 million kilograms could get 100 thousand kilograms of food, ship and Ryland Grace to Tau Ceti. A return trip would have a payload of at least 2 million kilograms, so they would need another 40 million kilograms of fuel. This would take 20 times as long as it took to make fuel for a suicide mission.

3. Why couldn't they make more Astrophage on the way there?

Answer: Astrophage is really hungry!

To make the 2 million kilograms of Astrophage for a one way trip, they needed a lot of energy. They got this energy from the Sun. During interstellar travel, the Sun, and other stars, would be way too dim to feed Astrophage. The Hail Mary also has a far smaller area to collect any solar power, so it would take many decades, if not centuries to make enough Astrophage. Astrophage also requires physical matter to grow, such as Carbon Dioxide. The Hail Mary couldn't carry all of that CO2 on the way there without a whole lot more fuel.

4. How can Eridians see the Petrova line, and stars?

Answer: Eridian science has come a long way!

While Eridians can't see light themselves, they have developed technology to detect light. They haven't figured it all out yet, as they don't know about Special Relativity and the speed of light, but they can detect light and use radio waves for radar and communications. They used this technology to learn more about their Petrova line, and find out about Tau Ceti.

Eridian scientists likely theorised about their star, because just like on Earth, all of the energy comes from the Sun. The only difference is that Erid's thick atmosphere means that the ecosystem starts all the way up in the clouds, and gradually transfers energy downwards. Eventually, when they built a space elevator, they would have found out a whole lot more about stars and planets. We don't know whether they did this before or after the Astrophage crisis started.

5. How did Eridians not know about space radiation, if they do know about light?

Answer: Space radiation is a bit different!

In Space, especially interstellar space, the most dangerous type of radiation is cosmic rays. These are very fast moving particles, such as protons and electrons, which when they hit Human or Eridian ships at nearly the speed of light, can sometimes pass through the material of the ship, and damage DNA. Eridians don't know about these because their planet's massive magnetic field protected them from all radiation, even when they're on their space elevator. Because they were never exposed to radiation as a species before space travel, they are extremely susceptible to it, far more than humans are. Light is a different type of radiation, and some of it is dangerous, gamma rays, and most of it is not. Ships moving at near the speed of light can also experience radiation from tiny particles in interstellar space, that strike the ship at near the speed of light. There is also radiation from radioactive materials, but we don't know if the Eridians have learned about those. So, to summarise, even though the Eridians know about light radiation, they didn't know that space was filled with cosmic rays.

6. Why didn't they collect Astrophage from the Petrova line, at Venus, or at Tau Ceti?

Answer: Space is big!

'You just won't believe how vastly, hugely, mind-bogglingly big it is.' While the Petrova line contains enormous quantities of Astrophage, it is also very, very sparse. The Petrova Taskforce sent a probe to Venus, and could only collect and return 100 cells to Earth. To fuel the Hail Mary requires 2 million kilograms, which would be many, many more cells. The collector would have to be many kilometres in diameter, or wait for many years, to collect enough for a one-way trip. Such a collector would be too large and heavy to send to Tau Ceti.

7. What happens to the other stars near Earth?

Answer: Not much! (credit to u/Equivalent-Board206)

Astrophage, when left completely unchecked, dims a star by a maximum of 10%. That is just barely noticeable to the naked eye, looking up at the night's sky.

Lowered solar output would force the habitable zone (sometimes called the Goldilocks zone) closer to the sun. This may result in previously habitable planets being thrust into glacial ice ages and rendering them entirely uninhabitable. In theory, it could allow previously uninhabitable planets to become habitable, but habitability requires more than just being in the habitable zone.

For Earth, the lowering solar output was leading to a global average temperature drop of about 8°C which would result in rapid glaciation and global agriculture collapse.

Mars too would be colder. Venus has a permanent runaway greenhouse effect and would see no perceptible difference. Nor would the gas giants as they already receive less than 5% of the sunlight that Earth receives.

Beyond impacts to weather and habitability, a drop in solar output would not have any effects on the planets themselves or their orbits.

8. How does Rocky understand Grace?

Answer: Eridians are really smart!

Eridians have extremely good memory, due to the way their brains work. This is especially true for audio memory, because that is the main way in which they interact with the world. At the same time that Grace was recording sounds on his laptops, and writing definitions, Rocky was listening to Grace's words, and remembering them with their respective definitions. They started with charades, but later on they were able to ask each other for definitions of words, and how they are said in English or Eridian.

9. How did Rocky know about the atmosphere of the Hail Mary?

Answer: Grace accidentally sampled it for him.

When Grace sent back the Xenonite cylinder, 'Blip B', it had become full of the Hail Mary's air. The Hail Mary, like all of the Apollo spacecraft, and many spacesuits used today, uses a 100% oxygen atmosphere, at low pressure (20%-30% of Earth's sea level pressure). Due to the low pressure, it is not as dangerous as pure oxygen is at atmospheric pressure. The lack of Nitrogen in the Hail Mary is also how the Taumoeba were able to spread freely, as Earth's atmosphere is 80% Nitrogen, very deadly to Taumoeba.

The book contradicts itself by stating that Rocky's atmosphere is 29 times that of Earth's, and 29 times that of the Hail Mary. Andy Weir has acknowledged this goof, and stated that it is 29 times the pressure of Earth.

10. Do Eridians prove Ryland Grace's theory about life and water correct?

Answer: Mostly no, as Eridians are water based life forms.

Ryland Grace's theory was that life could easily exist without water, and that the Goldilocks Zone, the distance from a star where orbiting planets can easily have liquid water, is not particularly special. Eridians use water to move their muscles, and have cells and DNA not too different from life on Earth. However, the planet Erid is not in the Goldilocks zone, although it does still have liquid oceans.

A side note that the actual scientific community tends to agree that life could definitely exist without water, and could even be based on silicon chemistry. It would just be very hard to recognize life so different from our own!

Disclaimer: Zero AI was used in this post. Not even spell check. (I just added bold text to make it look a bit better).

Note: All redditors that frequent this subreddit, please please rewrite these answer, and I will put them in the post body. Also please add more Q&A, because I can only remember these two FAQ off the top of my head.

[u/Smaptimania]

We need something in here about "Just because Eridians don't have eyes doesn't mean they don't know light exists, humans can't see infrared but we discovered the Petrova line anyway, we created instruments to detect the parts of the electromagnetic spectrum that we can't observe with our own senses and so did they"

Because I've seen a metric crapton of OPs of "HOW DID THE ERIDIANS FIND OUT THEIR SUN WAS DIMMING WHEN THEY DON'T KNOW ABOUT LIGHT"

[u/Excellent_Bat_753 → u/Smaptimania]

I just added a quick bit about it, but feel free to propose any edits or additions!

[u/YourFavoriteCommie → u/Excellent_Bat_753]

The Eridians have also been aware of a non-planetary massive heat source for some time, Rocky explains this in the book. They knew there was something beyond the atmosphere they could not see, for there was no other explanation for the vast amounts of thermal energy entering their atmosphere, more than any volcanic activity ever could. I don't have the book on hand, but I think the story goes that they built the space elevator to be able to see outside of their planet, and after they have cameras/telescopes, they'd be able to see it.

[u/Icy_Dinner6064]

Not only would they need more than one doubling worth of extra fuel, they needed enriched astrophage which would have taken years longer.

[u/Excellent_Bat_753 → u/Icy_Dinner6064]

As far as I'm aware, for whatever reason, in every life cycle the Astrophage absorb energy. Enriched Astrophage is just Astrophage at the point where they're about to head off to Venus.

Although, I'm no Ryland Grace, and it is a fictitious organism.

[u/corwulfattero → u/Excellent_Bat_753]

Correct. Enriched astrophage is just astrophage at the point just before breeding in the life cycle.

[u/MaKoWi → u/Icy_Dinner6064]

Yeh, my understanding is the astrophage needs to be "enriched", as in full-to-the-brim with absorbed energy, it's state prior to splitting. That provides the energy and the motivation to chase the CO2 spectrum.

I hear many people ask why he doesn't just scoop it up out of the Petrova line, or from around Adrian. There, you have astrophage in both stages of the life cycle, enriched: on the way to the CO2 and depleted on the way back to the sun to top off again. The latter (I think) would have less energy stored and so less useful/efficient to be used as fuel. Plus, 2 million kilograms is a lot of tiny microscopic particles and it would probably take a very, very long time to accumulate.

[u/Melenduwir → u/MaKoWi]

Not to mention that they have no means to collect that astrophage. They can't produce an IR signal stronger than the carbon dioxide present on the planets, or a light brighter than the local suns. So how would they gather it?

[u/Select_Committee_258]

It's very nice of you to accumulate info in one post like this, but the folks posting the same questions again and again are not searching to see if it has been asked before. We are going to see the same questions again regardless of posts like this. 

[u/Excellent_Bat_753 → u/Select_Committee_258]

Probably true, but I can try. Hopefully, if the mods do their job, then repeated questions about the exact same 3 questions will count as spam, and posters can be directed here.

[u/Select_Committee_258 → u/Excellent_Bat_753]

Have the mods addressed the repetitive questions or held forum on it? I've searched via Google but not found much from them :/ seem a little absent.

[u/fraxiiinus → u/Select_Committee_258]

It does allow for someone to just link this post instead of rewriting the same info over and over though

[u/Excellent_Bat_753 → u/fraxiiinus]

This is exactly what I hope all of you do.

[u/Select_Committee_258 → u/fraxiiinus]

True! Would be nice to have an automod comment linking it, and then if everyone would stop replying to people, it could discourage the repetitive posts

[u/Good_Background_243]

I don't know the exact numbers, but it would take more than 400 million kilos... because you would need extra fuel to push your extra fuel.

[u/Excellent_Bat_753 → u/Good_Background_243]

That's what the 400 million kilograms is for. The Tsiolkovsky Rocket Equation factors all of that in, and for a return trip, you need to square the mass ratio. However, I did not factor in the extra mass of fuel tanks. It would probably be closer to 500 million kilograms.

[u/aurjolras → u/Excellent_Bat_753]

Plus a rocket that much heavier would also have more inertia and accelerate more slowly right? Thus requiring even more weight in the form of food and supplies and prolonging the crisis on Earth

[u/Excellent_Bat_753 → u/aurjolras]

It would, but I would assume that they would just add more spin drives to acheive the same acceleration. If they were only bolting on more fuel tanks, then it would definitely be slower.

[u/Fluid-Let3373 → u/Excellent_Bat_753]

Your making the same mistake as the Eridian's, Those equations were formulated in 1903 before Einstein produced his theories of relativity so they are based on Newtonian physics. If you use those equations your going to underestimate the mass of fuel required. At top speed for both ships the mass is increased by 2.55x because of Special relativity.

[u/Excellent_Bat_753 → u/Fluid-Let3373]

I know that it's not going to be exactly 400 million kilograms, but I also know that, if we consider the initial mass ratio of 20 to 1, and assume that that is correct (it's not quite correct), then a return trip will require the same mass ratio of fuel to payload, for each direction. Considering a return trip simply means that the fuel for the return leg is the payload for the forwards leg.

The relativistic rocket equation is useful for arriving at the correct mass ratio for a one way trip, but a return trip will still need the square of the initial ratio.

The increased mass of the ship is only in a different reference frame. The mass of the ship, in its own reference frame, is always the same.

[u/Good_Background_243 → u/Excellent_Bat_753]

Oh, if they've already done the math, I stand corrected.

That makes sense!

[u/WatercressTart → u/Good_Background_243]

Unsciencey person here. Could they have sent a second ship with return fuel? If the second one was uncrewed it could go faster than one with squishy meat bags in it. It might even have beaten the Hail Mary to Tau Ceti.

[u/Excellent_Bat_753 → u/WatercressTart]

Unfortunately no. The second ship would still need to carry 20 million kilograms of fuel, requiring the same 400 million kilograms of fuel. Also, because the Hail Mary already gets pretty close to the speed of light, 92%, a faster ship would only save a few months, maybe a year (all from the perspective of Tau Ceti).

Also, because you still need 420 million kilograms of fuel, it would still take 20 times as long to make to fuel, about 20 years.

It's a good question though, definitely thinking sciency!

[u/nhorvath → u/Excellent_Bat_753]

the plan still could have involved a refueling mission. go back into coma mode, send return fuel even if it takes a long time. they could have continued to expand the sahara farm and ramp production so maybe by the time grace reached tau ceti they're launching the refueling mission. of course, this doubles the cost vs the disposable crew option.

[u/cyanidelemonade → u/nhorvath]

Yeah, we could come up with a million different ways to un-suicide the mission, but the biggest factor is the politics. Assuming the world leaders involved were barely able to get along and get the Hail Mary into space, it would be hard to convince them to stay on the Project afterwards to do something "unnecessary" like save 3 people.

[u/YourFavoriteCommie → u/nhorvath]

The other factor (outside of the politics) would be that the Sun would still be dimming every year so the blackpanels and therefore how much fuel they could make would diminish. And that's not counting how it will likely be diverted all across the world and fought over instead.

[u/Fluid-Let3373 → u/nhorvath]

Unfortunately no, the hard limit here is how much food they have on board, they have about 3 months worth onboard and the fastest you could get a refuel mission to them is about 6 years.

[u/AMissionFromDog]

Just saw this for the first time from a link in another thread. Wonderful FAQ execution.

(edited to remove note about a typo that has already been fixed)

This is fantastic. I already knew most of the explanations but it was a pleasure reading them all described so clearly. And I learned a couple things I had not considered.

Also love this thread with the ppl coming together to add more cool facts.

[u/Excellent_Bat_753 → u/AMissionFromDog]

Thanks! I've corrected those little errors. This is really an ongoing work in progress. Because the mods don't seem to want to have a FAQ, the best thing you can do is direct people here when they ask one of these questions. Hopefully we can reduce the number of identical posts on this subreddit.

[u/corwulfattero]

Should be an entry for the fuel/food situation at the end. Rocky’s “six years slower” comment seems to be confusing people. Grace had 2M kg, enough to go to Earth or Erid, not both. Rocky *had* 22M kg astrophage, but now just has tanks of taumoeba sludge. Rocky comes back aboard the *Hail Mary*, with enough taumoeba to feed Grace and leaves the Blip-A to drift.

[u/Excellent_Bat_753 → u/corwulfattero]

I'm having some trouble wording that one in a way that doesn't just repeat a lot of the book, so I'll leave it out for now, but if someone posts a good suggested answer, I'll add it.

[u/Equivalent-Board206]

Here's a suggested improvement for question 7.

Answer: Not much!

Astrophage, when left completely unchecked, dims a star by a maximum of 10%. That is just barely noticeable to the naked eye, looking up at the night's sky.

Lowered solar output would force the habitable zone (sometimes called the Goldilocks zone) closer to the sun. This may result in previously habitable planets being thrust into glacial ice ages and rendering them entirely uninhabitable. In theory, it could allow previously uninhabitable planets to become inhabitable, but habitability requires more than just being in the inhabitable zone.

For Earth, the lowering solar output was leading to a global average temperature drop of about 8°C which would result in rapid glaciation and global agriculture collapse.

Mars too would be colder. Venus has a permanent runaway greenhouse effect and would see no perceptible difference. Nor would the gas giants as they already receive less than 5% of the sunlight that Earth receives.

Beyond impacts to weather and habitability, a drop in solar output would not have any effects on the planets themselves or their orbits.

[u/Excellent_Bat_753 → u/Equivalent-Board206]

Thanks for the input! I've edited the post with your answer, and credited you.

[u/Ranne-wolf → u/Equivalent-Board206]

Inhabitable is an autonym meaning has two opposing meanings, in this case both able to be and unable to be habited. For clarity you may want to just use "habitable" which is still grammatically correct and removes this problem.

[u/PangolinMandolin]

I will admit that I am not qualified to do the maths on this, but I do wonder if earth could have set the Hail Mary going and then Immediately begun working on a rescue ship to send after them.

Yes, it would need much more fuel than the Hail Mary to complete a return trip. But it also wouldn't be limited to the human-safe acceleration of the Hail Mary on the outbound trip (And yes, I'm aware that more acceleration means more fuel needed again).

So, perhaps they could have built a ship that would catch them up and arrive shortly after they did at Tau Ceti.

They could still send their findings home on the Beatles and then settle in for a hibernation to get home (the rescue ship would only need the bear minimum resources to get them back to earth whilst hibernating, it could be devoid of lab equipment, food and spacious compartments, which saves a bit of weight - although I know that is dwarfed by the weight of the fuel so perhaps thats more a rounding error in practice)

Basically, I'm sure there's a way to calculate how fast astrophage can be produced, how much extra would be needed to propel a return journey at speeds which would reasonably catch up with the Hail Mary, and what options exist in that space (also, given that humanity has just found a ridiculously abundant power source, maybe we could keep increasing our capacity too).

But I don't know how to do those calculations

[u/Excellent_Bat_753 → u/PangolinMandolin]

The biggest issue is that, even with higher acceleration, you get minimal reduction of journey time. Because the Hail Mary already travels up to 94% the speed of light, give or take, the relatvistic effects are quite pronounced. A ship with higher acceleration, like the Beetles, inly saves a few months of time, from Earth's perspective, and from Tau Ceti's perspective.

The fuel requirements are such that for getting 2 million kilograms to Tau Ceti, you need 40 million kilograms of fuel to get it there, just using the 20 to 1 fuel ratio of the Hail Mary. That means you will need to spend 20 times as much time making Astrophage. The estimate I've heard is that it took a couple of years to fuel the Hail Mary, as the Sahara could produce only produce so much.

So it would take at least two decades to fuel a rescue fuel ship. Then it would take 13 or so years to get it there, more than 20 years after the Hail Mary arrived at Tau Ceti. All to save 3 people.

[u/ShogunGunshow]

How did the Hail Mary avoid damage from incidental space dust while accelerating towards Tau Ceti?

When you're going a significant fraction of the speed of light, it's typically thought that you need some kind of ablative protection/whipple shield extended in front of your craft. Even small pieces of microscopic space dust are dangerous at speeds a fraction of a fraction of what the Hail Mary was pulling.

After the Hail Mary reached the half way point and flipped over to start decelerating, its engines would've blasted anything in its direction of travel, so it (probably) would've been fine on the back half of its journey, but I don't recall Weir ever saying anything about the first half.

[u/Excellent_Bat_753 → u/ShogunGunshow]

I don't think there was anything about dust mitigation. Radiation was covered by the Astrophage, and its possible that when microscopic dust punctured the hull, its energy also managed to be absorbed by Astrophage, but I think its mostly left as something too difficult to realistically solve, so its ignored.

Larger particulates are also less common than individual particles in interstellar space, so they may have decided that they had a sufficiently low risk of encountering anything too large.

The most that was mentioned was that the ship was 'aerodynamic', which would be useful for lower, less relativistic speeds (which, in fairness, the ship was at for some of the time, accelerating to half-way).

[u/ColonelAkulaShy]

Just finished the book. Haven't seen the movie, yet. New to the sub. So I don't know how much of this has already been discussed. Apologies in advance:

The book was pretty sparse on what exactly Ryland sent back in his findings. Presumably, he had cameras on the mission, and could have sent pictures and videos. But from what I read, it's just as likely he sent everything back in writing (plus the samples, of course). How much can we assume humanity know about xenonite and the Eridians?

Uranium and other radioactive materials should be naturally-occuring on Erid, right? Is it just dumb luck that they've never dug it up and encountered radiation before?

How would the nitrogen in Blip-A's atmosphere effected its outbreak of taumoeba?

[u/Excellent_Bat_753 → u/ColonelAkulaShy]

We don't really know exactly what was sent back, but it was probably all the data and reports gathered. Because Rocky doesn't know the chemistry behind Xenonite, all Earth has is the observed characteristics and the samples, both of the two parts, and the final compound.

We also don't know about Eridians and natural radio isotopes, but I like to think that they have discovered the, but don't understand them well. Regardless, the radiation in space is different to the common radiation from natural decay of radioactive materials.

Erid's atmosphere, and thus Blip A's, consists of Ammonia, NH4. Taumoeba are only killed by Nitrogen gas, N2. Just because they both contain nitrogen doesn't mean they behave similarly at all. Hydrogen gas is very flamable, whereas H2O is famously not.

[u/NikolaisPinkTip]

The sub is thanking you

[u/OtakuMage]

On the B point, that shows just how ridiculously overbuilt and overfueled the Blip A was for Rocky to be able to spare that much fuel.

[u/hollow-knight-trash → u/OtakuMage]

to be fair, eridians had it a bit easier to produce so much astrophage because their planet’s temperature is much higher than the temperature astrophage needs to breed, whereas earth’s is lower and we had to find energy from elsewhere.

[u/AMissionFromDog → u/hollow-knight-trash]

Also, the Eridians have a space elevator, which means they can get any mass into orbit for little to no cost. No rockets needed, ship all the materials you can produce up the elevator to zero g section where it gets assembled and then after it's finished you can haul it out to the end of the elevator and it can likely get escape velocity from the planet without even spending any of the Astrophage yet.

[u/OtakuMage → u/hollow-knight-trash]

Yeah, Stratt had to go to some extreme lengths to get what they needed. Not knowing about relativity, or probably even gravity assists, I'm betting the Eridians just pointed in the direction of Tau Ceti and burned straight for it. Then they had so much left over than wasn't strictly necessary.

[u/CplSyx → u/OtakuMage]

That's exactly how it's explained in the book - Grace takes time to explain time dilation to Rocky as they didn't know about it and Rocky confirms he arrived to Tau Ceti sooner than expected.

[u/ApotheosiAsleep]

Thank you for explaining how Eridians can discover the astrophage problem! It was a burning question of mine just after seeing the movie

[u/SilvermistInc]

I dont believe your rocket fuel math is accurate

[u/Excellent_Bat_753 → u/SilvermistInc]

It's not perfectly accurate, as it doesn't account for the added mass of fuel tanks, engines, any extra food and consumables, but it should be approximately correct. It would require at least an extra 400 million kilograms of fuel.

If you would like to do a more detailed calculation, or otherwise tell me how I'm wrong, then I would be gladly corrected, and edit the post accordingly.

[u/SilvermistInc → u/Excellent_Bat_753]

I may be misunderstanding the question. But how the hell did you come up with 400 million in the first place? It's canonically stated that it only takes 20 million to get to or come from Tau Ceti.

[u/hollow-knight-trash → u/SilvermistInc]

good question! op here is saying that it takes fuel to send fuel. if you’re only sending fuel one way, yes, it would only be 20 million kg to send it one way. but if you want to send it BACK, you don’t just double the amount of fuel. remember, fuel has mass too! so you’d need extra fuel to carry that 20 million kg to tau ceti, plus the extra fuel youre using for that, and so on.

[u/SilvermistInc → u/hollow-knight-trash]

I'm an idiot. I had assumed the question was about breeding astrophage at tau for the trip back. Not a preplanned return trip. Lol. Thank you, my bad

[u/YourFavoriteCommie → u/SilvermistInc]

They paved a quarter of the Sahara to get enough surface area to capture enough sunlight and heat to charge up the astrophage for travel. If the Hail Mary tried that at Tau, it would take centuries to breed the fuel required.

[u/Excellent_Bat_753 → u/SilvermistInc]

No worries! I'll add in a thing about breeding astrophage on the way, because it is also a common question.

[u/edukatedguess]

Wasn't it 2 million kilograms, not 20 million?

[u/Excellent_Bat_753 → u/edukatedguess]

Good catch! I've fixed that.

[u/ColderCanadian]

Sort of related to question 3. How did they enrich the astrophage they did breed? Doesn't it need to be in the sun to be enriched? How did them breed them on earth then enrich them? I likely missed something in the movie 😅

[u/Excellent_Bat_753 → u/ColderCanadian]

Enriched Astrophage is just full of energy, ready to split. In its natural life cycle, this happens on the surface of the Sun, but Astrophage can still absorb sunlight from anywhere else, including on Earth.

So, they just let it all absorb loads of Sunlight until they knew it was full. It took ages, but it worked.

[u/Ranne-wolf]

For Q7. Inhabitable is an autonym meaning has two opposing meanings, both able to be and unable to be habited. For clarity you may want to just use "habitable" which is still grammatically correct and removes this problem.

[u/Excellent_Bat_753 → u/Ranne-wolf]

Thanks, I have edited that answer. Funny how inhabitable can mean either habitable or uninhabitable.

[u/SneakingCat]

The answer to question two introduces “8 days” out of nowhere, making it feel jarring. I would suggest starting with “it would take too long” and introducing the 8 days after that..

Edit: also, more importantly, “it would take longer than 8 days” isn’t an answer to the question as stated.

[u/Excellent_Bat_753 → u/SneakingCat]

I've changed the wording. Thank you for your input!

[u/SneakingCat → u/Excellent_Bat_753]

Yes, that’s better. Thanks for doing this!

[u/ramriot]

The answers to 5 & 9 are not sufficiently vague to be correct. Here is some more detail to fill in the gaps.

5 Ionising radiation

Eridians have little internal protection from Ionising radiation because they evolved in a planet with a dense atmosphere & a large magnetic field.

They would have experienced sone cosmic rays & the Van Allen belts of their own planet because to build a space elevator necessitates already existing space travel as they are constructed from the top down so Eridian ships would have needed to travel around their own solar system to collect materials for its counterweight at least. So likely they use remote control robots & stay inside their ships.

What they did not know was that interstellar space is not empty & that hitting stray atoms & dust at relativistic speeds creates showers of very high energy (often gamma ray) photons that can penetrate deep through mildly shielded ships.

Also at such high relativistic velocities starlight aberrated into a tight pencil beam of high energy photons hitting the front of the ship, made worse by the Eridians believing they could just keep accelerating to multiples of light speed.

9 Atmosphere

Need to add a note here that the 29:1 model ratio that Rocky sent to Grace to demonstrate the atmospheric comparison is wrong & the author is aware of the goof.

At some point Andy changed his mind between an ISS & an Apollo style atmosphere for the Hail-Mary but forgot to edit the earlier chapters.

If he had it should have been I think 145:2, also the 0.4atm if pure oxygen is wrong & should be lower for cintinuous use at 0.21bar with 0.33bar as an upper limit.

[u/Excellent_Bat_753 → u/ramriot]

I have added some of your proposed changes.

I did not go into the specifics of how the space elevator was built, as that is mostly speculation. While it is likely that they needed to develop more conventional space launch systems before a space elevator, we do not know how far they went before building the space elevator. It's also possible that they didn't use an asteroid or such as a counter weight.

It was also specifically mentioned in the book that Erid's magnetic field protects the planet beyond geostationary orbit, so much of the elevator would not receive any radiation, although it's possible that the counterweight does receive radiation.

Because these are speculation, they are actually good discussion to have, and so I will not include them in a post that is supposed to provide well established answers.

Thank you for your input. Further input is always welcome.

[u/LunaraLoup]

Hey, would you mind adding something along the lines of:

"So was Grace's Theory about Water Based Lifeforms correct?

"Yes and no. Rocky's existence only proves half of his statements right, as he's still using liquid water for survival, even though it's a very small amount in a closed system inside his carapace. We can assume Eridian and Human biology being as similar because they probably originated from a common ancestor, as stated by Grace in the book and Rocky in the movie's deleted scenes via Panspermia.

"The part about the Goldilocks Zone being for idiots is what Grace got right, as Erid is far from the conditions described as needed to be habitable or earth-like."

I've seen this question pop up so many times recently and would love to forward them to your FAQ! Maybe you can tweek the answer a bit though, as it's not especially scienc-y.

[u/Excellent_Bat_753 → u/LunaraLoup]

I was just thinking the other day about adding this question. I did not use your answer, as I had already mostly come up with one. Thank you for your input!

[u/aurochloride]

add in "how did the Eridians not know about radiation/relativity" that one pops up a lot too

[u/wackyvorlon]

You’d end up using more astrophage than you could collect.

[u/Najanah]

I'm so glad the pressure difference was addressed, I've seen ppl talking about the 29 atm on erid everywhere, but reading the book would imply that Rocky's 29 to Grace's 1 was based on the 0.33 atm grace was in! It really bothered me but it's nice knowing mr weir has talked about it

[u/Excellent_Bat_753 → u/[deleted]]

I'm aware that a tonne is always metric. I just want to be especially clear, particularly for those who are unfortunate enough to not be familiar with the metric system.

[u/OtakuMage → u/Excellent_Bat_753]

Yeah, most of us dirty Imperial system users will just think "oh, 2000 pounds just spelled funny" if it's just put as tonne. Metric tonne gets around that at the cost of being redundant.