# Project Hail Mary Stellar Map

## 2023-12-04 (originally 2021-11-31)

Project Hail Mary is a 2021 science fiction novel by Andy Weir - and a really good one. However, the book doesn't have a map of the relevant parts of space from the book. I think it needs a map, so here's a map!

The following map shows our local stellar neighborhood. Every dot represents a rogue planet or a star system (a star system is a star or set of stars that orbit each other, along whatever objects like planets that orbit those stars). For brevity, I'll call each rogue planet or star system a "stellar object". The name of the stellar object is next to it. Near the bottom center of the map, labelled in bold, is our Solar System (a star system centered on our own star, Sol). All distances in this map are measured in light-years (the distance light travels in 1 year in a vacuum). The directed lines show the travel of Astrophage (more about that in a moment).

Click on the map to see just the map (which will will enlarge it); more information follows the map.

# Representation challenges

One challenge with maps of space is that space is 3-dimensional, but printed maps are 2-dimensional. This map shows, on a 2-D plane, the location of each object on a plane going through the celestial equator (a plane that aligns with the equator of the earth in 2000). In front of every name are square brackets containing the number of light-years the object is above or below this plane. We'll center its height on our own Solar System (so its "height" is 0.0). This map only shows the stellar objects between -5 and +5 light-years of height in the celestial plane, to make the map simpler. Here's an example that might help you understand the map: if we added Polaris (the North Star) to this map, we would add a dot almost on top of the Solar System and write "[430] Polaris" to note that Polaris is 430 light-years in height.

I have made a minor exception with Alpha Centuri, the star system closest to our own. Alpha Centauri is a triple star system, and its star Proxima Centauri so much closer to our Solar System compared to its other stars Alpha Centauri A and B that their locations are visibly different even at the scale of this map. Thus, instead of one dot, the Alpha Centauri star system has two adjacent dots, one for Proxima Centauri and the other for the pair Alpha Centauri A and B.

# Directed lines represent Astrophage

The directed lines show the spread of Astrophage per pages 95-96 (all page numbers are per the hardcover 2021 release). Centered on each directed line is the distance of that trip (shown in italics and measured in light-years). Note that the trip distance between each of these stellar objects is less than the 8 light-year maximum transmission range of Astrophage. Some are close, such as the 7.8 light-year distance from Epsilon Eridani to Sirius, but they are all less than 8 light-years.

Let's follow the directed lines from their source.

• Tau Ceti. It's eventually revealed in the book that Astrophage started at Tau Ceti. On this map this star is notated in bold near the right center. Tau Ceti is a main sequence single star with a mass of 0.783 the mass of Sol. Astrophage then spread to Epsilon Eridani.
• Epsilon Eridani. This star is only 10.5 from Sol, making it the third-closest star system visible to the naked eye. It's a single star. It's also a relatively young star, estimated to be less than one billion years old. In the (different) science fiction TV series Babylon 5, the Babylon 5 space station is located in this system.
• 40 Eridani. Astrophage eventually ended up on Rocky's system of 40 Eridani (per page 204). This star system is noted in bold text on the top right of this map. The novel doesn't expressly say it, but by looking at the locations of stars it's almost certain that Astrophage spread from Epsilon Eridani directly to Rocky's home system of 40 Eridani. 40 Eridani is actually a triple star system. This is probably the location of the planet Vulcan, in the (different) science fiction series Star Trek (and thus 40 Eridani would be the sun of Mr. Spock's home world). The science fiction series Star Trek never officially gives the location of Vulcan, but Vulcan's Sun (Sky & Telescope, July 1991) notes that the two main candidates are 40 Eridani and Epsilon Eridani, and Epsilon Eridani is a far more likely answer. The letter was co-signed by Gene Roddenberry, creator of Star Trek, and that seems pretty canonical to me.
• Sirius. Astrophage also spread to Sirius from Epsilon Eridani. Sirius is the brightest star in our sky (it's bright to us in part because Sirius is relatively close to us). It's really a pair of stars, Sirius A and Sirius B.
• WISE 0855-0714. Astrophage spread to WISE 0855-0714 from Sirius. There are so many strange things about this that I've created a whole separate section discussing WISE 0855-0714.

Finally, from WISE 0855-0714, Astrophage spread to at least 4 other star systems: Wolf 359, Lalande 21185, Ross 128, and our own Solar System (page 95). The novel goes back and forth chronologically, but almost all the events occur after the humans detect the effects of Astrophage on our own Solar System (the system gravitationally bound to Sol). Astrophage has almost certainly spread to other star systems as well, but these are the only one identified in the book. Let's deal with the weird one: WISE 0855-0714.

# WISE 0855-0714

That path seems straightforward. However, WISE 0855-0714 is where things get weird on this map. First, a quick acknowledgment: the full designation of this system is WISE J085510.83-071442.5. That's too much, so we'll shorten it (sort of) to WISE 0855-0714 or sometimes WISE.

WISE 0855-0714 was only discovered in April 2014 and isn't actually a star. Its mass is so low that it can't fuse any atoms like a main-sequence star or a brown dwarf. It's only 3-10 Jupiter masses (MJ). Normal hydrogen thermonuclear fusion reactions require .08 mass of the Sun - so a main-sequence ("normal") star requires at least around 89 Jupiter masses. Its mass is even below 13 Jupiter masses, the minimum (limiting) mass for thermonuclear fusion of deuterium that is necessary to create a brown dwarf. Its temperature is not like a typical star either: -48 to -13 °C (-55 to 8 °F). Objects like these are often classified as a sub-brown dwarf. It could also be called a rogue planet. It's easy to quibble about categories. However, since it can't fuse any atoms, it's more like a massive rogue planet than a star in the usual sense, so that's what I'll call it here.

But could WISE 0855-0714 support Astrophage reproduction? All the other stellar objects on this journey are star systems. It's easy to imagine that they each have a star and at least one planet with the carbon dioxide necessary for Astrophage to reproduce. However, WISE 0855-0714 is different! Could Astrophage regain enough energy (possibly by reproducing) to travel to other stars?

Let's first deal with Nitrogen. For unclear reasons, normal Astrophage is killed by Nitrogen. In the real world I haven't found information about Nitrogen and WISE 0855-0714. Its atmosphere seems evidence of sulfide clouds below water ice clouds, along with methane. So we don't know if there's a lot of Nitrogen on this dwarf planet. A tiny percentage of Nitrogen is fine; Sol has a little Nitrogen and it didn't kill Astrophage.

To see if it's justifiable that Astrophage could reproduce there, let's first look at the Astrophage lifecycle (primarily from pages 84-85 and 90-91):

1. Get energy. This requires a temperature over 96.415 °C (page 219), but it's faster at higher temperatures and Astrophage are basically immune to heat (they absorb it when they aren't full and they emit the rest). Typically this will occur on a star surface. This will continue until they're ready to breed (aka "full" or "enriched"), which is when it's consumed 1.5 megajoules (17 nanograms) of energy (page 97). For some unexplained reason the process is self-limiting once the star dims by 10% (page 95). There are many ways biological systems might end up with such a limitation, so while unexplained it's reasonable enough.
2. Travel along a north magnetic pole. This lets it find a source of carbon dioxide (otherwise it could only see a star if it's on one).
3. Search and travel towards a carbon dioxide source. This is done by looking for light at 4.26 and 18.31 microns. In our Solar System, that source is Venus.
4. Reproduce. Once it's at the source of carbon dioxide (such as Venus) it will make a copy using its remaining energy, carbon dioxide, and other materials available.
5. Both mother and daughter travel to an energy source. Say, a star. Now the process repeats.

So, could Astrophage at WISE 0855-0714 eventually end up at other stars? At the very least this requires the Astrophage to find a location on WISE 0855-0714 with a temperature over 96.415 °C, so it can get energy. That way, Astrophage looking for an energy source (like a star) will have one to find. At first this looks impossible, since WISE 0855-0714's average temperature is -48 to -13 °C. However, averages hide a lot; in 2021 Earth's average temperature was 14.7 °C (58.4 °F) yet the highest and lowest air temperatures on Earth are likely more than 55°C apart (per "Climate Change: Global Temperature" by Rebecca Lindsey and Luann Dahlman, June 28, 2022, Climate.gov). What's more, there are places on Earth (like lava flows) that have locations with far higher exposed temperatures. WISE 0855-0714 is probably a gas giant, so Astrophage could simply fall into it until it found pockets where that temperature (energy) is available. In short, it's very plausible that it could find spots above 96.415 °C even though its average isn't that high. What's more, Astrophage would be attracted to such sources.

After Astrophage fills up on energy, it will need to find a way away from its energy source (its "star") by using a strong magnetic field. Again, this seems plausible. WISE 0855-0714 is likely to have a strong magnetic field. After all, all four of our gas giants have magnetic fields. So once it's filled with energy, it can simply follow the magnetic field that is almost certainly present.

Could Astrophage have entire reproduction cycles completely at WISE 0855-0714? That's important, because if it could reproduce on the rogue planet, that would greatly increase the likelihood that it would spread to other worlds (simply because there's more trying).

I think it's plausible that Astrophage could reproduce completely at WISE 0855-0714. The key reason is that it's plausible that the rogue planet has carbon dioxide. If that's true,, the rogue planet could fulfill the role of both "star" (energy source) and "planet" (carbon dioxide source). Any Astrophage leaving the rogue planet looking for the "other role" has a good chance of rediscovering the rogue planet. The lack of energy on the rogue planet might encourage Astrophage looking for energy to go looking elsewhere, as the rogue planet wouldn't "look" as much like a star, resulting in faster spread. It's not clear if lower-energy Astrophage could make the trip to another star, given its lack of evergy, but it might. We know that some Astrophage (likely the full energy ones) can make the trip.

No matter what, WISE 0855-0714 will have lower available temperatures compared to a star. As a result, it would have taken far longer for Astrophage to have gathered energy from this rogue planet instead of its usual diet of stars. I haven't tried to calculate that, but I suspect it would be a long time depending on the temperature available. Perhaps it's best if we don't examine this question too closely :-). Still, both humans and Eridians were able to do it, and Astrophage would have sought out energy sources, so I think it's at least plausible.

# Miscellaneous

StarForce: Alpha Centauri is a board game published in 1974 by Simulations Publications Inc. (SPI) that uses a game map based on the real world positions of stars. My page Project Hail Mary and StarForce: Alpha Centauri shows how Project Hail Mary maps to positions on the StarForce game board.

This map is based on data from the Wikipedia List of nearest stars and brown dwarfs. Putting Your Stars in Their Places by Greg Scalise (2003) explains how to convert the usual astronomical coordinate system into the X-Y-Z system used here. In the X-Y-Z system used here, Right Ascension of 0 degrees is directly to the right ('east') of the point (0,0). I created the map using a very common set of data analysis tools (which I already knew how to use), including Python, pandas, numpy, and matplotlib. The code is public and available as open source software if you want to examine or improve it.

Many science fiction stories use real star systems as part of their plot. In some cases those star systems are too far away to be included. For example, in Dune, the ancestral home of House Atriedes is the planet Caladan (Dan) in the star system Delta Pavonis (19.9 ly from Sol), the ancestral home of House Harkonen is the planet Giedi Prime (Gammu) in the star system 36 Ophuichi (19.4 ly from Sol), and the planet Arrakis (also Dune and Rakis) is in the Canopus star system (312.7 ly from Sol).

I love hard science fiction (where there's an effort to make the plot believable based on our current knowledge of science). Project Hail Mary manages to combine science, a sense of adventure, and a sense of humanity in one awesome book. I appreciate gems like this book!