Why You Probably Won't Be Living on Mars Anytime Soon

In one sentence: Kelly and Zach Weinersmith, authors of A City on Mars, walk through what it would actually take for humans to settle Mars soon — and on almost every front (bones, radiation, food, reproduction), the honest answer is "we don't know yet."

Key Findings

• Bone loss adds up fast, by the month. In zero gravity, hip bone density drops "about 1% ... per month" — Zach Weinersmith stresses "per month." The longest anyone has spent continuously in space is 437 days; almost no one has been up for a full year, so long-duration data is thin.
• Mars gravity is 40% of Earth's — nobody knows if that's enough. Orbital stations put the body in something close to zero gravity ("like on a roller coaster"). Mars surface gravity sits at roughly 40% of Earth's, somewhere between weightlessness and Earth-normal. Zach: "there's some world in which 40% Mars gravity makes things just fine" — or it might take a weighted suit. "But we don't know is the problem."
• Radiation pushes settlement designs underground. The Earth-to-Mars transit takes "a minimum of six months" with current technology, and spacecraft shielding isn't quite enough. Once on Mars, there's no planet-wide magnetosphere and the atmosphere is "only 1% as thick" as Earth's, so most radiation reaches the surface directly. Kelly: most serious proposals involve "burying our habitats under a few meters" of Martian dirt — "we're living like mole people underground," not the glass-domed cities of concept art.
• Perchlorates in the Martian soil complicate both farming and meat. Regolith contains perchlorates — "endocrine-disrupting hormones" that disrupt adult metabolism and fetal development — and plants grown in perchlorate-laced soil pull the chemical into their tissue, so settlers would need to strip it from soil and water or switch to hydroponics entirely. Livestock fares worse: chickens and cows are "too big," "too messy," "too hard to keep alive," leaving vegetarianism or insect protein as the main options in most proposals.
• Reproduction research in space is nearly nonexistent. What exists is, in Zach's words, "a grab bag of experiments" — some invertebrates, some vertebrates, "a very small number of ... rodent studies," nothing close to human data, and no generational study of vertebrate mammals at all. Rat mothers sent to space during pregnancy came back experiencing labor contractions "twice as often." Kelly: "I wouldn't want to be the first woman on Mars, hoping my hips are going to hold out when labor kicks in."
• Nearly all existing spaceflight data comes from middle-aged men. Women weren't allowed into the U.S. astronaut program until 1978 (Russia flew women twice earlier); to date, only about 10% of everyone who has been to space is a woman. Data on how children or teenagers would develop in space is simply absent.

Scientific Background

In low Earth orbit — aboard the ISS, for instance — the spacecraft is still within Earth's gravitational field, but it and everyone aboard are in continuous free fall around the planet, which feels essentially like zero gravity. That's the main driver of rapid bone- and muscle-density loss in astronauts. Mars is a different regime: surface gravity is about 40% of Earth's, somewhere between weightlessness and home, and there's almost no real data on how the human body responds there — only speculation.

Radiation protection works on a separate logic. Earth's global magnetosphere deflects most charged particles; ISS astronauts largely share in that protection and don't stay up long enough to accumulate much dose. But the six-month transit to Mars offers only limited shielding from the spacecraft itself, and Mars has neither a global magnetosphere nor a thick atmosphere (just 1% of Earth's), so radiation reaches the surface at much higher, sustained levels. That's why "live underground" is the near-universal conclusion of serious settlement proposals, rather than the sunlit glass domes of concept art.

Bone loss connects to a second chain: bone formation depends on vitamin D and calcium, and vitamin D synthesis depends on sunlight — which underground living mostly eliminates. Kelly points out that if this chain breaks down in adult women who are already losing bone density under reduced gravity, and they're also trying to grow a fetal skeleton, the risks compound — part of why she worries "Mars babies" could be born with more fragile bones than intended.

People

Kelly Weinersmith — biologist and science writer, trained as an ecologist specializing in parasitology (she describes herself as someone who loves "the weird, creepy things you find inside of the more charismatic megafauna"). She and her partner Zach spent four years researching space settlement for a book the episode refers to as City on Mars (i.e., A City on Mars), covering whether humans can — and should — settle Mars at scale. Throughout the interview she repeatedly says the honest answer to key questions is "we don't really know."

Zach Weinersmith — Kelly's partner and co-author on the book. In this episode, he's the one puncturing the romanticized image of Mars colonization — noting that the "guy walking toward the horizon dramatically" GIF that circulates on social media bears little resemblance to actual ISS life ("scrubbing the toilets," managing mold, cooking, "managing interpersonal relationships"). He delivers the episode's heaviest closing line: even $10 trillion "could not... make Earth as bad as Mars" — "this is the only option, and it will be that way for a very long time."

Odd Details & Trivia

• The social-media shorthand for "going to Mars" is a GIF of someone walking dramatically toward the horizon; Zach says real ISS life is mostly scrubbing toilets, managing mold, cooking, and "managing interpersonal relationships" — "a lot like Earth."
• Vision loss in space is unexplained and happens "even on short trips"; the mild fix is "space adjustment goggles," but the worry is undiagnosed nerve damage with possible long-term cognitive effects.
• Astronauts lose the "baroreflex" — the mechanism that prevents dizziness when standing up — to the point that simply walking after a few months back on Earth is considered "very impressive."
• CO2 concentration on the ISS runs "orders of magnitude" higher than on Earth, and astronauts often get headaches from it; certain bacterial infection rates also rise in space, possibly from stress or from bacteria floating more freely in zero gravity — the episode doesn't settle on a cause.
• Production credits: produced by Jessica Yung, edited by showrunner Rebecca Ramirez, fact-checked by Tyler Jones, engineered by Gilly Moon; Beth Donovan is senior director, Collin Campbell is senior VP of podcasting strategy.

Cross-Domain Connections

This episode and shortwave-mars-settlement (2024-11-13) are both interviews with Kelly and Zach Weinersmith about the same book, but they're separate recorded conversations with about 23% content overlap and otherwise distinct material — this page covers only the 2025-04-21 interview; see related in the frontmatter for the cross-link.

Between the science domain and the acquired (business history) domain, there's no honest connection to draw yet: the acquired domain currently holds only a raw Tier 0 mirror of a SpaceX episode (sources/acquired/2020-05-26_spacex_en.md), not yet extracted into a Tier 1 page, so there's nothing in the library to link. Zach's line about Elon Musk "putting on a cowboy hat and flying off to Mars" would be a natural point of resonance if that episode is ever extracted — but that's speculative for now, not forced here.

Pages to Build

• space-radiation — the radiation-exposure and shielding problem once humans leave Earth's magnetosphere spans orbital flight, Earth-Mars transit, and the Mars surface alike; it also surfaces in the library's existing Artemis/lunar roundup episodes, making it worth a standalone page.
• perchlorates — the perchlorate contamination in Martian regolith is this episode's most concrete, reusable chemistry/physiology concept; sources/shortwave/ holds two more unextracted 2025 Mars episodes (2025-09-05, 2025-09-12) likely to reference it again.

Source · shortwave