Chemistry Without Gravity: What Changes on the International Space Station, and How Student Experiments Get There

Gravity is so constant in every chemistry lab you've ever worked in that it's easy to forget it's doing any chemical work at all. It is: gravity drives convection, settles denser fluids below lighter ones, and pulls flames into their familiar teardrop shape. Remove it, and reactions that look identical on paper can behave completely differently in practice. That's exactly what happens on the International Space Station's National Lab, where student-designed experiments — alongside professional research — fly routinely aboard SpaceX cargo missions. I serve on one of the review boards that decides which student proposals are actually ready to fly, which means this reading is really two things at once: the chemistry of what changes without gravity, and an honest look at how "an experiment" becomes "a payload."

What gravity is actually doing in a normal reaction

In a beaker on a lab bench, gravity drives two transport processes that chemists mostly take for granted: convection (bulk fluid movement caused by density differences — hot, less-dense fluid rises; cool, denser fluid sinks) and sedimentation (denser particles or precipitates settling downward). Both processes mix reactants and remove products far faster than molecular motion alone would. In microgravity, both effects drop out almost entirely. What's left is diffusion — the slow, random thermal motion of individual molecules — as the dominant, often only, transport mechanism. A reaction that is normally convection-mixed within seconds can, in orbit, depend on diffusion happening over minutes or hours instead, and that change in timescale and uniformity changes what actually forms.

Three real, published examples of what changes

Protein crystal growth. On Earth, a growing protein crystal generates a "depletion zone" of lower solute concentration around itself, and convection constantly resupplies fresh protein to the growing surface — often too fast and too unevenly, producing smaller, defect-riddled crystals. In microgravity, without convection, that depletion zone persists undisturbed, and molecules arrive at the crystal surface by diffusion alone, slowly enough that each one has time to find its lowest-energy, best-fit position before the next one arrives. The published result, replicated across many ISS macromolecular crystal growth studies, is measurably larger and more ordered crystals — which matters enormously for structural biology, since crystal quality directly limits the resolution at which scientists can determine a protein's 3-D structure by X-ray diffraction.

Combustion and flame shape. A candle flame on Earth is teardrop-shaped because hot combustion gases are less dense than the surrounding air and rise by convection, dragging the flame into its familiar upward point and pulling in fresh oxygen behind it. ISS combustion studies of diffusion flames (including spherical ethylene diffusion flames burning around a small fuel source) show flames that are far more spherical and symmetric, because without buoyant convection, oxygen reaches the flame front only by diffusion from all directions equally. These studies also reveal shape and luminosity asymmetries invisible in Earth-bound combustion, information that feeds directly back into spacecraft fire-safety engineering.

Solidification and alloy segregation. When a molten alloy cools and solidifies on Earth, convection currents constantly stir the melt, which changes how different components segregate as the solid forms. ISS solidification experiments using model transparent alloys, tracked in real time with optical videomicroscopy, let researchers isolate diffusion-driven segregation patterns from convection-driven ones for the first time — distinguishing two effects that are impossible to separate in a gravity-bound lab because they always happen at once.

From an idea to a launch: how a proposal actually gets reviewed

None of this happens because a student mails an experiment to NASA. Programs like the Student Spaceflight Experiments Program (SSEP) run a formal two-step review process before anything is cleared to fly, and I sit on one of these proposal review boards, which is a different job than being a chemistry teacher and worth explaining honestly.

In Step 1, a local review board — researchers and science educators from the sponsoring community — evaluates every student proposal against a formal, published set of criteria and narrows the field down to a small number of finalists. In Step 2, a national review board, organized into teams that each pair working researchers with experienced science educators, evaluates the finalists from multiple communities side by side and selects the actual flight experiments — the ones that will be built, integrated into a certified flight hardware unit, and carried to the ISS National Lab on an upcoming resupply mission. What a reviewer is actually checking for at both steps is not "is this a cool idea," but specific, answerable questions: does the proposal identify a real, testable hypothesis about how microgravity specifically changes the outcome, rather than just moving an ordinary experiment to orbit for novelty's sake? Is the procedure something that can genuinely be executed by an astronaut, inside a small, sealed flight tube, with no ability to improvise if something goes wrong? Are all reagents and materials safe to fly on a crewed spacecraft? And is there a real Earth-based control the student team can run in parallel, so the microgravity result actually means something scientifically rather than being a single, uncontrolled data point?

That last question is usually where the most promising-sounding proposals fail on a first pass, and where the review board's feedback matters most: a strong idea with a vague mechanism gets sent back for revision, not rejected outright, because the goal of this kind of review isn't gatekeeping for its own sake — it's making sure that what does fly is good enough science to be worth the extraordinary cost and limited opportunity of a real launch.

Microgravity

The condition of continuous free-fall experienced in low Earth orbit, in which objects and fluids experience only extremely weak apparent gravitational effects; not the complete absence of gravity, but the near-absence of its usual effects like convection and sedimentation.

Convection

Bulk fluid movement driven by density differences, typically caused by temperature differences (buoyancy); the dominant mixing process in most Earth-bound liquids and gases, and largely absent in microgravity.

Diffusion-limited growth

A growth or reaction process whose rate is controlled by how quickly molecules arrive by random thermal motion (diffusion) rather than by bulk fluid transport (convection); the dominant regime for crystal growth and many reactions in microgravity.

Proposal review board

A panel of scientists and educators who evaluate research proposals against formal criteria — scientific rigor, feasibility, and safety — before resources (such as a spaceflight opportunity) are committed to them.

Check your understanding

Sources: NASA, "Macromolecular Crystal Growth" (ISS National Lab mini-book); Buffalo HWI, "Microgravity as an Environment for Macromolecular Crystallization"; PMC, "Tracing Transport of Protein Aggregates in Microgravity versus Unit Gravity Crystallization"; MDPI Fire, "Spherical Diffusion Flames of Ethylene in Microgravity: Multidimensional Effects"; PMC, "Microgravity Studies of Solidification Patterns in Model Transparent Alloys Onboard the International Space Station"; Student Spaceflight Experiments Program (ssep.ncesse.org), Step 1 and Step 2 Proposal Review Board documentation. Review-process description reflects the author's own experience serving on an SSEP proposal review board. DRAFT — verify current 2018 Virginia Science Standards of Learning chemistry codes with the current Curriculum Framework before publishing.