The International Space Station Turns 20
Hurtling around Earth at 17,500 miles per hour, the International Space Station (ISS) is the largest manned object ever blasted into space. Visible from most places on Earth at least once a day, the station—which is the size of a soccer field—is also the only science laboratory with permanent microgravity. Since the ISS launched, over 1500 experimental setups have been placed on board, one-fifth of which have been directly related to the physical sciences and astronomy. Astronauts have captured photographs of thunderstorms, studied how bubbles coalesce in the absence of gravity, and monitored the unusual shapes of flames in space. But astronauts aren’t the only ones making measurements. Most of the experiments on the ISS are designed by researchers and housed in freezer-drawer-sized boxes that are operated remotely from Earth. (One exception is the Alpha Magnetic Spectrometer, a particle detector hunting for dark matter that is attached to the ISS’s facade.) Here are five experiments from the ISS’s first 20 years that you may—or may not—have heard about.
A Dusty Plasma Contains a Hole
The space station’s first physics experiment, which launched in 2000, involved a dusty plasma—an ionized gas enriched with dust particles. Dusty plasmas exist in comet tails and in the rings of Saturn, for example. But they are hard to study in ground-based labs, where the tug of gravity causes particles to begin falling to the floor in a matter of milliseconds. On Earth, researchers hold the dust in place using strong electric fields, but that technique can introduce experimental artifacts. On the ISS, where gravity is one millionth of that on Earth’s surface, the dust stays suspended without intervention.
The first plasma experiment on the ISS aimed to create dusty-plasma crystals. These structures were predicted to form in dense, dusty plasmas made of similarly sized dust particles, says Hubertus Thomas of the German Aerospace Center, who has been involved in the ISS’s plasma experiments since their inception. The experiment succeeded—a variety of ordered patterns formed in the dust—but they also kicked up a surprise, Thomas says. An unexpected particle-free void developed in the center of their plasma, leaving theorists puzzled. Researchers now attribute the void’s formation to “ion drag”: A positive electric potential develops at the plasma’s center, setting up an outward-moving current that is strong enough to carry away the dust particles.
Fluid Flows Faster Through a Wet Tube
Controlling the flow of fluids in space is tricky, which poses performance problems for equipment on spacecraft. In fuel tanks, for example, drops of propellant may bounce around inside the tank and interfere with the fuel transfer process. “Nothing works in space,” says Mark Weislogel of Portland State University in Oregon. He adds that NASA continually ferries new parts to space for toilets, refrigerators, and the like to keep them functioning.
Weislogel studies fluid flows in zero-g. He designs and tests tapered conduits with cross sections that are triangules and other polygons, exploiting surface tension and wetting effects to guide fluids along some path. In a series of experiments that launched in 2004, he and others monitored how different fluids moved through these conduits, and they found some quirky behaviors. For example, prewetting a conduit can "radically" change the way it transports a fluid, Weislogel says. Fluids can switch from flowing along one wall of a polygonal conduit to flowing along another wall, and flows can speed up or slow down, all as a result of prewetting. Understanding these behaviors has allowed engineers to design better urine-processing equipment, heat exchangers, and even coffee cups. “If we're going to send people to Mars, these systems have to work perfectly,” Weislogel says.
Space Ants Are Poor Searchers
Humans aren’t the only living creatures orbiting Earth. Fruit flies, zebra fish, and ants have all made it aboard the ISS. In 2014, Deborah Gordon of Stanford University in California and colleagues launched eight ant habitats, each containing around 80 pavement ants, to probe whether the collective search behaviors of the six-legged critters changed in microgravity conditions. The study, it was hoped, would help scientists design and control future space robots that would work together in similar environments.
The ants were initially housed in a small corner of a plastic box. Over a few minutes, barriers in the box were removed, giving the ants additional space to explore. The same experiments were conducted on Earth. With access to new territory, Earth ants adjusted the shape of their paths—from spirals to straight lines—and within five minutes surveyed the whole box. Space ants, however, retained curvy trajectories and explored much less of their habitat over the same time period. In a 2014 TED talk, Gordon explains why. Ants adapt their search strategy to the density of the colony, which the insects monitor via the frequency of their interactions. In a low-gravity environment, Gordon says, the ants are “working so hard to hang on” that this relationship between density and meet-up frequency is changed, altering the ants’ search patterns.
Colloids Abound on the ISS
Studies of colloidal dispersions—tiny particles suspended in liquids—are popular on the ISS, amounting to more than 30 experiments to date. Many household products are colloids, including paints, face creams, and shampoos. The particles in those products are typically nanometers in size, which makes them buoyant but hard to image. So, instead, researchers study model systems containing larger, micrometer-sized, particles. “But for larger particles, gravity becomes a force to deal with,” says Matthew Lynch of the US-based personal care company Proctor & Gamble. The ISS takes gravity out of the equation.
Lynch helped design the ISS’s first colloid experiments, which launched in 2012. The experiments studied suspensions containing particles of two different sizes. By snapping images of the colloids, Lynch and his colleagues determined the conditions under which the particles form stable, connected structures and when they don’t—information that will help companies develop face creams with longer shelf lives, for example. The initial images captured only 2D projections of the structures, but 3D information will be provided by a new confocal microscope on the ISS. The new imaging possibilities provide “a whole different level of ability,” Lynch says.
Quantum Gases That Last for Seconds
Launched in May, the Cold Atom Laboratory (CAL)—a Bose-Einstein-condensate-creating machine—is the ISS’s newest physics setup. Robert Thompson of the Jet Propulsion Laboratory in California, and CAL’s project scientist, says that he and others first discussed creating BECs in space back in 1995, shortly after physicists first produced them on Earth. Doing so, they hoped, could up the lifetime of the cold quantum gases from milliseconds to around ten seconds.
“Initially it was a really crazy idea,” Thompson says. Producing a BEC typically requires a whole room of equipment, including specialized optical and magnetic traps. The setups are also very hands on, so stuffing everything into a freezer box and running experiments remotely seemed like a daunting task. But it became possible thanks to several technological advances. For example, in place of the chunky wire coils normally needed to generate the trapping magnetic fields, CAL employs a compact silicon chip consisting of just a few tiny wires. Designing and building the setup was a monumental but worthwhile task, Thompson recalls. CAL is still in test mode, but the system is producing and analyzing sub-nanokelvin BECs that last for seconds, Thompson says. He describes having a BEC experiment on the space station as “a dream come true.”
–Katherine Wright
Katherine Wright is a Senior Editor for Physics.