Space

74 cards about Space, with explanations, methods and source links.

Cards about Space

• What happened to an astronaut’s genes after a year in space, next to his identical twin on the ground?

  Scott Kelly spent close to a year on the space station while his identical twin Mark, himself a retired astronaut, stayed on Earth. That is about as good a control as human biology ever offers. What did the comparison show?

  Source: Garrett-Bakelman et al., Science

• Why is every look at the sky a look into the past?

  Light has a finite speed, which sounds like a technical footnote. But if it left a star a thousand light-years away, it reaches your eye a thousand years later. What does that make of what you are seeing right now?

  Source: NASA Science

• How is the distance to the Moon known to the millimeter?

  The tides are raised by the Moon’s gravity, and the relationship runs both ways: the moving water lags slightly behind the Moon and tugs it forward along its orbit. Which means the Moon is leaving. How do you measure something like that?

  Source: Dickey et al., Science

• How long do Saturn’s rings have left?

  Saturn’s rings are almost entirely water ice, with hardly any dust in them. That is odd, because space is not clean and microscopic dust should have been collecting in them and darkening them for as long as they have been there. What does their brightness give away?

  Source: Waite et al., Science

• What is actually glowing when you see the northern lights?

  The Sun puts out a constant stream of charged particles called the solar wind, and the Earth’s magnetic field deflects most of it well away from us. At the poles it does the opposite and funnels them in along the field lines. What happens when they reach the atmosphere?

  Source: NASA Science, solar storms and flares

• How many sunrises does an astronaut get through in one day?

  The International Space Station travels at about 28,000 kilometers an hour, some four hundred kilometers up. At that speed one full lap of the Earth takes about ninety minutes. So how often does the crew cross the line between light and shadow?

  Source: NASA, International Space Station

• Who wrote down a star exploding almost a thousand years ago?

  In 1054 observers in China recorded a "guest star" that appeared in the sky and stayed bright enough to be seen in daylight for weeks. A record like that reads like a legend, and there is nobody left to ask what they saw. So how do you check it?

  Source: Blandford & Bühler, Handbook of Supernovae

• What does a dust devil on Mars sound like passing right over a microphone?

  Dust devils on Mars have been photographed many times from above, and a picture says nothing about what is going on inside one. In September 2021 one passed directly over a rover whose microphone happened to be running. What did it pick up?

  Source: Murdoch et al., Nature Communications

• The surface of the Moon swings from extreme heat to extreme cold. What happens inside a deep pit?

  The lunar surface near the equator heats to about 127 degrees by day and cools to about minus 173 at night, so almost nothing can stay on it. A collapse pit has walls and shade. What goes on inside one?

  Source: Horvath, Hayne & Paige, Geophysical Research Letters

• Where did the carbon dioxide on Europa come from?

  Europa is a moon of Jupiter with an icy shell and a hidden ocean beneath it. Carbon dioxide has been identified on its surface, and it could have arrived from outside impacts just as easily as from within. How do you decide between the two?

  Source: Trumbo & Brown, Science

• Why does the storm at Saturn’s pole have six nearly straight sides?

  Voyager and Cassini saw the same structure decades apart: Voyager found a nearly perfect hexagon around Saturn’s north pole back in the 1980s, and Cassini found it still there more than thirty years later. A geometric shape that stable in an atmosphere is a strange thing. What holds it?

  Source: Morales-Juberías et al., The Astrophysical Journal

• What falls in Jupiter’s storms instead of ordinary hail?

  Juno’s microwave measurements revealed that ammonia is not spread evenly deep in Jupiter’s atmosphere, and in some layers it is simply missing. Something is carrying it downward. What falls there?

  Source: Guillot et al., Journal of Geophysical Research: Planets

• What actually flies off a comet when the sun warms it?

  A comet is sometimes described as a dirty snowball, as though it were mostly ice with a little dust in it. The Rosetta spacecraft stayed with comet 67P as it approached the sun, and measured the dust and the gas leaving it separately. What is the real ratio?

  Source: Rotundi et al., Science

• What did Cassini find inside the ice grains fired out of a moon of Saturn?

  An ocean hides beneath the icy crust of Enceladus, and cracks at the south pole fire material from it straight into space. Cassini passed through the jets from the pole and collected ice grains. What was in them?

  Source: Postberg et al., Nature

• How fast is material fired out of the volcanoes of Io?

  Io is stretched and squeezed over and over by the gravity of Jupiter and by its resonance with Europa and Ganymede, and the internal friction heats it without pause. The result is the most volcanic moon in the solar system. How powerful does that get?

  Source: de Pater et al., Annual Review of Earth and Planetary Sciences

• Water bears were fired from a laboratory gun. Which crash did they survive?

  A dried water bear enters a resistant state called a tun, in which it survives vacuum, radiation and extreme cold. The open question was whether there is any limit at all. So one was fired from a gas gun into sand bags. What did it survive?

  Source: Traspas & Burchell, Astrobiology

• A snowfield turns into a forest of blades. Why do they all face the same way?

  Penitentes are blades of snow and ice that grow in dry air at altitude, and they all face the same way. They do not pile upward: they are what remains when the snow around them vanishes. What sets the spacing between them?

  Source: Claudin et al., Physical Review E

• A nacreous cloud glows in a sky that has already darkened. What is happening high above us?

  A nacreous cloud glows in intense colors when it is already dark on the ground, and most clouds at that hour are in shadow. So it has to be far higher than they are. What goes on up there?

  Source: Tritscher et al., Reviews of Geophysics

• A forest fire grows a storm cloud above the flames. How far up can it reach?

  Above a fierce fire, scorching air and smoke rise in a strong column, and as the air cools water vapor condenses into cloud. The condensation releases latent heat that accelerates the rise further. How far up can that go?

  Source: Fromm et al., Bulletin of the American Meteorological Society

• The blue circle of Belize sits in the middle of the sea. Why are there cave stalactites inside it?

  The Great Blue Hole of Belize is a round sinkhole in the middle of a shallow lagoon, and stalactites have been found on its floor and its walls. Stalactites can grow only in a dry cave with water dripping through it. So what happened there?

  Source: Gischler et al., The Depositional Record

• Why does lightning return again and again to the same corner of Venezuela?

  Lightning is thought of as a chance event that appears wherever there is a storm. Over the mouth of the Catatumbo river in Venezuela it returns to the same corner night after night, with a steady seasonal pattern. What produces something like that?

  Source: Bürgesser et al., Journal of Atmospheric and Solar-Terrestrial Physics

• How can a distant ship look as though it is floating above the sea?

  Light travels in a straight line, so a ship beyond the horizon simply cannot be seen. Sometimes it is seen anyway, hovering above the sea and stretched into layers. What bends the rays?

  Source: Dettwiller, Comptes Rendus Physique

• How is a piece of black glass born from an impact out of space?

  Tektites are pieces of black glass found scattered across whole fields, and the usual assumption is that what fell from the sky is the impacting body itself. Except that they are made of terrestrial rock. So how did they form?

  Source: Howard, Proceedings of the Geologists’ Association

• Are the dark streaks on Mars flowing water?

  Recurring dark streaks appear in summer on steep Martian slopes and fade afterwards, which made them a possible sign of liquid water. Researchers simulated the temperature and humidity in Hale crater. What did they find?

  Source: Gough et al., Geophysical Research Letters

• What is left behind when a star swallows one of its own worlds?

  In 2020 a star in our galaxy brightened, faded, and wrapped itself in gas and dust. More than two years later the Webb telescope looked at what remained, in infrared, the light our eyes cannot see, and measured the leftovers. Which event fits what it found?

  Source: The Astrophysical Journal

• How does a world that wanders alone hold on to clouds?

  PSO J318 drifts through the galaxy with no star to light it, and it still glows with the heat left over from its own formation. Webb gathered that glow from 1 to 18 micrometers and spread it into a spectrum. What is hiding in the atmosphere?

  Source: Astronomy and Astrophysics

• What wraps a planet whose day side is permanently molten?

  The rocky planet 55 Cancri e sits so close to its star that its equilibrium temperature runs near 2,000 kelvin, hot enough for magma at the surface. Earlier observations had already argued against a leftover hydrogen and helium envelope. Webb then measured the heat coming off the planet from 4 to 12 micrometers. What does the spectrum hide?

  Source: Nature

• What enormous wave is moving through the stars around us?

  Three-dimensional dust mapping revealed a 2.7-kiloparsec chain of dense gas clouds near the Sun, bent into the shape of a wave. A shape is not a motion, though, and early searches for matching movement came back inconclusive. Researchers then added the velocities of the clouds and of the young star clusters inside them. Does the structure actually move?

  Source: Konietzka et al., Nature

• What was waiting inside a sample from an asteroid that was never opened on Earth?

  Meteorites carry organic matter, and they also land in a field, sit in the weather and pick up whatever is around them, so nobody can be sure what arrived with the rock. OSIRIS-REx collected material from the asteroid Bennu and sealed it in space. Laboratories then opened it under contamination control. What was in there?

  Source: Glavin et al., Nature Astronomy

• How do you hear two black holes colliding?

  Gravitational waves are stretches and squeezes of spacetime itself, predicted by Einstein decades before anything could measure one. On 14 September 2015, at 09:50:45 UTC, both detectors of the Laser Interferometer Gravitational-Wave Observatory caught a brief signal that swept upward in frequency. What makes a sound like that?

  Source: LIGO and Virgo collaborations, Physical Review Letters

• What does it take to move an asteroid off its course?

  Dimorphos circles a larger asteroid called Didymos, and before September 2022 one lap took 11 hours and 55 minutes. A spacecraft hit it on purpose, and the mission counted anything from 73 seconds of change as a success. Then telescopes on Earth measured the new lap time.

  Source: NASA

• Can a planet orbit two stars at once?

  A pair of stars circling each other makes a messy gravitational neighborhood, and whether a planet could hold a stable orbit around both was an open question with a famous fictional answer. Then Kepler watched a planet cross both stars in turn. What did the geometry turn out to be?

  Source: Doyle et al., Science

• What does months in orbit do to an astronaut’s eyes?

  Long-duration spaceflight produces physiologic and pathologic neuro-ophthalmic findings that do not look like anything in terrestrial medicine. They turn up often enough to have been given a name and a definition. The question for a Mars-length mission is what exactly is on the list.

  Source: Lee et al., npj Microgravity

• Why is the Sun’s atmosphere hotter than its surface?

  Move away from a fire and it gets cooler. Above the Sun’s surface the opposite happens: the surface simmers at about 10,000 degrees Fahrenheit while the corona above it spikes upward of 2 million. Decades of measurements from a distance have not settled why, and NASA calls it one of the greatest unanswered questions in astrophysics.

  Source: NASA Science

• What did the water at Jezero crater do before it left?

  Orbital images showed a fan-shaped body of sedimentary rock at the western margin of Jezero crater, which is the signature of a river delivering sediment into standing water. Orbit cannot see the faces of that fan. Perseverance landed in February 2021 and could.

  Source: Mangold et al., Science

• Has anyone ever seen the Oort Cloud?

  Long-period comets arrive from every direction, on orbits that take them far beyond the planets. In 1950 Jan Oort proposed where they come from: a vast, extremely distant spherical shell of icy bodies surrounding the solar system. The proposal explains the comets. The shell itself is another matter.

  Source: NASA Science

• What is hiding inside a cloud of cosmic dust?

  The Cosmic Cliffs look like craggy mountains on a moonlit evening, and they are the edge of a gaseous cavity in the star-forming region NGC 3324 in the Carina Nebula, about 7,600 light-years away. Dust hides what forms inside such a region. Infrared gets through it.

  Source: NASA, ESA, CSA, James Webb Space Telescope

• How far away was the camera that took the Pale Blue Dot?

  On Valentine’s Day in 1990, Voyager 1 turned around and photographed the planet it had left. The picture is famous for what it shows, which is almost nothing: Earth as a speck inside a band of scattered sunlight. The number that gives it meaning is the distance.

  Source: NASA Science

• Has starlight ever been caught building a molecule elsewhere?

  Photochemistry is a fundamental process of planetary atmospheres, regulating what they are made of and how stable they stay. It is how ozone exists above our heads. In the atmosphere of another star’s planet, no unambiguous photochemical product had been identified at all.

  Source: Tsai et al., Nature

• On which planet does the year end before the day does?

  A planet spins while it orbits, and on Earth those two clocks are separated by a factor of 365. Nothing requires them to stay that far apart. The rotational period of one of our neighbors stayed unknown for a long time, because its clouds never let anybody watch the surface turn.

  Source: Royal Belgian Institute for Space Aeronomy

• What did the strongest SETI candidate ever recorded look like?

  On 15 August 1977 the Ohio State Radio Observatory was doing what it did every day, scanning overhead skies for narrow-band signals as part of a search that would run 22 years. Nobody noticed anything that night. The record of what happened is a line of characters on a printout.

  Source: SETI Institute

• What would make low orbit unusable without a single new launch?

  Most space debris large enough to cause a catastrophic collision, meaning larger than 10 cm, comes from the more than 500 on-orbit fragmentation events in spaceflight history. Launches add to the population. The worrying part is what the population does on its own once it is dense enough.

  Source: ESA Space Debris Office

• How old is the sunlight falling on your face?

  Sunlight takes just over 8 minutes to cross the 150 million kilometers from the Sun to Earth, which is the number everybody knows. The energy carrying it was made in the core, and the core is 695,700 kilometers below the surface. Getting out of the Sun is a different journey from crossing space.

  Source: ScienceAlert

• What is pulling the Milky Way, and why can we not see it?

  Major deviations from universal expansion turned up in the 1970s and 1980s, with our galaxy and thousands of others moving toward a diffuse concentration of mass some 250 million light-years away. Astronomers named it the Great Attractor. The direction it lies in is the one direction we cannot look.

  Source: Staveley-Smith et al., Astronomical Journal

• Why does the bright spot beside the sun always sit in the same place?

  Water drops and ice crystals in the atmosphere can act as a prism, splitting visible light into the colors that make it up. That is what produces halos, sundogs and sun pillars. A sundog looks like an accident of the clouds, yet it turns up at a fixed distance from the sun every time.

  Source: National Weather Service, La Crosse

• Is that column of light above the sun actually a beam?

  A sun pillar appears as a shaft of light standing vertically above the sun, most often at sunrise or sundown. It looks like a searchlight pointing up out of the horizon. Nothing in the sky is emitting a beam, and the explanation is the opposite of the one that produces a rainbow.

  Source: National Weather Service, La Crosse

• How much smaller is Jupiter’s Great Red Spot than it used to be?

  The Great Red Spot is an anticyclonic storm larger than Earth, a deep red eye set in swirling layers of pale yellow, orange and white, with winds reaching several hundred kilometres per hour. Astronomers have known since the 1930s that it is shrinking, which raises the obvious question of by how much.

  Source: ESA/Hubble

• What does a photograph of a black hole actually show?

  A black hole emits no light, so the thing itself cannot appear in any image. General relativity predicts something adjacent: immersed in a bright region such as a disc of glowing gas, a black hole should carve out a dark region. Nobody had ever seen it.

  Source: ESO

• Who discovered dark matter?

  Dark matter is a central element of modern cosmology, and the standard model of the universe does not work without it. The story of how it got there is usually told as an anecdote about a few pioneering scientists. Bertone and Hooper set out to check that telling against the record.

  Source: Bertone and Hooper, Reviews of Modern Physics

• What makes the clouds on Neptune come and go?

  Neptune orbits thirty times further from the Sun than Earth, which makes sunlight there faint enough that it is easy to treat as irrelevant to the weather. The planet’s brightness in the near-infrared rises and falls over years. Archival images going back to 1994 are enough to ask what that tracks.

  Source: Chavez et al., Icarus

• Why is a rainbow in fog white?

  A rainbow forms when sunlight enters a water droplet, slows and bends, reflects off the inside surface and bends again on the way out. What comes back is a spectrum, aimed at an observer who has the sun behind them. Fog is water droplets too, and the geometry still works. What comes out is pale and almost colorless. What changed?

  Source: Met Office, Rainbows

• Why do the colored rings follow your own shadow and nobody else’s?

  Look down from a mountain or an aircraft onto cloud or fog with the sun behind you, and you may see a set of colored rings on the cloud below. They sit around the head of your shadow. Somebody standing beside you sees their own set, around their own head, and not yours. Why is it fixed on the observer?

  Source: NOAA National Weather Service

• Does a rainbow made by moonlight have colors?

  Moonlight is reflected sunlight, so it carries the same spectrum, and a water droplet does not care which direction its illumination arrived from. Rainbows do occur at night, made by the moon rather than the sun. They are rare, and they need a nearly full and low moon, a dark sky and water droplets opposite it. To the naked eye they look white.

  Source: Met Office, Rainbows

• Why do sunbeams fan out from a point in the clouds?

  Through a gap in cloud, sunlight sometimes arrives as distinct slanted bands, alternating with shadow. They are crepuscular rays, named from the Latin for twilight, when they show most clearly. They appear to spread outward from a single point behind the cloud, which would mean the sun is close enough for its light to diverge noticeably. It is not. So what is going on?

  Source: NOAA National Weather Service

• What was the band of rainbow colors over Spokane with no rain anywhere?

  On the afternoon of the third of June, two thousand and six, the sky over Spokane and Post Falls carried what looked like a rainbow. There was no rain. It also failed to form the high arch a rainbow makes near a shower. Sun dogs were the popular guess, and they are far less vivid and much smaller than what people saw. So what was it?

  Source: NOAA National Weather Service, Spokane

• Why is the day not the same length it has always been?

  Tides and the Earth-Moon system are coupled over geological time. Energy dissipated by tides on Earth slows the planet’s rotation and pushes the Moon’s orbit outward, while dissipation inside the Moon pulls in the other direction on its inclination, eccentricity and distance. Daher and colleagues integrated the whole system backwards for four and a half billion years. What does that give?

  Source: Daher et al., Journal of Geophysical Research: Planets

• Why are there eighteen moons on one early photographic plate?

  A daguerreotype of about two and a half inches by two, recently discovered and now at the Metropolitan Museum, carries eighteen tiny but detailed impressions of the moon scattered across its metal surface. It was made somewhere between eighteen forty-six and eighteen fifty-two, when photography was barely a decade old. Why eighteen of them on one plate?

  Source: The Metropolitan Museum of Art

• What was the hiss two radio engineers could not get rid of?

  At Bell Telephone Laboratories in the early nineteen sixties there was a receiver built for satellite communications, with remarkably low noise of its own. When it became available for research, Penzias and Wilson pointed it at the sky to study the faint microwave background, tuned to seven centimeters, where the intensity should have been very low. They measured a comparatively high one.

  Source: The Royal Swedish Academy of Sciences

• How much would a spoonful of a collapsed star weigh?

  A neutron star packs more than the mass of the Sun into something about twenty kilometers across, which is a city rather than a star. Matter cannot normally be compressed anywhere near that far, and the usual way to convey what it means is to imagine taking a small sample of it. So take a teaspoon. What are you holding?

  Source: Britannica

• What is blowing past the Earth at four hundred kilometers a second?

  Comet tails point away from the Sun regardless of which way the comet is heading, and the Earth’s magnetosphere trails off in the same direction. Something is streaming outward past both of them, continuously, and it is not light. It comes off the Sun hot enough to escape the Sun’s own gravity. What is in it?

  Source: Britannica

• Why do some worlds keep an atmosphere and others cannot?

  Air is not held down by a lid. The molecules of a gas are moving at speeds that depend on temperature, in every direction, and a world keeps them only if its gravity can outrun them. That gives a sharp criterion with two numbers in it: how fast you need to go to leave, and how fast the gas is already going.

  Source: Britannica

• Where does the Sun end?

  The Sun has no surface to land on, so saying a spacecraft reached it needs a definition. Physics supplies one: a boundary called the Alfven critical surface, below which the magnetic field is strong enough to dominate the plasma and hold it in causal contact with the star. Above it the solar wind has broken away for good. In April two thousand and twenty-one, something crossed it.

  Source: Kasper et al., Physical Review Letters

• What is the bright heart on Pluto made of?

  New Horizons flew past Pluto in two thousand and fifteen and sent back a world with a pale heart-shaped region on one side, far brighter and smoother than the cratered ground around it. Pluto’s surface is geologically complex because its ices move on seasonal and longer timescales. Lewis and colleagues mapped which ice is where. What is the heart?

  Source: Lewis et al., Icarus

• How does the planet closest to the Sun keep water ice?

  Mercury has almost no atmosphere and daytime temperatures that would boil lead. It also has craters near its poles whose floors never see sunlight, because the planet’s axis barely tilts. Radar from Earth found bright deposits inside them decades ago. Eke, Lawrence and Teodoro set out to measure how much is actually there.

  Source: Eke et al., Icarus

• Why does one of Saturn’s moons look like a flying saucer?

  Pan orbits inside a gap in Saturn’s rings. Photographs show a small moon with a smooth ridge running right around its equator, wide enough to make the whole thing look like a dumpling or a saucer. Atlas has one too, and Daphnis. A ridge that sits exactly on the equator is a clue about where the material came from.

  Source: Quillen et al., Icarus

• What did we bolt to the outside of the spacecraft leaving the solar system?

  Voyager one launched in nineteen seventy-seven to visit Jupiter and Saturn, and kept going. In nineteen ninety-eight it overtook Pioneer ten to become the most distant human-made object, and in two thousand and twelve it crossed the heliopause into interstellar space. Each of the two Voyagers also carries a greeting to whoever might eventually find it. In what format?

  Source: Britannica

• What are the brilliant white patches on the floor of a crater on Ceres?

  Ceres is the largest asteroid and the first to be discovered, and it is not bright. Its surface reflects about nine percent of the light that hits it, which is darker than asphalt. When Dawn arrived in twenty fifteen it found two patches inside Occator crater that are dazzling by comparison, bright enough to be visible from tens of thousands of kilometers away.

  Source: Britannica

• How long does a footprint last where there is no weather?

  The photographs of boot prints in lunar dust are sharp enough to show the tread pattern, which tells you the surface takes an impression well. On Earth such a print would be gone in a season, erased by rain and wind. The Moon has neither. What it does have is a surface built entirely by something slower.

  Source: Britannica

• What does a solar eclipse look like from above?

  From the ground a total eclipse is an event in the sky, and the experience lasts at most seven and a half minutes. From orbit it is something else entirely: a dark patch lying on the Earth and moving across it. The patch has a structure, with a sharp core and a soft surround, and the geometry explains both.

  Source: Britannica

• How do you show that time runs slow for a fast particle?

  Cosmic rays hitting air nuclei high in the atmosphere make muons, which decay with a known mean lifetime. Moving at nearly the speed of light, a muon should not survive the trip to sea level. Plenty arrive anyway. Coan, Liu and Ye built a detector small enough to carry up a mountain and measure how many stop in it at each altitude.

  Source: Coan et al., American Journal of Physics

• Why does the Moon turn red instead of going dark?

  In a lunar eclipse the Moon enters the Earth’s shadow, opposite the Sun, and the shadow sweeps across its surface. Unlike a solar eclipse, this one is visible from everywhere on Earth that the Moon is above the horizon. If the Earth simply blocked the sunlight the Moon would go black. It usually does not.

  Source: Britannica

• How do you measure an aurora from inside it?

  An aurora happens between about eighty and a few hundred kilometers up, which is too high for balloons and too low for satellites to linger. Ground instruments see it from below, radar probes it at a distance, and neither gets inside the plasma. The answer is to fire a rocket through one, and to make it come apart on the way.

  Source: Giono et al., Journal of Geophysical Research Space Physics

• What happens if you drop a hammer and a feather where there is no air?

  Everyone is taught that gravity accelerates all objects equally, and everyone has also watched a feather drift while a hammer drops. Air resistance accounts for the difference, which means the claim can only be tested somewhere without air. In August nineteen seventy-one the commander of Apollo fifteen was standing in exactly such a place, holding both objects.

  Source: Britannica

• If the universe is endless and full of stars, why is the night sky dark?

  Take two assumptions that sound harmless. The universe is endless, and it is uniformly populated with luminous stars. Then every line of sight you could pick must eventually end on the surface of a star, so the night sky should be bright everywhere with no dark gaps between the stars. It plainly is not. The question is which assumption the darkness is refusing.

  Source: Britannica

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