Questions to Ask About Space
For curious children, students, parents and teachers who want good questions about space to put to a science teacher, an astronomer, a planetarium guide or each other. The list travels outward the way the conversation tends to: the solar system, stars and black holes, the size and age of the universe, life elsewhere, going to space, and then what to do next. Each question has a note with the short answer, what is still open and where to take the conversation from there.
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The questions
Each question, and why to ask it
Solar system
Why does the Moon look different every night?
Why ask it
Half of the Moon is always lit by the Sun, and what changes over the month is how much of that lit half is turned toward us as it circles Earth. Be wary of an answer that blames Earth's shadow, which only explains eclipses. A lamp, a ball and a dark room will show it faster than any diagram.
How many planets are there, and why is Pluto not one of them anymore?
Why ask it
Eight. In 2006 the International Astronomical Union voted that a planet must orbit the Sun, be round and have cleared its path of other bodies, and Pluto, which shares its region with many icy neighbors, became a dwarf planet. Nothing about Pluto changed, only a definition that some astronomers still object to, so follow up by asking who decides what a word in science means.
How far away is the Sun, and how long does its light take to reach us?
Why ask it
About 93 million miles, or 150 million kilometers, which sunlight covers in a little over eight minutes. The follow-up that lands with children is that we never see the Sun as it is now, only as it was eight minutes ago, and that sets up every later question about looking back in time.
Why is it hot in summer and cold in winter?
Why ask it
Most people guess that Earth swings closer to the Sun, and it is the tilt instead: the axis leans about 23 degrees, so each half of the planet tips toward the Sun for part of the year and away for the rest. The check on the distance idea is that Earth is nearest the Sun in early January, in the middle of northern winter. Then ask what season Australia is having in July.
How big is Earth compared with the other planets and the Sun?
Why ask it
Jupiter, the largest planet, is about eleven Earths wide, and the Sun is about 109 Earths wide, with room inside for more than a million. Widths and volumes get mixed up, so ask which one a figure means. Then pace it out on a playing field: with a basketball for the Sun, Earth is a seed less than a tenth of an inch across, nearly thirty yards away.
What is an eclipse, and why is there not one every month?
Why ask it
In a solar eclipse the Moon passes between us and the Sun, and in a lunar eclipse the Moon slides through Earth's shadow. The Moon's path is tilted about five degrees against Earth's, so in most months the three miss lining up. Ask when the next one can be seen from where you live and how to watch it safely: a lunar eclipse needs nothing, while looking at the Sun takes proper solar filters.
Why is the Moon covered in craters when Earth has so few?
Why ask it
Earth has been hit at least as often. Rain, wind, moving crust and living things wear its scars away, and the Moon has none of those, so a crater there can last billions of years. The detail people remember is that the boot prints the Apollo crews left are, as far as anyone knows, still sitting in the dust.
Why does Saturn have rings, and do other planets have them?
Why ask it
All four giant planets have rings, and Saturn's are simply the bright ones, made mostly of pieces of water ice that run from grains up to the size of a house. How old they are and what made them is still argued over, so a good answer gives the leading ideas and stops short of picking a winner.
Which planet would be the worst one to stand on?
Why ask it
Venus is the usual choice: a surface hot enough to melt lead, air pressure like being more than half a mile down in the sea, and clouds of sulfuric acid. The gas giants are a trick answer, since they have no solid surface to stand on. Ask for the least bad planet next and you have compared the whole solar system in two questions.
Was Mars once warm and wet like Earth?
Why ask it
Rovers and orbiters have found dried river channels, old lake beds and minerals that form in water, so the case for ancient rivers and lakes is strong. Where the water and the thicker air went is partly understood, and whether anything ever lived there is not known at all. Keep those two apart, because people tend to answer the first and imply the second.
What is the difference between an asteroid, a comet and a meteor?
Why ask it
An asteroid is rock or metal, a comet is ice and dust that grows a tail as it nears the Sun, and a meteor is the streak of light when a small piece burns up in our air. One that survives to the ground is a meteorite. Worth sorting out before a meteor shower, when all four words get used as if they were one.
Could a big asteroid hit Earth, and is anyone watching for one?
Why ask it
Dust and pebbles arrive daily and burn up, while large impacts are separated by enormous stretches of time. Telescope surveys track the big objects that pass near Earth, and in 2022 the DART spacecraft struck a small asteroid moon and shifted its orbit, the first test of nudging one aside. With a worried child, begin with the people whose job is to watch, then ask an astronomer what the current watch lists show.
How did the Sun and the planets form in the first place?
Why ask it
The accepted outline is a cloud of gas and dust that collapsed about 4.6 billion years ago, with the Sun forming at the center and the planets building up from the disk left over. The details are live research, such as how far the giant planets wandered and where Earth's water came from. Listen for which parts the person calls settled and which they call a best guess.
Stars and black holes
Is the Sun a star, and will it ever burn out?
Why ask it
Yes, an ordinary middle-sized one, close enough to show as a disk. It is roughly halfway through a life of about ten billion years, after which it is expected to swell into a red giant and then shrink to a white dwarf, being too small to explode. Give a nervous child the timescale first: about five billion years.
What is the Sun made of, and what keeps it shining?
Why ask it
Mostly hydrogen and helium, as a ball of glowing, electrically charged gas with no solid surface. It is not on fire: deep in the core, hydrogen nuclei are squeezed together into helium, and that fusion releases the energy that reaches us as sunlight. A reply about burning is the one to question, since a fire needs oxygen and a Sun that burned like coal would have gone out long ago.
Why do stars twinkle when planets mostly do not?
Why ask it
A star is so far off that it reaches us as a single point of light, and moving air bends that point back and forth on its way down. A planet is a tiny disk, so the flicker averages out. Use it the same evening: the bright 'star' that holds steady is very likely a planet.
How many stars can you see on a clear night, and how many are there?
Why ask it
From a truly dark place the eye picks out a few thousand at once, and far fewer from a city. The Milky Way alone is estimated to hold a hundred billion or more, and nobody has counted them, so the better half of the answer is how such an estimate gets made.
What is a constellation, and are its stars really close together?
Why ask it
A constellation is a picture people drew on the sky, and its stars usually lie at wildly different distances, lined up only from where we stand. Astronomers recognize 88 official ones, and other cultures joined the same dots into other figures. A planetarium guide can often tell you what one familiar pattern was called somewhere else.
Why are some stars red and others blue?
Why ask it
Color is temperature, and it runs opposite to a faucet: blue-white stars are the hottest and red ones the coolest. Orion shows both to the naked eye, with reddish Betelgeuse at one shoulder and blue-white Rigel at the opposite foot. In a planetarium, ask the guide to point out one of each.
How can anyone know what a star is made of without going there?
Why ask it
Spread a star's light into a rainbow and dark lines appear in it, a different pattern for each element, like a barcode. Helium was spotted in sunlight that way before anyone had found it on Earth. Bring this one out whenever an answer sounds too sure of itself, since it is the 'how do you know' behind most of astronomy.
What happens when a star dies?
Why ask it
It depends on the star's mass. One like the Sun sheds its outer layers and leaves a white dwarf, while a much heavier one collapses and explodes as a supernova, leaving a neutron star or a black hole. The follow-up to save for last: the calcium in bones and the iron in blood were made inside stars that died before the Sun formed.
What exactly is a black hole?
Why ask it
Not a hole: a great deal of mass squeezed so small that, inside a boundary called the event horizon, nothing gets back out, light included. Watch for the vacuum cleaner picture, which is wrong. From a distance a black hole pulls no harder than a star of the same mass, and if the Sun were swapped for one of equal mass, Earth would keep to its orbit in the dark.
If a black hole gives off no light, how do we know any exist?
Why ask it
By what happens around them: stars at the center of our galaxy whipping around something massive and unseen, gas glowing as it spirals in, and since 2015 the ripples in space picked up when two of them merge. In 2019 a network of radio telescopes produced the first picture of a black hole's shadow. Four separate kinds of evidence is what a strong answer sounds like here.
What would happen to someone who fell into a black hole?
Why ask it
The approach is understood physics: gravity pulling harder on the feet than on the head, and a clock that seems, to anyone watching from outside, to slow down. What lies at the very center is not known, because the two best theories physicists have stop making sense there. An honest answer says where the knowledge runs out.
The universe
What is a light-year?
Why ask it
A distance, not a time: how far light travels in a year, close to six trillion miles. Astronomers need it because miles stop meaning anything past the planets. Check that it has landed by asking how far a 'light-minute' would be, and then how many of those away the Sun is.
When we look at the stars, are we looking into the past?
Why ask it
Yes, because light takes time to arrive: from a few years to a few thousand for the stars you can see, and about two and a half million years for the Andromeda galaxy, the farthest thing most people can spot unaided. One popular claim needs correcting, though. Nearly all the stars visible to the eye are still there, since even a few thousand years is nothing in the life of a star.
What is a galaxy, and which one do we live in?
Why ask it
A galaxy is a vast island of stars, gas and dust held together by gravity. Ours is the Milky Way, a flat spiral, with the Sun well out from the center and taking over two hundred million years to go around once. The pale band across a dark sky is that disk seen from inside, which is the thing to go and look at afterwards.
How big is the universe, and does it have an edge?
Why ask it
The part we can observe is about 93 billion light-years across, a limit set by how far light has been able to travel and not by a wall. Whether the whole universe goes on forever is unknown. The answer to trust keeps 'the observable universe' and 'the universe' as two separate things.
How old is the universe, and how did anyone work that out?
Why ask it
About 13.8 billion years, from two lines of evidence: how fast galaxies are moving apart, and the faint glow of heat left over from the early universe. Different ways of measuring the expansion rate currently disagree by a small but stubborn amount, and nobody yet knows why. That disagreement makes a better follow-up than the number does.
Why is the sky dark at night if there are so many stars?
Why ask it
It sounds silly and it has a name, Olbers' paradox: in a universe that was endless, ageless and still, every line of sight would end on a star and the whole sky would glow. The accepted way out is that the universe has a finite age, so light from most of it has not had time to arrive, and expansion stretches much of what does arrive out of visible range. Ask it straight after the age of the universe, because a dark sky is evidence for that answer that anyone can check from a window.
What was the Big Bang, and what was there before it?
Why ask it
Not an explosion at one spot, whatever the name suggests. The evidence is for all of space being hot and dense and then expanding everywhere at once, which is why there is no center to point at. 'Before' has no agreed answer and may not be a question that makes sense, so prefer the person who says so over the one with a confident story.
Is the universe still expanding, and what is it expanding into?
Why ask it
Still expanding, and since the late 1990s the measurements say it is speeding up. It is not spreading into empty room: the space between galaxies is itself stretching, like raisins moving apart in rising dough. Every such picture breaks somewhere, so ask where this one does (the dough has an edge, and the universe has none that we know of).
What are dark matter and dark energy?
Why ask it
Names for two things nobody has identified. Galaxies spin too fast to be held together by the matter we can see, and the expansion is accelerating when gravity ought to slow it, so something is doing each job. In the standard picture the two make up about 95 percent of the universe, which makes this the largest open question on the list.
How will the universe end?
Why ask it
The leading expectation is a long cooling: expansion continues, stars use up their fuel and the sky goes dark over spans far longer than the universe's present age. Other endings have been proposed, and which is right depends on dark energy, which is not understood. Say the timescale out loud if a child asks, because the honest answer is also the reassuring one.
Could there be other universes besides this one?
Why ask it
Some theories in physics allow for them, and no observation so far supports the idea or rules it out. For an older student the better question is the next one: if an idea can never be tested, is it still science? Researchers answer that differently, and hearing them disagree is the point.
Life elsewhere
Has life been found anywhere other than Earth?
Why ask it
No: not a fossil, not a microbe, not a signal. Possible hints are announced from time to time and so far each has stayed unconfirmed. The care is in the wording, because 'none found' after searching a tiny fraction of places is very different from 'none there'.
Where in our solar system would you look for life first?
Why ask it
The usual shortlist is Mars, for traces of ancient life, and the moons Europa and Enceladus, where there is strong evidence of salt-water oceans under the ice. Titan, with its lakes of liquid methane, is the wild card. Ask which missions are on their way or planned, since that list changes from year to year.
What does a planet need before life could start there?
Why ask it
The working list is liquid water, a source of energy, the right chemical ingredients and a long stretch of stable time. Notice the phrase 'life as we know it' when it comes up: every item is drawn from one example, Earth. Asking what life might do without water takes the conversation somewhere scientists themselves disagree.
How do astronomers find planets around other stars?
Why ask it
Mostly indirectly: a star dims slightly each time a planet crosses in front of it, or wobbles as the planet tugs on it. Thousands have been confirmed since the 1990s. Follow with 'has anyone found another Earth?' and expect a careful reply, since a rocky planet at the right distance is not the same as one known to have air and water.
If there are other civilizations, why have we heard nothing?
Why ask it
This puzzle has a name, the Fermi paradox, and no agreed solution. The candidates include distances too great to cross, civilizations that do not last, life being rarer than hoped, and our having listened for only a few decades to a small slice of sky. In a class, have each person pick one and argue for it.
What would alien life probably look like?
Why ask it
Nobody knows, though biology offers one sober hint: for most of Earth's history all life was single cells, so a first discovery is more likely to need a microscope than a handshake. With younger children, turn it into a drawing. Pick a world with heavy gravity or no sunlight and design something that could live there.
Are UFO sightings evidence of aliens?
Why ask it
Unexplained is not the same as extraterrestrial, and so far no sighting has come with evidence that scientists could examine and confirm. The reply to respect does not sneer at the question. It says what kind of evidence would be convincing, such as a physical sample or a signal that several observatories record independently.
Should we send messages into space, or only listen?
Why ask it
People have already done both: a radio message was beamed from the Arecibo telescope in 1974, and the two Voyager probes carry gold-plated records of sounds and pictures from Earth. Scientists are divided on whether calling attention to ourselves is wise. The livelier half is what you would put in the message, and who gets to decide.
Going to space
Where does space begin, and how high up is it?
Why ask it
There is no sharp edge, because the air just keeps thinning. By the most widely used convention space starts 100 kilometers up, about 62 miles, and some agencies in the United States use 50 miles. Ask for a comparison: at highway speed, straight up, the drive would take about an hour, and the space station circles only about 250 miles overhead.
Why do astronauts float in space?
Why ask it
Not for lack of gravity: at the height of the space station it is still about nine tenths as strong as on the ground. Crews float because the station is falling around Earth the whole time and they are falling with it, the way you go light for a moment as a swing or an elevator drops. 'Zero gravity' in an answer is a sign to ask again.
Is there sound in space?
Why ask it
Almost none: sound is a vibration passed along through air, water or something solid, and space has too little matter to carry it, so an explosion out there would be silent. Astronauts on a spacewalk talk by radio, which needs no air. Ask whether two of them could hear each other by pressing their helmets together (they could, because the sound would travel through the helmets).
How cold is space?
Why ask it
Trickier than it sounds, because temperature belongs to matter and space has almost none. Far from any star an object would settle a few degrees above absolute zero, about minus 455 Fahrenheit, yet near Earth the sunlit side of a spacecraft can run hotter than boiling water while its shaded side sits far below freezing. An answer with a single number is too simple, so ask: in sunlight or in shadow?
How does a rocket move when there is no air to push against?
Why ask it
The engine throws gas out of the back at great speed and the rocket is shoved the other way in return, which works better in a vacuum than in air. Let go of an untied balloon to show it. A reply that needs something outside the rocket to press on has it backwards.
How long does it take to reach the Moon, Mars and the nearest star?
Why ask it
About three days to the Moon for the Apollo crews, something like seven months to Mars with today's rockets, and tens of thousands of years to Proxima Centauri, the nearest star, at the speed of the probes now leaving the solar system. Put the three side by side. The gap between the second figure and the third is why other stars are studied by telescope and not by visit.
Why did people stop going to the Moon after Apollo, and who is going back?
Why ask it
Twelve people walked on the Moon between 1969 and 1972, and the flights ended for reasons of cost and politics, not because anyone forgot how. Several countries and companies now have lunar programs, and their dates slip often. Ask what has flown this year instead of trusting any schedule, including one you read here.
Could people live on Mars?
Why ask it
Nobody has tried, and the list of problems is long: air too thin to breathe and mostly carbon dioxide, deep cold, radiation, toxic dust, and months of travel with no quick way home. Sort the reply into problems already handled on a space station and problems that are not. Enthusiasts and skeptics both exist among the experts, so find out which one you are talking to.
What is the farthest that anything built by people has traveled?
Why ask it
Voyager 1, launched in 1977, which left the bubble of the Sun's influence in 2012 and is now more than 15 billion miles away. Its radio signal takes about a day to arrive. Ask for today's distance, since it grows by close to a million miles daily, and whether the probe is still answering.
What can a telescope in space see that one on the ground cannot?
Why ask it
Two things. Above the air there is no shimmer to blur the picture, and kinds of light the atmosphere blocks, such as much of the infrared, the ultraviolet and X-rays, come through. Ask to see one object photographed in visible light and in infrared, because the pair explains it better than words do.
What is space junk, and how much of a problem is it?
Why ask it
Dead satellites, spent rocket stages and fragments from collisions, circling at speeds where a fleck of paint can chip a window. The larger pieces are tracked and spacecraft steer around them. Who should pay to clean up is unsettled, which gives a class something to argue over.
Could a spaceship ever travel faster than light?
Why ask it
Every test of relativity so far says that nothing with mass can reach the speed of light, let alone pass it. Warp drives exist as mathematics that calls for forms of matter nobody has found. Listen for the difference between 'not with any physics we have tested' and 'never', because careful people say the first.
Is exploring space worth the money?
Why ask it
A debate question, and it goes better split in three: satellites that forecast weather and guide phones, robot probes and telescopes, and sending people. The case for each is different, and so is the cost. Have whoever raised it argue the side they disagree with first.
Next steps
What can I see in the sky tonight, and do I need a telescope?
Why ask it
It depends on the date and where you are standing, so put it to a local astronomer, a planetarium or a sky app and not to a book. The Moon, the bright planets and a few constellations need no equipment, and binoculars add lunar craters and Jupiter's moons as tiny dots. Go outside the same night, and ask whether a club or library near you lends telescopes before buying one.
What have we learned about space since you were in school?
Why ask it
Aimed at a teacher, parent or grandparent, this shows how fast the subject moves. Depending on their age the list may include planets around other stars, the first picture of a black hole, ripples in space from colliding black holes and close-up photographs of Pluto. It also teaches a child that the textbook is a snapshot.
Is there real space research that someone my age can help with?
Why ask it
Yes. Volunteers with no training sort pictures of galaxies and comb telescope data for planets on citizen science websites, and amateur observers still send in counts of meteors and measurements of stars that change in brightness. Projects open and close, and some have age rules or need a parent's account, so ask a teacher, a library or a local astronomy club which ones are running now.
What jobs are there in space science besides being an astronaut?
Why ask it
Far more people work on space from the ground than ever fly: engineers, software developers, telescope operators, planetary geologists, mission planners, teachers and writers. Routes in and what they require differ by country and employer, so ask how it works where you live. For a student, the follow-up to add is which school subjects the person still relies on.
What question about space do you most want answered in your lifetime?
Why ask it
Save it for the end of a talk or a visit. It moves an expert from reciting to wondering, and their choice shows what they find exciting, whether that is life elsewhere, dark matter or the inside of a black hole. Have your own answer ready, because a good speaker will ask it back.
Getting a real answer to a question about space
Practical guidance for the conversation itself
Before you ask
Match the question to the person
A classroom teacher, a planetarium guide and a research astronomer are good for different things. The teacher can tie an answer to what the class is studying, the guide can point at the sky while answering, and the researcher can say what is being argued about this year. A parent who does not know can say so and look it up with the child, which is a fine answer too.
Start with something you can see
The Moon's changing shape, a twinkling star beside a steady planet and the plain fact that the night sky is dark can all be checked from the back step the same evening. Open with one of those before the Big Bang, especially with younger children, so the far-off answers have something to hang on.
Guess first
Before the answer comes, have everyone say what they think. A wrong guess said aloud (the seasons come from distance, the Moon's phases from Earth's shadow) is what makes the right answer stick, and it tells the person answering where to begin.
Bring one question and a follow-up
At a public talk you may get a single turn. Choose the question in advance and have a second one ready behind it. 'How do we know that?' works after almost any answer in astronomy, and it usually gets a better story than the first question did.
How to listen to the answer
Sort it into measured, estimated and open
The distance to the Sun is measured. The number of stars in the galaxy is an estimate. What dark matter is remains open. A careful speaker marks which is which, and when they do not, ask: is that measured, or is it a best estimate?
Ask for a comparison with every big number
Billions of miles and billions of years slide straight past most listeners. Ask for a scale model instead: if the Sun were the size of a beach ball, how big would Earth be and how far off? If the whole age of the universe were squeezed into one year, when would people show up?
Treat 'nobody knows' as a result
What came before the Big Bang, whether anything lives beyond Earth and what sits at the center of a black hole have no answers yet. Someone who says so, and then explains what would settle it, has given the best answer available. Be more careful with a confident reply to an open question.
Check the date
Missions launch, slip and end, probes travel farther every day and new planets are announced all the time. Anything on this page about what has flown or been found was true when it was written. Ask what has changed since.
With children and classes
Model it with what is in the room
A lamp and a ball show the Moon's phases, a tilted globe carried around the lamp shows the seasons, an untied balloon shows how a rocket moves, and a basketball and a seed paced out across a field show how much of the solar system is empty. Do the model after the guess and before the explanation, and let the child hold the ball.
When a question is really a worry
Asteroids, the Sun burning out and black holes can frighten a young child. Answer the fear first with the plain fact that helps: people are watching for asteroids, the Sun has billions of years left, and the nearest known black hole is far too distant to affect us. Then give the science.
Keep a list of the ones nobody could answer
Write the unanswered questions on a sheet on the wall or in a notebook. Those are the ones to take to the next museum visit, school talk or library trip, and a child who arrives with a written question tends to get a fuller reply.
Give the opinion questions to the group
Whether to send messages to other worlds, whether space is worth the cost and what aliens might look like have no expert answer. Put them to everyone, ask for reasons, and leave the expert's own view until last so it does not close the discussion.
Where it goes wrong
A quiz in place of a conversation
'How many moons does Jupiter have?' gets a number that goes out of date as more are found, and it leads nowhere. Why and how questions give the person something to explain, and the number usually turns up along the way.
Movie physics
Films put sound in space, fire in a vacuum and asteroid fields too crowded to fly through. Asking which film gets space most wrong is a good-natured way to raise it, and kinder than correcting a child who is quoting one.
The UFO standoff
The question is fair and the argument that follows is rarely useful. Move it to evidence: what would count, who could check it and what has been checked so far. That turns a standoff into a lesson in how any claim gets tested.
Too many at once
Six questions fired in a row get six one-line answers. Two or three, each followed up, will fill a twenty-minute session and leave people wanting to come back.