Explore the big questions

Reaching orbit is far less about altitude than about sideways speed. A rocket that flew straight up and then stopped would simply fall back down — being high above Earth doesn't keep you there. The trick to staying in orbit is to move sideways so fast that, as gravity pulls you down, the curve of your fall matches the curve of the Earth. You keep falling, but you keep missing the ground.

In low Earth orbit that means travelling roughly 7.8 kilometres every second — about 28,000 km/h. So a launch climbs vertically only long enough to clear the thickest, most draggy part of the atmosphere, then pitches over into a long, mostly horizontal acceleration called a gravity turn. By the time the engines shut down, the vehicle is moving nearly parallel to the surface, hundreds of kilometres up.

This is why getting to space is the easy part and getting to orbit is the hard part. The hop to the edge of space is a few minutes of climbing; orbit demands an extraordinary, sustained sideways burn. It's also why rockets are mostly propellant by mass — the speed required is so high that the fuel needed dwarfs the payload, the structure, and everything else combined.

An orbit is best understood as continuous falling. Newton imagined a cannon on a very tall mountain: fire the ball gently and it lands nearby; fire it harder and it lands further away; fire it hard enough and the ground curves away as fast as the ball falls — so it never lands, it circles the planet. That is an orbit.

Nothing holds a satellite up; it is in constant free-fall, which is also why astronauts float. They are not beyond gravity — the International Space Station feels almost 90% of Earth's surface gravity — they are simply falling around the planet together with their spacecraft.

Counter-intuitively, higher orbits are slower. A satellite just above the atmosphere laps the Earth in about 90 minutes, while the Moon takes roughly a month. Geostationary satellites sit at about 35,786 km, where one orbit takes exactly a day, so they appear to hover over a fixed spot — ideal for communications and weather.

Changing orbits costs energy measured in 'delta-v', the change in velocity a spacecraft can achieve. Every manoeuvre — raising an orbit, rendezvousing with a station, escaping to another planet — is a careful delta-v budget, which is why mission designers obsess over it as much as over fuel.

Twelve people walked on the Moon between 1969 and 1972, and then human lunar exploration simply stopped for over half a century. The reason is not that we forgot how — it's that Apollo was, at its heart, a Cold War sprint. At its peak NASA consumed more than 4% of the entire US federal budget, a level of spending that was never going to last once the race was won.

When the political urgency faded, so did the money, and the giant Saturn V production line was shut down. For decades human spaceflight focused on cheaper, nearer goals: the Space Shuttle and the International Space Station in low Earth orbit. Going back sustainably proved genuinely hard and costly, and every few years a new administration set a new direction, cancelling the previous plan before it could mature.

What's different now is that the goal has shifted from 'flags and footprints' to staying. NASA's Artemis programme aims not just to land again — including the first woman and first person of colour on the surface — but to build a lasting presence, with a small station called Gateway in lunar orbit and reusable landers.

It's also no longer a solo American effort: international agencies and commercial companies are deeply involved, which spreads the cost and, hopefully, the staying power.

Artemis is NASA's programme to return humans to the Moon and, this time, to establish a lasting presence as a stepping stone toward Mars. Named after the Greek goddess of the Moon and twin sister of Apollo, it is deliberately a successor to Apollo — but with very different goals.

It is built around a few big pieces. The Space Launch System (SLS) is a giant rocket that sends astronauts beyond Earth orbit; Orion is the crew capsule they ride in; and Gateway is a small station planned for lunar orbit, where crews and landers can gather. For the final descent, NASA has contracted commercial human landing systems, including a lunar version of SpaceX's Starship.

Artemis I flew in 2022 as an uncrewed test, sending Orion around the Moon and back. Later missions aim to carry crew around the Moon and then land near the lunar south pole — a region of interest because permanently shadowed craters there may hold water ice, a potential source of drinking water, breathable oxygen and rocket propellant.

Artemis is explicitly international and commercial: agencies from Europe, Japan and Canada contribute hardware and crew, and private companies build much of the equipment. The schedule has slipped repeatedly, which is normal at this scale, but the aim is consistent — not a brief visit, but a sustained human return to the Moon.

Honestly, we don't know yet — but it's one of the most actively investigated questions in science, and the search is far more concrete than science fiction suggests. We have not found life anywhere beyond Earth, not even microbes. What we have found is that the ingredients and conditions for life appear to be common.

Astronomers have confirmed thousands of exoplanets orbiting other stars, and statistics suggest the galaxy holds billions — many of them rocky worlds in the 'habitable zone' where liquid water could exist. Closer to home, the search focuses on a few promising places: Mars, which was warmer and wetter billions of years ago, where rovers hunt for chemical signs of past life.

Then there are ocean worlds like Jupiter's moon Europa and Saturn's moon Enceladus, which hide vast liquid-water oceans beneath icy shells. Enceladus even sprays plumes of that water into space, which spacecraft have flown through. Missions like Europa Clipper are designed to assess whether such oceans could be habitable.

For life beyond the Solar System, the approach is to study exoplanet atmospheres with telescopes like JWST, looking for 'biosignatures' — gas combinations that life might produce. So far there is no confirmed detection, and extraordinary claims demand extraordinary evidence. The honest summary: we have gone from wondering whether other worlds exist to systematically checking specific ones, even though the headline answer remains a careful 'not yet'.

It seems contradictory: sunlight in space is fierce and unfiltered, yet space itself is described as bitterly cold. The resolution is that how warm an object actually gets depends on how heat can reach it. On Earth, heat moves mostly by conduction and convection — molecules of air, water or solid bumping into one another and passing energy along.

Space is a near-perfect vacuum, almost empty of molecules, so those two mechanisms barely work. That leaves radiation: energy carried by light. The Sun pours out radiation, and an object in sunlight absorbs it and heats up — sometimes ferociously.

A spacecraft's sunlit side can climb well above the boiling point of water, while its shaded side — radiating its own heat away into the dark with nothing to warm it back — can plunge to nearly minus 150°C. This is why spacecraft and suits are wrapped in shiny multi-layer insulation, and why probes rotate to balance the heat.

The 'temperature of space' itself, far from any star, is about 2.7 degrees above absolute zero — the faint afterglow of the Big Bang. So space isn't cold the way a freezer is; there's simply almost nothing there to hold or transfer heat. Managing that imbalance is a central challenge of spacecraft engineering.

A rocket is mostly a tank of fuel with engines bolted on, and the single biggest enemy of reaching orbit is dead weight. As a rocket burns through propellant, the empty tanks and now-useless engines become exactly that — mass the remaining engines must keep dragging up to orbital speed.

Staging is the elegant fix: build the rocket as a stack of self-contained sections and, as each one runs dry, drop it. Shedding that empty mass lets the upper stages accelerate far more efficiently. This is why almost every orbital rocket has two or three stages — a powerful first stage to get off the ground and through the atmosphere, then lighter upper stages optimised for vacuum.

The historical downside is that discarded stages were simply thrown away — millions of dollars of hardware lost on every flight, like scrapping an airliner after a single trip.

The big shift of the last decade is reusability: rockets like SpaceX's Falcon 9 fly their first stage back to a controlled landing on a pad or a drone ship, refurbish it and launch again — dramatically cutting the cost of reaching space. Fully reusable vehicles are the next frontier, and a key reason launch is becoming cheaper and far more frequent.

The International Space Station is a crewed laboratory orbiting roughly 400 km above Earth, circling the planet about every 90 minutes at around 28,000 km/h — fast enough that its crew sees a sunrise or sunset every 45 minutes. It is one of the most complex engineering projects in history, assembled piece by piece in orbit since 1998 from modules launched by multiple nations.

About the size of a football field including its solar arrays, it has been continuously inhabited since November 2000 — humanity has not been entirely absent from space for over two decades. It is a partnership chiefly between the United States, Russia, Europe, Japan and Canada.

Its purpose is research that can only be done in sustained weightlessness: how the human body adapts to long spaceflight (vital for future Moon and Mars missions), how fluids, flames and crystals behave without gravity, and Earth and space observation. Crews of around seven rotate roughly every six months, arriving on Crew Dragon and Soyuz, and exercise for hours daily to fight bone and muscle loss.

The ISS is now in its later years; plans are underway to retire it safely around 2030 and hand the low-Earth-orbit baton to commercial stations and lunar outposts like Gateway. For a generation, it has been our permanent home in space and a working symbol of international cooperation.

Looking far out into space is the same thing as looking back in time. Light travels fast but not infinitely fast, so when you see a galaxy a billion light-years away, you see light that left it a billion years ago — the galaxy as it was, not as it is now. Telescopes are, in a very real sense, time machines.

Seeing such faint, distant objects comes down to gathering enough light. A big telescope mirror is a giant light bucket: the larger it is, the more of those ancient, feeble photons it collects, and the fainter and farther the objects it can reveal. Putting telescopes in space, like Hubble and JWST, removes the blurring of Earth's atmosphere for razor-sharp views.

The most distant galaxies are seen in infrared rather than visible light, because the expansion of the universe stretches their light to longer, redder wavelengths on its journey to us — an effect called redshift. That is exactly why JWST was built to see in the infrared.

Occasionally the universe lends a hand: a massive foreground galaxy cluster can bend and magnify the light of something behind it, acting as a natural 'gravitational lens'. By combining huge mirrors, the clarity of space, infrared eyes and these cosmic magnifying glasses, astronomers can now see galaxies that formed only a few hundred million years after the Big Bang.

A scrubbed launch is frustrating to watch, but a delay is almost always the system working exactly as it should. Spaceflight runs on tiny margins: a rocket is a controlled explosion balanced on the edge of failure, so engineers set strict 'go/no-go' criteria for thousands of parameters. If even one drifts out of bounds, the launch is held rather than risked.

Weather is the most common culprit. Beyond obvious dangers like lightning, rockets worry about high-altitude winds that could push the vehicle off course, and even thick clouds that can trigger static discharge as the rocket climbs through them.

Then there's the launch window. Many missions can only fly during a precise stretch of time — sometimes a single instant — because the target orbit, a rendezvous with the Space Station, or an alignment with another planet is moving. Miss it and you wait, often a full day, for the window to come round again.

Hardware issues and last-second software aborts also stop the clock, sometimes with the engines already lit; modern rockets shut themselves down safely if anything looks wrong in the final seconds. When a countdown stops, the layered safety system usually caught something and chose to wait — and those abundant chances to say 'no' are a big reason modern launches are so reliable.

Space junk, or orbital debris, is the growing cloud of dead and broken human-made objects circling Earth: spent rocket stages, defunct satellites, lost tools and — overwhelmingly — countless tiny fragments from past collisions and explosions. Networks track tens of thousands of objects larger than about ten centimetres, but models suggest well over a hundred million pieces are too small to track.

It matters because of speed. In low Earth orbit, debris travels at several kilometres per second, and a collision happens at the combined velocity of both objects — so even a fleck of paint carries the punch of a bullet, and something marble-sized hits with the energy of a small bomb.

Operational satellites and the International Space Station occasionally manoeuvre out of the way of tracked debris, and the ISS crew has at times sheltered in their return capsules as a precaution. The real long-term fear is a chain reaction, the Kessler syndrome: a collision creates more debris, causing more collisions, potentially making some orbits unusable for generations.

To prevent this, operators are increasingly required to de-orbit satellites at end of life, design rockets not to explode in orbit, and experiment with clean-up missions that capture dead hardware. As launch gets cheaper and mega-constellations go up, keeping orbit sustainable is becoming one of spaceflight's defining challenges.

Mars is the obvious next great destination for human exploration, and the reasons are part science, part long-term ambition. Scientifically, it is the most Earth-like world within reach: a day almost the same length as ours, polar ice caps, ancient dried-up riverbeds, and clear evidence it was once warmer and wetter.

That makes it the best place in the Solar System to ask whether life ever arose beyond Earth — and if it did on a second world so close by, the implication for life elsewhere is profound. Robotic rovers are already there hunting for answers, even caching rock samples for a future mission to bring home.

The longer-term motivation is about humanity's resilience and reach: becoming able to live on more than one world. But Mars is brutally hard. It's tens of millions of kilometres away, so a round trip takes years; the thin, unbreathable atmosphere and absent magnetic field expose the surface to radiation; and the journey bombards crews with cosmic rays and prolonged weightlessness.

Landing is notoriously difficult — the atmosphere is too thin to rely on parachutes alone but thick enough to cause searing heat. This is why so much of today's spaceflight, from the Space Station's health studies to the Artemis return to the Moon, is framed as practice: learning to keep humans alive far from home before the hardest trip people have ever planned.