Space exploration is the study of the universe using rockets, telescopes, satellites, space probes and crewed spacecraft such as the International Space Station. Rockets lift our instruments above the atmosphere, telescopes collect light and other radiation from distant objects, and satellites and probes send back the data. Together, they show us how we explore space and what lies beyond Earth.
For most of history, people could only look up. Then, in the twentieth century, engineers worked out how to leave the ground, and everything changed. In this guide you will learn how rockets work, how telescopes see across the universe, why satellites stay in orbit and what astronauts do on the International Space Station. We finish with probes, rovers and the missions planned for the future.
If you are studying GCSE physics, you will meet forces, orbits and the electromagnetic spectrum along the way. If you are simply curious, the numbers alone are worth the read.
How do we explore space?
We explore space in two main ways: by sending machines or people there, and by studying what reaches us from far away. Both rely on the same toolkit.
- Rockets are the only vehicles able to reach orbit and beyond.
- Telescopes collect light and other radiation from distant objects, on the ground and in space.
- Satellites orbit Earth or other bodies to observe, navigate and communicate.
- Space probes and rovers travel to other worlds and send back measurements and photographs.
- Crewed spacecraft and space stations let humans work and live in orbit.
Each tool answers a different question. A telescope tells us what a galaxy is made of, but only a probe can scoop up and test the soil of Mars. Rockets sit at the start of nearly every mission, so we begin there.
Rockets: how we get into space
Every object that has ever reached space got there on a rocket. Konstantin Tsiolkovsky worked out the mathematics of rocket flight (the rocket equation) in 1903. In 1926, Robert Goddard launched the first liquid-fuelled rocket. Today’s launchers are far larger, but they rely on the same physics.
How a rocket works
Many people ask how do rockets work, and the answer is Newton’s third law of motion. A rocket burns fuel and pushes hot exhaust gases out of the back at high speed. The gases are pushed backwards, so the rocket is pushed forwards.
action → equal and opposite reaction
This works in a vacuum. Rockets do not push against the air; they carry their own oxidiser, so they need no air to burn their fuel. If you want the full set of laws, see our guide to Newton’s laws of motion.
Liquid-fuel rockets store a fuel and an oxidiser in separate tanks. Common pairs are liquid hydrogen with liquid oxygen, and RP-1 (a type of kerosene) with liquid oxygen.
Orbit vs escape
Reaching space is not the same as staying there. To stay in low Earth orbit, a spacecraft must travel sideways at about 7.8 km/s, which is roughly 28,000 km/h. To leave Earth’s gravity completely, it needs the much higher escape velocity.
| Goal | Speed needed | Approx. speed in km/h |
|---|---|---|
| Low Earth orbit | ≈ 7.8 km/s | ≈ 28,000 km/h |
| Escape from Earth | ≈ 11.2 km/s | ≈ 40,300 km/h |
Notice that escape velocity is only about 1.4 times orbital speed, but the fuel needed grows quickly with the speed required. That is why rockets are so large, and why most of their mass at launch is propellant.
Multi-stage and reusable rockets
Carrying an empty fuel tank all the way to orbit wastes energy. Engineers solve this with multi-stage rockets: when one stage runs out of fuel, it is dropped and the next stage fires. The rocket gets lighter as it climbs, so the remaining fuel goes further.
The next step was to reuse the stages instead of throwing them away. In 2015, a Falcon 9 first stage landed upright after launch, and boosters now land and fly again. Reuse lowers the cost of reaching orbit, which is one reason the number of launches keeps rising.
Milestones of the space age
The first sixty-odd years of spaceflight produced some famous firsts.
- 4 October 1957: the USSR launches Sputnik 1, the first artificial satellite.
- 12 April 1961: Yuri Gagarin becomes the first human in space.
- 20 July 1969: Apollo 11 lands on the Moon, and Neil Armstrong and Buzz Aldrin walk on its surface.
- 1969–1972: in total, 12 people walk on the Moon.
- 1998: assembly of the International Space Station begins.
- 2 November 2000: the ISS is occupied for the first time, and it has been continuously occupied since.
- 2015: a Falcon 9 first stage lands upright after launch.
Not every mission went well. Explosions, lost spacecraft and tragedies taught engineers hard lessons, and you can read about them in our article on failures in space travel.
Telescopes: seeing across space
Rockets take us into space, but most of what we know about the universe comes from light and other radiation that reaches us from far away. Telescopes collect that radiation and make faint, distant objects bright enough to study. They are the main tool of space exploration for anything we cannot visit.
There are two basic designs. Refracting telescopes use lenses to bend light to a focus. Reflecting telescopes use curved mirrors, which can be made much larger. Galileo pointed a telescope at the sky in 1609–1610 and spotted Jupiter’s four large moons, proof that not everything orbits the Earth.
Light is only one part of the electromagnetic spectrum. Stars, galaxies and gas clouds also give out radio waves, infrared, ultraviolet, X-rays and gamma rays, so astronomers build different telescopes for each.
| Type of telescope | Detects | Where it must be |
|---|---|---|
| Optical | Visible light | Ground or space |
| Radio | Radio waves | Ground (for example, the Very Large Array in New Mexico) |
| Infrared | Infrared radiation (heat) | Best in space |
| Ultraviolet | Ultraviolet radiation | Space |
| X-ray | X-rays | Space |
| Gamma-ray | Gamma rays | Space |
Why put telescopes in space?
Radio waves pass through the atmosphere, so radio astronomers can work on the ground. The Very Large Array in New Mexico links 27 dishes to act as one giant telescope.
Other radiation is a problem. The atmosphere blurs visible light, which is why stars twinkle, and it absorbs X-rays completely. An X-ray telescope therefore has to be carried above the atmosphere on a rocket or satellite. A space telescope also avoids clouds, weather and light pollution.
Hubble and Webb
The Hubble Space Telescope was launched in 1990 and orbits in low Earth orbit at roughly 475 km today (it was higher at launch and slowly sinks). Its 2.4 m mirror has given us sharp images of galaxies, nebulae and planets for decades.
The James Webb Space Telescope launched on 25 December 2021. It has a 6.5 m gold-coated beryllium mirror and observes mainly in the infrared, which lets it see through dust and detect very distant, redshifted light. Webb does not orbit the Earth in the usual way. It orbits around a point called L2, about 1.5 million km away. You can read more in our article on NASA’s James Webb Space Telescope.
Telescopes also help us map the sky. If you want to know how astronomers divide it up, see how many constellations there are.
Satellites: our eyes in orbit
A satellite is any object that orbits a larger body. The Moon is Earth’s natural satellite. Artificial satellites are machines we launch, and more than 15,000 active ones now circle our planet.
They do many jobs:
- Communications: phone, internet and TV signals.
- Navigation: GPS satellites tell your phone where you are.
- Weather: images and measurements that feed forecasts.
- Earth observation and climate: tracking ice, forests, oceans and cities.
- Science: telescopes such as Hubble.
- Spy and military: surveillance and secure communications.
Why satellites don’t fall down
Gravity does not switch off in orbit. A satellite is constantly falling towards Earth while moving sideways so fast that it keeps missing it. The Earth’s surface curves away beneath it as it falls.
Gravity provides the centripetal force that keeps the satellite moving in a circle. Without that force, it would fly off in a straight line. At low Earth orbit, the speed needed is about 7.8 km/s, around 28,000 km/h.
Types of orbit
The height of an orbit decides how long it takes to go round and what the satellite is useful for.
| Orbit | Height above Earth | Typical uses |
|---|---|---|
| Low Earth orbit (LEO) | Roughly 160–2,000 km | ISS, Hubble, Earth observation |
| GPS (medium Earth orbit) | About 20,200 km | Navigation |
| Geostationary | About 35,786 km, above the equator | TV, weather |
A satellite in geostationary orbit takes about one day to go round, so it stays over the same spot on the equator. It is why a fixed satellite dish can point at one place in the sky. Strictly, the period is one sidereal day, about 23 hours 56 minutes.
All these orbits are getting busier. Dead satellites and fragments of rockets stay up there as space junk, and the problem is growing. Our article on how to solve the problem of space junk explains the options.
The International Space Station
The International Space Station (ISS) is the largest structure humans have ever built in space, and the best example of what space exploration looks like when countries work together. Its first parts were assembled in 1998, and people have lived on it without a break since 2 November 2000. That is a long run of continuous human presence beyond Earth’s surface.
Five space agencies share the project: NASA (USA), Roscosmos (Russia), ESA (Europe), JAXA (Japan) and CSA (Canada). Each contributes modules, equipment, astronauts or launches, and the station is run as a joint laboratory rather than as one nation’s property.
ISS key facts
- Orbit height: about 400–420 km above Earth, in low Earth orbit.
- Speed: about 28,000 km/h (roughly 7.8 km/s).
- One orbit: about 90–93 minutes, so roughly 16 orbits every day.
- Size: about the size of a football pitch once the solar arrays are included.
- Mass: more than 400,000 kg.
- Occupied: continuously since 2 November 2000.
- Planned retirement: around 2030, when it is due to be deorbited.
Because the station circles Earth about 16 times a day, the crew sees about 16 sunrises and 16 sunsets in every 24 hours. Their day is organised by clocks, not by the Sun.
Life on board: microgravity
Astronauts experience microgravity, where objects and people appear weightless. It is fun to watch, but it is hard on the body. Without gravity pulling on them, muscles and bones are not worked in the usual way, so crew members can lose muscle and bone mass during long stays.
To limit this, astronauts exercise for about two hours a day using a treadmill, a bike and a resistance machine. Even sleeping, eating and washing need special equipment, because water and crumbs float rather than settle.
That is exactly why the ISS is such a useful laboratory. Research on board includes how microgravity affects the human body, how plants grow, how materials behave and how medicines can be developed. Some of the results help us understand health problems on Earth, such as bone loss, as well as preparing for longer journeys to the Moon and Mars.
The station also needs a lot of power. Huge solar panels convert sunlight into electricity, which is why the arrays are such a large part of its football-pitch size. If you want to know how sunlight is turned into electricity, see our guide to solar energy and how it is used.
How far does the ISS travel?
You can test the numbers above with a quick calculation. Enter a speed and a time below to see how far the ISS covers, using distance = speed × time.
distance = speed × time
Try the station’s speed of about 28,000 km/h with a time of one hour, then with the 90–93 minute orbit time (about 1.5 hours), to see how much ground it covers on every lap of Earth.
Space probes, landers and rovers
Crewed missions are only one way of exploring. Robotic spacecraft can travel for years, survive radiation and temperatures that would be fatal to people, and cost far less because they do not need life support. Most of what we know about other planets comes from them.
Robotic missions fall into four main types:
- Flyby: passes a target once, taking measurements as it goes.
- Orbiter: goes into orbit around a planet or moon for long-term study.
- Lander: touches down on the surface and stays put.
- Rover: a wheeled vehicle that drives across the surface and explores.
Famous Mars rovers include Sojourner (1997), Spirit and Opportunity (2004), Curiosity (2012) and Perseverance (2021). Each one is bigger and more capable than the last, carrying instruments that test rocks and soil for signs of the planet’s past.
Six missions show how different these spacecraft can be. Try to guess each one before you tap the card.
Cassini’s work on Saturn’s moons is a good example of why probes matter. One of those moons, Titan, is a world of its own, and it raises a question many students ask: is Titan a planet or a moon? Spacecraft data helps scientists answer questions like this by showing us what these worlds are really like.
Together, the ISS, probes and rovers show the two sides of exploration: people living and working in orbit, and robots travelling to places we cannot yet reach ourselves.
Beyond our solar system
Everything we have visited so far sits inside one small neighbourhood. Our Sun is just one star among an estimated 100–400 billion in the Milky Way, and the Milky Way is just one galaxy. The observable universe contains hundreds of billions of galaxies, and perhaps up to a couple of trillion.
No spacecraft can reach another star within a human lifetime yet. Even Voyager 1, launched in 1977 and in interstellar space since 2012, is still only at the very start of that journey. So for the stars and galaxies beyond, space exploration relies on telescopes collecting light and other radiation that has travelled for years, centuries or billions of years to reach us.
Exoplanets: worlds around other stars
An exoplanet is a planet that orbits a star other than the Sun. Astronomers have now confirmed more than 6,000 of them, and the count keeps rising as telescopes and surveys improve. Each one is a chance to ask whether our own planetary system is typical or unusual.
To understand how planets form around any star, it helps to know the story of our own. You can read about it in our guide to the origin of our solar system.
The future of space exploration
The next few decades look busy. NASA’s Artemis programme aims to return humans to the Moon. Artemis I flew uncrewed in 2022. Artemis II carried four astronauts on a crewed flight around the Moon in April 2026, the first crewed lunar flyby in over 50 years. Between 1969 and 1972 only 12 people walked on the lunar surface, and later Artemis missions aim to land crews there again.
Robotic missions are heading further out. ESA’s JUICE probe launched in 2023 and is bound for Jupiter’s moons. NASA’s Europa Clipper launched in 2024 to study Europa. Bringing Mars rock samples back to Earth for study in laboratories has long been a goal, although the NASA–ESA Mars Sample Return programme lost its funding in 2026.
Commercial spaceflight
Space is no longer only the business of national agencies. Reusable boosters now land and fly again, which has helped to change the economics of launching satellites and people. Read more in our articles on private space flight and the future of commercial space flight.
Whoever builds the rocket, the physics stays the same: Newton’s third law, gravity and orbital speed decide what is possible.
Try this typical exam question, then tap to check your answer.
Explain why a satellite in orbit does not fall to Earth. (3 marks)
Gravity pulls the satellite towards Earth (1 mark). The satellite is also moving sideways at high speed (1 mark). It is constantly falling towards Earth but moves sideways fast enough that it keeps missing it, so it stays in orbit; gravity provides the centripetal force (1 mark).
Test yourself
Key points
- Rockets work by Newton’s third law and carry their own oxidiser, so they work in a vacuum.
- Orbital speed in low Earth orbit is about 7.8 km/s; Earth’s escape velocity is about 11.2 km/s.
- Telescopes detect different parts of the electromagnetic spectrum, and some must be in space.
- Satellites stay in orbit because they are constantly falling while moving sideways fast enough to keep missing Earth.
- The International Space Station has been continuously occupied since 2 November 2000.
- Probes and rovers explore places people cannot yet reach, and Voyager 1 is the most distant of all.
- Artemis, Europa Clipper, JUICE and commercial spaceflight shape the next chapter.
Frequently asked questions about space exploration
How do rockets work in space with no air?
Rockets work by Newton’s third law. They push exhaust gases backwards, and the gases push the rocket forwards with an equal and opposite force. Because rockets carry their own fuel and oxidiser, they do not need air to burn it, so they work perfectly well in a vacuum.
What is escape velocity?
Escape velocity is the minimum speed an object needs to break free from a planet’s gravity without further propulsion. For Earth it is about 11.2 km/s. That is faster than the roughly 7.8 km/s needed to orbit Earth in low Earth orbit.
Why put telescopes in space?
Earth’s atmosphere blurs light and absorbs some types of radiation, including X-rays. A telescope in space avoids both problems and gives a clearer view. Hubble, launched in 1990, and the James Webb Space Telescope, launched on 25 December 2021, are well-known examples.
What do satellites do?
Artificial satellites do many jobs. They carry communications and TV signals, provide GPS navigation, forecast the weather, observe Earth’s climate, support science and serve military needs. More than 15,000 active satellites now orbit Earth, so we rely on them every day without noticing.
What is a geostationary orbit?
A geostationary orbit is about 35,786 km above the equator. A satellite there takes about one day to circle Earth, so it stays over the same spot on the ground. This makes it ideal for TV broadcasting and weather monitoring.
How fast does the International Space Station travel?
The ISS travels at about 28,000 km/h at an altitude of roughly 400–420 km. One orbit takes about 90–93 minutes, so the crew sees around 16 sunrises and 16 sunsets every day.
How long has the ISS been occupied?
People have lived aboard the International Space Station continuously since 2 November 2000, after assembly began in 1998. Five partners share it: NASA, Roscosmos, ESA, JAXA and the Canadian Space Agency. It is planned to be retired and deorbited around 2030.
What is the difference between a space probe and a rover?
A probe is a robotic spacecraft that flies past, orbits or lands on a target. A rover is a type of lander with wheels that drives across a planet’s surface. Curiosity and Perseverance are Mars rovers, while Voyager 1 is a probe.
What is the future of space exploration?
The future includes NASA’s Artemis programme to return humans to the Moon, proposals to bring Mars samples home, and robotic missions such as ESA’s JUICE and NASA’s Europa Clipper to Jupiter’s moons. Commercial spaceflight is also growing, with reusable rockets making launches more affordable over time.
