Learning About Space: A Beginner's Guide

Where space begins

There is no sharp line where the air stops and space starts. Earth's atmosphere simply thins out with height, until there is almost nothing left to breathe or to hold up an aircraft wing.

A commonly used edge of space sits about 100 km above the ground, roughly an hour's motorway drive pointed straight up. Compare that with Earth's mean radius of about 6,371 km and the layer of air that keeps everyone alive turns out to be remarkably thin, a little like the skin on an apple.

Reaching space is one thing; staying there is another. To stay up rather than fall straight back, a spacecraft must also travel sideways at about 7.8 km/s, which is how the International Space Station keeps circling about 400 km up.

A quick tour of the Solar System

The Solar System is the Sun and everything that travels around it. The Sun sits about 150 million km from Earth, a distance astronomers call one astronomical unit, and it holds almost all of the Solar System's mass.

Here is the neighbourhood, from the centre outwards:

• The Sun: an ordinary star, a vast ball of hot gas that shines because of nuclear fusion in its core.
• Rocky planets: Mercury, Venus, Earth and Mars, small worlds with solid surfaces.
• The asteroid belt: a wide ring of rocky leftovers between Mars and Jupiter.
• Gas giants: Jupiter and Saturn, enormous worlds made mostly of hydrogen and helium, with no solid ground to stand on.
• Ice giants: Uranus and Neptune, colder, bluer worlds rich in water, ammonia and methane.
• Dwarf planets: Pluto and other small, round worlds, many of them in the icy region beyond Neptune.
• Moons and comets: Earth has one Moon, the giant planets have many, and comets are icy wanderers that grow glowing tails as they near the Sun.

How far is far?

Distances in space are so large that kilometres stop being helpful. Instead, astronomers often describe them by how long light takes to cross them. Light travels at about 300,000 km per second, fast enough to circle Earth several times in a single second.

At that speed, the Moon, an average of 384,400 km away, is about 1.3 light-seconds from Earth. Sunlight takes about 8 minutes and 20 seconds to arrive, so the Sun you see is always a little over eight minutes old. Mars is about 3 to 22 light-minutes away, depending on where the two planets are in their orbits, and Jupiter about 33 to 54 light-minutes.

Stars are much further still. A light-year is the distance light travels in a year, and even the nearest stars beyond the Sun are a few light-years away. Looking at them means seeing light that set off years ago, which makes stargazing a gentle kind of time travel.

These delays matter for exploration too. A message to a rover on Mars can take many minutes to arrive, so robots far from Earth need to handle some decisions for themselves. The space signal delay calculator shows how long a message takes to reach the Moon, Mars, Jupiter and beyond.

How people learn about space

Almost everything known about space comes from collecting light and other signals, then reading them carefully. Telescopes on the ground gather far more light than the human eye, and many sit high on mountains, above much of the air's haze.

Telescopes in orbit go one better. Above the atmosphere they avoid the shimmer that makes stars twinkle, and they can see kinds of light, such as much of the infrared and ultraviolet, that the air blocks.

Then there are the robotic visitors. Probes fly past or orbit other worlds, landers touch down on their surfaces, and rovers drive across them, studying rocks, soil and weather. People have explored in person too, with six crewed Moon landings between 1969 and 1972.

Satellites looking back at Earth complete the picture. Since the first artificial satellite in 1957, they have become essential for weather forecasts, maps, farming and disaster response.

Gravity and weight in a minute

Gravity is the pull that every object with mass has on every other object. The more massive a world is, and the closer you are to its centre, the stronger the pull at its surface.

Your mass, the amount of stuff you are made of, is the same everywhere. Your weight, the pull of gravity on that mass, changes from world to world. Surface gravity on the Moon is about 0.17 of Earth's, roughly a sixth, so jumping would feel wonderfully easy. On Mars it is about 0.38, a little over a third, while at Jupiter's cloud tops it is about 2.53 times Earth's.

Planets also take different lengths of time to travel round the Sun. A year on Mars lasts 687 Earth days, while Neptune takes about 165 Earth years, so on Neptune almost everyone would still be waiting for a first birthday. The weight and age on other planets tool works out your own numbers.

Gravity also decides how fast a satellite must travel to stay in orbit. Lower orbits need more speed and go round more quickly, while higher orbits are slower and take longer. Pick an altitude in the orbit calculator and watch the speed and orbit time change.

Start stargazing tonight

You do not need expensive equipment to enjoy the night sky. A clear night, a little patience and a few simple habits go a long way.

• Find somewhere dark. A park, beach or stretch of countryside away from streetlights reveals far more stars than a bright town centre.
• Give your eyes about 20 minutes to adjust, and keep bright screens out of sight. A red torch helps you read notes without spoiling your night vision.
• Check the Moon's phase. A full Moon washes out faint stars, so nights near the new Moon are best for faint objects, while a half-lit Moon shows craters beautifully along the line between day and night.
• Start with binoculars. An ordinary pair shows lunar craters, star clusters and the brightest moons of Jupiter as tiny points of light.
• Use a star-chart app. Many show what is overhead when you hold your phone up, making it easy to tell a planet from a star.
• Look out for the International Space Station. It appears as a steady, bright light gliding across the sky over a few minutes.

Keep learning about space: where to go next

Once the basics click, every corner of space has a deeper story. These pages pick up where this guide leaves off:

• Satellites and orbits, explained simply: why satellites stay up and what they do for daily life.
• How rockets work and why reusable rockets matter: thrust, stages and the journey to orbit.
• The Moon, Mars and beyond: the next era of exploration, from lunar bases to the outer planets.
• The space and investing glossary: plain-English definitions of the terms that come up most often.

Nicole Junkermann's perspective

Nicole Junkermann follows space with the same curiosity she brings to artificial intelligence and health. She presents the AI Overview podcast, and in her view the best way into any big subject is to start with a few clear ideas and keep asking good questions.

She often describes the real story of AI and work as reinvention rather than replacement, and learning about space fits that spirit. A grasp of orbits, light delays and satellites makes it easier to follow how space connects to daily life, to new careers in AI and space and to the patient, long-term thinking behind the sector's future.