Rockets launched into space can be suborbital (brief visit to space) or orbital (staying in motion around the Earth) or can escape Earth’s gravity to travel deeper into space.
What is space?
The official beginning of space is 100 km above the Earth’s surface. The gases that make up the Earth’s atmosphere thin out rapidly as height increases. If you were in a rocket travelling upwards, at a height of 11 km, you would have passed 77.5% of the Earth’s atmosphere. At 31 km, you would have passed 99%. At this height, you would see the blackness of space above you rather than the blue of the sky.
Gravity and space
The Earth is a massive object (6 x 1024 kg), and its gravitational influence extends well into space. The gravitational influence of Earth changes only a little for heights at which most low Earth orbit (LEO) satellites and the International Space Station operate.
| Height above Earth’s surface (km) | Weight force on a 10 kg mass (N) | |
| 0 | On Earth’s surface | 98 95 88 82 6 2 |
At a height of 1000 km above the Earth’s surface, the gravitational force on a 10 kg mass is about 75% of its value on Earth.
This means that, even if a rocket reaches space, the force of gravity will still pull it back towards the Earth.
Suborbital rockets – visiting space briefly
Rockets that reach space and then return to Earth are classed as suborbital rockets. They are usually sounding rockets (‘sounding’ refers to taking measurements).
Sounding rockets are designed to take measurements of the atmosphere or to perform microgravity experiments as they travel up through the atmosphere and then fall back to Earth.
Sounding rockets can travel much higher than weather balloons (another way of measuring the atmosphere), which can only travel up to an altitude of 40 km. Some sounding rockets travel up to 950 km or higher before falling back to Earth.
Getting rockets into orbit
To get rockets into orbit, they need much more thrust than the amount that will get them up to the required altitude. They also need sufficient thrust to allow them to travel with very high orbital speed.
For example, the International Space Station (ISS) is at an altitude of about 360 km. To build the ISS, each rocket carrying people and parts needed enough thrust to give it an orbital speed of 28,000 km/h. At this speed, an object at that height will stay in orbit around the Earth. If speed is less than this, an object will fall back to the Earth.
To help visualise this, the famous astronomer, mathematician and physicist Sir Isaac Newton imagined a cannon placed on a very high mountain. If the cannon ball is fired with enough speed, it will travel so fast that it falls towards the Earth but misses it entirely. This is what satellites are doing.
Around 3000 satellites made by humans currently orbit the Earth. Each of these has been placed into orbit by a rocket. Some are as small as a washing machine, others the size of a bus, and the International Space Station is as roomy as a 5-bedroom house and, with its solar panels extended, is the size of a rugby field.
Getting deeper into space
To travel beyond the Earth towards other planets and beyond, very large multistage rockets are needed to give a probe such as Voyager 1 enough thrust to escape the gravitational pull of the Earth and the Sun.
Voyager 1 is a 722 kg space probe that was launched in 1976 on a tour of the Solar System. More than 30 years later, it continues to transmit data and to travel into interstellar space.
In March 2004, the European Space Agency (ESA) launched the Rosetta spacecraft from Earth. It then took Rosetta 10 years to catch Comet 67P, where it then landed a probe.
NASA launches have included the Juno mission on 5 August 2011. Juno will take 5 years to reach Jupiter, having travelled a distance of 2800 million km.
Getting more satellites into space
In May 2017, Rocket Lab launched the first test flight of their Electron rocket. The test was part of the build up towards running a commercial service, launching rockets with small payloads into space. In January 2018 Rocket Lab celebrated when it became one of only 11 countries to have successfully launched a satellite into orbit. They have developed innovative new technology to make rockets more affordable for a wider range of organisations and companies. Learn more about Rocket Lab here.
Nature of Science
Scientific discoveries often lead to the development of new technologies. For example, studies of rocket propulsion have led to more efficient rockets allowing bigger payloads (such as satellites) to be placed into orbit. New technologies often lead to new scientific discoveries. For example, deep space probes help us learn about the nature of our solar system.
Related content
Explore some of the ideas that are fundamental to understanding rockets:
Activity idea
In the activity Investigating rocket motion students investigate the motion of a rocket using a spreadsheet with graphs of motion included. They adjust variables and investigate how height and speed are affected.
Explore the ideas of force and thrust further in the Rocket launch challenge activity. Your challenge is to make the rocket go as high as possible and launch a payload 400 km above the ground.
Useful links
Investigate Newton's cannonball thought experiment – how fast would a cannon ball need to be if it was fired from the top of a mountain to put it into orbit around the Earth. See this simple physics HTML5 simulation or see the animations on this Wikipedia page.
View this video of the Ariane 5 lift off on 20 May 2011.
Find out more about the Juno mission to Jupiter on the NASA website.
Learn more about the Voyager missions.
Check out current NASA missions.
