Challenges of Space Travel

The video below shows the conditions NASA's new Orion Capsule will experience during a mission : 

Manoeuvering

Once a spacecraft leaves the Earth's atmosphere, the method of controlling speed and direction becomes completely different. The major change, however, is that engines are required only for manoeuvring; they are not required to maintain speed.

Newton's first law states:-

Also, without air resistance, a rocket cannot use manoeuvring flaps like an aircraft would; instead, it uses small rocket thrusters to change direction :

In the atmosphere, a vehicle must have its engines active to provide thrust, overcoming the frictional forces which act to slow it down. Without the engine thrust, the vehicle would slow down, due to the unbalanced Force acting against motion, until its forward Velocity became zero. 

Once outside the atmosphere, there is no air resistance acting to slow the craft down. So once out of the atmosphere, a spacecraft will travel at a constant Velocity in a straight line, without the need of engine thrust. 

Without air resistance to slow down a vehicle, retro rockets must be used. A retro rocket is simply a rocket engine which applies a force upon a rocket in the opposite direction to its motion, causing a negative Acceleration. 

The diagram below shows a Command module (on the Left) attached to a satellite (on the right):-

If the combined craft is assumed to be stationary to start, to move the spacecraft from position A to position B requires several engine burns:-

1. Command Module engine burn - Causes the spacecraft to Accelerate towards B.

2. Command Module engine cutoff - Spacecraft travels at a constant Speed towards B.

3. Satellite engine burn - Applies a negative Acceleration to the spacecraft, reducing its Velocity.

4. Satellite Engine cutoff - Spacecraft is now stationary at B. 

The above maneuver required only linear engine burns, but in order to turn the spacecraft, small maneuvering thrusters must be used. These are small clusters of rockets which can apply a Force perpendicular to the motion of the spacecraft, to change its direction. 

Distances 

Due to the distances involved with space, high velocities (around 16 km/s) are required to make travel between objects practical :

There are two main engine types used to accelerate a spacecraft to a high velocity : 

• Rocket Engines

• Ion Engines

A Rocket Engine provides a force by directing a stream of hot gases from a combustion reaction. The reaction force on the engine due to the gases provides the forward force of the spacecraft. The Rocket Engine provides kN of force, and therefore a large acceleration; however, the fuel will be rapidly used up, making rocket engines a challenge for long range exploration. 

The Ion Engine provides a force by accelerating charged ions in an electrical field to very high velocities. The reaction force on the thruster provides the forward force of the spacecraft. The Ion Engine proves only nN of force and therefore a tiny acceleration; however, it can do this for weeks or months, causing a large change in velocity. 

Velocity Change Without Engines

Another Method to accelerate a spacecraft is to use the gravtiational attraction of large bodies (such as planets) to change a vehicle's velocity.  This is known as a 'Gravity Assist' or a 'Slingshot'  manoeuvre :

As the spacecraft approaches the planet or large asteroid, the craft accelerates due to the gravitational effect of the planet and changes its velocity. On passing the planet, it will be travelling at a different velocity than before. 

A Gravity Assist manoeuvre was used by the Artemis II spacecraft in April 2026 (Timestamp : 'Days since launch = 1.0' in the gif below) to leave Low Earth Orbit and move towards the Moon at high velocity : 

Energy 

Extended travel through space requires substantial energy reserves to provide power to the spacecraft.

There are two main ways to provide energy to a spacecraft : 

• Radioisotope Thermoelectric Generator (RTG) 

• Solar Cells

In a Radioisotope Thermoelectric Generator (RTG), Heat energy is generated through radioactive decay. A thermocouple converts the heat energy into electrical energy, which is used to run on-board electronics. If a radioactive source with a long half-life is used, the RTG can provide power for a very long time.