Wayne Turner, of Dunedin, asks :-
Is it necessary for a rocket, while in space, to be jet-propelled?
John Campbell, a physicist at the University of Canterbury who wrote a brief biography of Bill Pickering at www.rutherford.org.nz under Other Places - Havelock, responded.
No, but it depends on why it was sent. A rocket must carry all its fuel and oxidiser, and not, like an aircraft, rely on the oxygen in the atmosphere to burn the fuel. But once up there it can coast at constant speed.
If a rocket is fired into space in order to go into orbit around the Earth, it may need to be fired again to get it into the correct orbit. Then it needs no more rocket firing unless it is in a low orbit, i.e. the shortest distance for optical spying or monitoring the Earth's surface. At those altitudes there will be sufficient gas particles in space to slow it down such that it spirals downward and every now and again it will need to briefly fire rockets to raise it up again.
Similarly if it is being sent to land on the Moon or other body it can coast there but will need to fire rockets to slow it during the landing.
If a rocket is fired off from the Earth's surface with sufficient speed that its initial kinetic energy just matches the potential energy that bound it to the surface, then when the spent rocket is a long way from the Earth it will keep going at ever diminishing speed. If it is sent off with a higher speed then it will travel at a higher speed in space (e.g. if you wish to get somewhere more quickly.)
In outer space there are many weak forces acting on coasting spacecraft that can alter its speed and/or direction. These can include a "wind" of particles ejected by the Sun, and even the pressure caused by photons (light) from the Sun being reflected or absorbed by the craft. Sometimes, a brief rocket firing is needed to correct for these forces. Or they can be used to "sail" the craft in space.
I recall an early propulsion method proposed whereby the spacecraft drops a nuclear bomb behind it and the high speed particles emitted propel the spacecraft forward at higher speed. That has many technical and health issues.
A spacecraft is also affected by the gravitational attraction of nearby planets etc. If it closely approaches a planet it will have a slingshot effect, whereby well before and after the speeds are the same but the direction has changed hence the speed relative to the Sun also changes. Under the direction of Bill Pickering, the New Zealander from Havelock who led the USA's deep space exploration, the spacecraft Gallileo flew by Venus to speed it up to enable it to travel farther from the sun on its way to Jupiter. That mission also required two close encounters with earth to have enough speed to get to near Jupiter.