# How do space probes find their way and how much fuel do they use to travel?

How do space probes find their way, for example, to explore dangerous planets like Jupiter or Saturn that have powerful magnetic fields? Do scientists control it from stations in NASA or they have a built in predefined program?

My second question is, how much fuel do probes (e.g. Voyager 1) use to travel? (Explain its equivalent with car consumption). Are they all powered by their own solar panels?

• I think this question should remain open. There is a clear explanation, so I think the close vote is inappropriate. I will answer in time, but I hope some of the more orbitally inclined folks will answer before me. Commented May 24, 2023 at 19:43
• What a fantastic question. Beginner questions are exactly as valuable as expert questions, so long as (obviously) they are clearly written. This is particularly the case on technical sites like say SO programming sites. Commented May 25, 2023 at 13:58
• The first question especially is a good one, but the second question should have its own home. One question per... er... question, please. Commented May 26, 2023 at 19:38
• Dangerous planets, pfft. Try flying too close to the sun. The Insane Engineering of the Parker Solar Probe - They redid the math so well that they got two gravity assists per pass of Venus. Amazing. How much fuel? too much; they had to add a third stage to the second largest existing rocket, the Delta IV Heavy (Falcon is the largest). Commented May 26, 2023 at 22:58

As to whether space probes are controlled, or follow a predefined program, it's yes to both.

One way to think about trajectory planning is that space probes are like billiard balls. By being very accurate with the initial hit (in spaceflight: the rocket flinging the probe away from Earth), the future course of the billiard ball can be made so it visits several interesting locations as it bounces around (in spaceflight: encounters the gravitational fields of solar system bodies), with no further propulsion needed.

You can, however, bring some rocket propellant along with you. It eats into the mass budget of things you would rather have brought, like scientific instruments. This extra propellant can be used for small adjustments, or bigger stuff like slowing down when arriving at a planet (otherwise you just fly by).

As for how much fuel space probes use to travel, this is hard to answer. For most of the time, they just coast along, using no fuel at all. The only times a spacecraft needs propulsion is when it need to change its velocity. Reading about delta-v budgets would perhaps be enlightening, but really, no meaningful comparison with car consumption can be made.

Finally, yes, solar panels is the most common way to get power onboard. Other options include 1) Just bringing batteries and run on them until they are empty (short missions) and 2) Bringing an RTG, a small piece of radioactive material giving off some heat and electricity.
Keep in mind that propulsion changing the velocity and course of a spacecraft is completely separate from powering equipment, like cameras and computers, onboard.

• "SE - stop firing the good guys" - what kind of visual tools do NASA engineers use to determine the probe's current position and its position relative to the planets at any point of time in the future? Commented May 24, 2023 at 20:32
• great analogy, already upvoted, but one nit to pick: solar panels predate satellites, and the first solar-powered satelliteis from 1958 predate the first interplanetary missions Commented May 24, 2023 at 22:10
• ... 1. Billiards suggests that planets are big and fairly easy to hit. Space is more like billiard on a football-field-sized table. 2. There's a lot of friction on a billiard table (cloth). Space is very different in that regard. 3. On a billiard table one typically points in the direction one wants the cue ball to go initially. In space, one more typically points in the opposite direction, anticipating how gravity will bend the trajectory. Even "short" travel between neighbouring orbits always involves strong gravitational bending, unless you have vast amounts of fuel to spare. Commented May 25, 2023 at 16:06
• @leftaroundabout If you've ever played air hockey on one of those tables with a curved bed, I totally recommend that as an analogy. (Plus it's a lot of fun! :) Commented May 25, 2023 at 19:50
• Related story: the Pioneer 10 and 11 probes unintentionally steered using RTGs. Their heat produced an unimaginably tiny but off-center force against the probe. Over the course of 30 years, it was enough to push them 400,000 km off course. In the vacuum of space, there's nothing to slow you down and every little nudge counts.
– bta
Commented May 26, 2023 at 23:37

To elaborate on fuel consumption: Space is mostly empty, there is almost nothing to slow you down. Vehicles on Earth constantly have to fight against friction and air resistance. In contrast a space probe launched at several kilometers per second will just keep going and will only change its speed or deviate from its course if gravity acts on it (or if it hits something, even if it’s just light). For example because it comes close to a planet. If the probe wants to change its course it has to fire its thrusters which basically work by throwing stuff (expanding gasses or accelerated ions) out the back as fast as possible. There is nothing to “push” against, so something like wheels or propellers simply can’t work.

Remember that orbiting objects (e.g. artificial satellites orbiting the Earth) travel at several kilometers per second and can do so basically indefinitely without expending any fuel as long as they are high enough up to not encounter any stray atmosphere.

Edit: As for how much fuel is required: You really need the Rocket Equation for that. It basically depends on how efficient (Isp) your engines are and how much of your rocket’s mass is propellant. For rockets launching into Low Earth Orbit it’s not rare that 90% of the launch mass is just propellant. For example a Falcon 9 is 549t at launch, weighs ~30t when empty and can only put 22.8t into Low Earth Orbit. All of that only to get through the Earth’s atmosphere and achieve 7.5km/s delta-v.

• My intend is "If the probe wants to change its course .." part. or If the probe wants to scape from planets gravity/magnetic field gravity ... Commented May 25, 2023 at 7:48
• @C.F.G Probes aren't typically responsible for breaking orbit - the launching rocket puts them on their course and they only have to handle minor corrections from that point forward. There are exceptions; the Hayabusa missions left Earth orbit under their own power, and later returned. These probes used ion engines that ultimately derive their power from solar panels. Commented May 25, 2023 at 8:56
• The planets are moving on their course around the sun without needing fuel. Once moving, neither do the probes. Commented May 25, 2023 at 16:00
• @supercat That is a pretty opaque way to say "In any direction away from Y, if you go fast enough you won't fall back to Y".
– user51397
Commented May 27, 2023 at 11:29
• Not only that one won't fall back to Y, but the total magnitude of delta V produced by Y will be bounded. Commented May 27, 2023 at 13:12

Midflight, the navigation is done on earth. The deep space network can with very high precision calculate:

• what direction the probe is from earth: measuring the angle of the radio signal from the probe.
• how far away the probe is from earth: a radio signal is sent from earth, received by the probe and returned. By measuring the time delay (radio signals moves at a known speed through space) the distance can be calculated
• what speed the probe: a radio signal with known frequency is sent from earth and is returned from the probe. As the probe moves away from earth the frequency will be lower (doppler shifted) which allows the speed to be calculated.

In effect: direction, distance and speed allows us on earth to calculate any necessary orbit changes compared to where we want the probe to go.

The probe gets a command, something like: align your spacecraft towards a specific point (using its star finder telescope), light the propulsion engine for x seconds.

• That's the nice thing about space. Everything is so large, spaced so far apart, and moves so predictably that there are no sudden surprise obstacles. Objects can be seen and their courses predicted years in advance, so controlling everything remotely is rarely a problem.
– bta
Commented May 26, 2023 at 23:44

To answer the question on navigation, the Deep Space network allows ground crews to monitor those craft constantly. If there are issues with navigation, ground crews can send commands to order the guidance systems to change course. Crafts typically have some sort of telemetry and sensors to maintain course.

With regards to fuel, most of the fuel is burned during launch and separation. On deep space missions, the craft pick up speed via gravity assists from other planets along the way. So essentially these crafts mainly consume fuel for on-board systems as opposed to actually thrust. The Voyagers have isotopes on board that are powering systems. However, Deep Space 1 was the first deep space craft to use an ion engine to build thrust during its mission. It carried about 250 lbs. of hydrazine and xenon to fuel its thrusters.

• To clarify: the fuel burned during launch and separation is on the rocket, not on the spacecraft. Commented May 25, 2023 at 6:52
• So essentially these crafts mainly consume fuel for on-board systems as opposed to actually thrust - What? Have any probes used consumable fuel (via a fuel cell?) for onboard electrical power? The normal options are solar panels or RTGs. RTG have limited life-time due to the radioactive stuff decaying, but drawing more power doesn't make that happen faster so I wouldn't really call that "fuel", especially not without distinguishing it from propellant. Commented May 25, 2023 at 10:53