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Would it be technically possible, to build any 'small' space craft that could reach Mars, launching from earth?

The requirement would only be for the craft to make impact with the planet, crashing into it is totally fine. There is no requirement to carry cargo or passengers, or to land/orbit. There are no budget constraints. The challenge is simply to start on the surface of Earth, and end up on Mars. It would be necessary however to verify that the object did indeed reach its target.

If so, what are the limits of 'small'? The size of a car? The size of a cell phone?

What would the main limiting factors be?

As this is a very broad question, I am not looking for specific technical detail, but rather what the general issues/constraints might be.

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closed as too broad by Phiteros, Organic Marble, Nathan Tuggy, Hohmannfan, Jan Doggen Nov 2 '16 at 7:38

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ What are your expectations for it for reaching Mars? Does it need to get into orbit? Does it need to land? This is much too broad. $\endgroup$ – Phiteros Nov 2 '16 at 1:05
  • $\begingroup$ Ok, thanks, let me edit it down a bit. First question here, wasn't sure how broad I could get with a general interest question. $\endgroup$ – Ryan27 Nov 2 '16 at 1:07
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    $\begingroup$ Well, if you're asking about a lower bound with no specifications, you could technically just launch a sub-atomic particle at Mars using a particle accelerator and call it a spacecraft. $\endgroup$ – Phiteros Nov 2 '16 at 1:08
  • $\begingroup$ Good point. I will add one more requirement. I'd like to be able to verify that the object reached the planet. $\endgroup$ – Ryan27 Nov 2 '16 at 1:12
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    $\begingroup$ Thanks for the reference @uhoh. I think I will try to reask this with a narrower scope. Would launching from an earth orbit lift a lot of the limitations here? $\endgroup$ – Ryan27 Nov 2 '16 at 11:45
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The main challenge is interplanetary navigation, to actually be able to hit Mars. Spacecraft do not navigate by themselves - navigation is done on the ground, with help of Deep Space Network(s), precisely timing the radio signals and calculating the trajectories correspondingly.

Normally, a launch vehicle upper stage sends a spacecraft en route to Mars, ground tracking determines the trajectory and calculates the necessary corrections. Hence, your spacecraft needs a radio to be trackable. Radio needs power as well, normally solar power, so you need to add some solar panels. In theory, it would be possible to carefully calculate power required for every communication session and just use primary batteries, which might further simplify things, but it would be quite risky.

Next, you need to be able to perform a few trajectory correction maneuvers. For that you need a propulsion system with enough delta-V budget to perform the corrections, which regularly is 3-4 maneuvers in total. With precise insertion, good tracking, the trajectory corrections should be limited to tens of meters/second of delta-V total, so the propulsion system does not need to be big or powerful. To correctly perform TCMs, you do need attitude control with fairly accurate pointing capability, which you likely need for radio communications anyway.

This should be sufficient to make something impact Mars, and if you add it all up, you are going to find that a regular 1U cubesat, weighing 1Kg is not enough to fit everything in. You'll need something bigger, on the order of a hundred or few hundred kilograms, or with really advanced design maybe a 12U cubesat. There are multiple teams around the world trying to figure out how to make something as small work for interplanetary flight, and it's going to be a few years at least before this can happen. And even then, these will first use a "mothership" approach, where a bigger spacecraft takes the smaller ones to the target orbit first. Note that there are even some rare commercial opportunities available to launch interplanetary cubesats in up to 12U sizes, but not on a Mars-bound trajectory.

To detect if the impact took place, you'll know exactly how Shiaparelli team learned about it - radio transmission will terminate, abruptly.

For some examples of how such mission might be accomplished, you can look at early history of interplanetary flights. Mariner 2 weighed about 200 kilograms and was the first probe to make it to another planet. Some of the more modern small spacecraft in the same weight class have been SMART-1 and Lunar prospector. Also Japanese Sagigake and it's sister Suisei, weighing in at only 140kg.

Interestingly enough, there is an extra barrier for hitting Mars: planetary protection. Whatever you intend to impact on Mars, needs to adhere to some agreed upon cleanliness requirements. For more info, you can refer to this paper titled Interplanetary Cubesat Navigational Challenges

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For verification, let's say that we're launching 100kg of cyan powder which will be explosively deployed at low altitude shortly before impact. We will need a small computer, altimeter, pyrotechnics, heat shield, parachute decelerator, and so forth - probably at least another 100kg there. Radiothermal generator or solar panels, communication antenna, 50kg. So our total entry-descent-and-splatter package comes to ~250kg. You could probably make it smaller at additional cost.

So now you need a launcher that can send that package to Mars. From Earth orbit you could do the Mars transfer with a 2-3 ton stage. So any launcher that can put a small satellite into orbit could do it. The Vega launcher could do it for relatively cheap. https://en.m.wikipedia.org/wiki/Vega_(rocket)

A smaller launcher like the Japanese Lambda 4S couldn't do it -- it puts only 26kg in Earth orbit, and there's no practical way to build an interplanetary stage that small. https://en.wikipedia.org/wiki/Lambda_4S

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