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Another run of kerbal got me thinking about what, in reality, makes up the limit for small, but still useful space probes.

The rationale should be obvious: The rocket equation does not hurt that much when we can make our payloads smaller and smaller. Assume a 10kg dry mass. With 260s ISP you get close to 6km/s delta-v out of a 100kg probe, enough to bring you into Mars orbit (and thus probably also somewhere in the belt). On a Falcon 9 you can start 220 of such probes at once, bringing launch cost down to about 300 000$. Probe production would also benefit from economics of scale. Finally, a certain risk of loss is not big deal anymore.

However, the obvious question is: Are such probes useful? Or rather: What is the reason we do not launch such Microprobes today? Is it:

  • The engine (is the smallest possible liquid fuel engine much bigger)?
  • Power supply (solar panels, batteries)
  • Controls
  • Scientific instruments

What is driving the payload weight currently?

edit: David's answer below focused on optics and communication. So I try to be a little bit more precise: A large CMOS chip is roughly 16cm². Does one need that much more for in-orbit photography (because there is so little light)? According communication: Is antenna size really the issue (not wattage)? If so, why shouldn't you fold it? Say we put a conventional, lightweight camera, top notch solar cells, a small battery, a raspi and a conventional GSM like sender on a probe and send it to a Phobos polar orbit. This "science package" should weigh under 1kg. That gives us another 9kg for attitude control, shielding etc. what am I missing?

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    $\begingroup$ Antenna size. to transmit useful info back? $\endgroup$ May 23, 2016 at 21:54
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    $\begingroup$ Not all satellite applications require communication, or even GPS for navigation. But if one does, a tight beam (for high gain) will need a small $\frac{\lambda}{d}$ fraction, and there are several ways to get that. One way is to use a larger denominator by making the antenna larger than the spacecraft - something unfolds or a wire with a weight at the end and slow rotation for example. Or a smaller numerator - just go to shorter wavelength - light for example. This has already been demonstrated at least twice - from Mars and from the ISS. Space and Optics were meant for each other! $\endgroup$
    – uhoh
    May 24, 2016 at 1:31
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    $\begingroup$ With such tiny probes, delta-V becomes so "inexpensive", that you could do with just a very rudimentary antenna capable of communication from LEO - and perform a fully autonomous round-trip to bring the data (and samples!) back in range! $\endgroup$
    – SF.
    May 24, 2016 at 8:19
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    $\begingroup$ ...OTOH the limiting factors are still power, chip density (your fancy 14nm technology CPU won't last a day out of Earth's magnetosphere), and machining capability for micromechanisms (drive, RCS, gyro.) Plus retaining fuel of high energy density. $\endgroup$
    – SF.
    May 24, 2016 at 8:25
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    $\begingroup$ @uhoh: Yes, but in that 10kg include 10g of return mass and 100g of departure mass (from, say, Jupiter orbit) - just memory and bare-bones radio + a pair of solar cells to power it up once it's near enough; all science instruments and heavyweight stuff needed to go "there" are dumped upon beginning of the return trip; you're getting a return trip of the same delta-V on 1% of payload mass. The key here is going to extremes with the wet:dry mass ratios. $\endgroup$
    – SF.
    May 24, 2016 at 9:02

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Aside from optics and communications, there are other limitations to miniaturization.

For example, smaller objects become harder to track if communication is lost. Our current orbital debris tracking system only tracks objects down to about the size of a marble, and with greater numbers of small items it becomes harder to avoid collisions with larger objects.

Electronic components also suffer from reliability issues from radiation. Scaling down many components results in higher component density, and thus higher error rates as shown in Radiation-induced soft errors in advanced semiconductor technologies - Baumann, R.C. and Soft-Error Rate of Advanced SRAM Memories: Modeling and Monte Carlo Simulation - Autran, et al.

As explained in another answer:

Miniature components are more affected by this radiation and that can produce unexpected consequences: Computer resets, memory corruption, noise on instrumentation readouts (especially for imaging satellites).

Down-scaling amplifies these consequences:

With the constant downscaling of microelectronic devices, the sensitivity of integrated circuits to natural radiation coming from the space or present in the terrestrial environment has been found to seriously increase

(Autran, et al.)

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What is driving the payload weight currently?

  • Optics.
    You need a big camera with a big lens if you want pretty, high resolution pictures. There is no way around this. It's physics.

  • Communications.
    You need a big antenna with all the big stuff that goes along with that if you want to communicate at a high rate. There is no way around this. This too is physics.

  • The cube-square law.
    In some places, this benefits smaller payloads. But in many other places, this benefits larger rockets, larger payloads. Physics, baby!

In short, its physics. What drives payloads to be smaller than optimal is economics.

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  • $\begingroup$ Adding to this, the communications part comes down to the Shannon-Hartley theorem and the free space path loss equation and to a lesser extent by the formulae describing antenna gain as a function of size (which are different depending on the type of antenna involved, such as a parabolic reflector antenna -- don't forget its feed antenna characteristics!). $\endgroup$
    – user
    May 25, 2016 at 11:19
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For interplanetary probes, the drivers are probably economic as much as physical.

Even for a tiny probe, your mission is going to start with an orbital launcher, likely costing upwards of \$100 million for the launch. It doesn't make sense to send less than several million dollars worth of scientific instruments in that case.

Your suggestion of sending a large number of small probes to a single destination has a big drawback; all the control, guidance, and communication systems must be replicated on each one. Ten microprobes each carrying 3kg of instruments is going to wind up heavier than one miniprobe carrying 30kg of instruments.

New Horizons is a good design example to study. 400kg dry, 30kg of scientific payload.

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I see several constraints to the size related to the space environment, spacecraft design, as well to scientific instrumentation. Although micro-probes are being considered, in space, EVERYTHING is a trade-off. I'll start first with the restrictions, but I also add some details after that about what is currently enabling those kinds of probes.

Radiation outside the Earth's magnetosphere is relatively high. A considerable amount of cosmic rays that are usually deflected by the magnetic field will hit your spacecraft. This causes several headaches to spacecraft designers, but one of the most prominent is that the reliability and lifetime of electronics drop considerably. Something like a raspberry pi or any regular consumer electronics has no hope to survive in space for any useful amount of time. Effects of radiation on electronics

You can improve your mission chances of success by adding considerable shielding which will definitely add weight, but most importantly, you MUST use radiation-hardened electronics, which are neither cheap nor anywhere as diverse as regular commercial products. (for example something like this: Rad Hardened FPGA). And this does not only applies to semiconductors, but also to PCBs, batteries, data busses, and communication systems, so you either buy the spacecraft components available on the market, or you build custom ones, which are even more expensive.

To give you an idea, the cheapest onboard computers for CubeSats cost around $5000 and will survive maybe 3 years on Low Earth Orbit which is highly protected from radiation compared to interplanetary space or other planets. Something like the Juice probe that is soon to be launched to Jupiter and will travel for 6 years before arriving, needs to survive for decades in space and requires very specific and even custom components to achieve its mission.

Added to all this, because of the manufacturing processes needed for radiation hardening, those electronics are also bulkier and less capable than their regular counterparts.

Then there is the temperature as well, consumer electronics are made to operate at a very mild range of temperatures (-20 to 40 degrees Celcius). Space electronics must be capable to operate in considerably higher ranges. (-40 to 85 degrees Celcius with proper thermal management in the Spacecraft), because in space when the sun hits you is REALLY hot, and when it doesn't is REALLY cold.

Then you have the payload part. You mention a camera. Well, any camera needs to be radiation hardened as well so you can't simply put any commercial device, but you must also ask why you want to send a camera.

You can take pictures for sure, but the resolution is highly reduced, if you want better resolution for example to study geological processes on another planet, you're going to need better bigger equipment. But most of the time, the scientific payloads are not cameras in the commercial sense.

Yes, you can have pictures that are useful, but most payloads are designed with a very specific type of research in mind, we're talking about ground composition studies, searching for subsurface water, etc. Those kinds of payloads are not small at all. The Mars Reconnaissance Orbiter has like 4 different types of cameras for very different types of data collection, a spectrometer, subsurface radar, etc (MRO FactSheet), and those are big, shoebox big.

As the payloads increase in size, also the power requirements, which will increase the battery size, solar panels... Sometimes you even need shielding between payloads so one does not interfere with the other.

The reasoning with most missions is that if you are already sending something, let's try to maximize the useful mass of your spacecraft.

Another limitation I can think off is communication. NASA uses the Deep Space Network, which consists on 3 sites around the globe with huge dishes, to communicate with anything that is far from the Earth. They even plan to use it for Artemis. But the probe must be able to reach them, there are limitations regarding the power of a transmission amplifier and the gain of the antenna, and their size, so if you go too small you risk not being able to communicate at all. Yes you can do deployable antennas, but that adds complexity and risks to the mission (design, testing) and makes it most expensive.

Now, one important point is that nanosatellites didn't really become mainstream when it comes to science until very recently. There was no availability of components or payloads to build such a device, but that has changed. Along with Artemis 1, there are 10 CubeSats that will be deployed around the Moon to perform scientific experiments Artemis Cubesats. Along with NASA's InSight Mars lander, there were 2 CubeSats that were designed to act as relay satellites for a technology demonstration mission (MarCO). ESA's Hera mission will also have 2 CubeSats that will collect additional data (Hera). So nanosatellites are starting to be considered for this type of mission.

But so far there are restrictions, you usually would need to stick to only one payload, if you want very specific data, probably a custom payload that will take years and a lot of money to design, test and manufacture. Sometimes you would need communications relay infrastructure in place, for example, the MRO could be used as a relay on Mars.

Then, due to the small size, propulsion is really an issue, is not only ISP, you need the right thrust as well. You cannot perform an orbital insertion with a very high Isp engine if it takes a month to actually achieve the required DeltaV (for example ions, which are very efficient but very very slow). The selection of thrust and Isp is also a trade-off, increase one and the other one will decrease, which limits the actual useful alternatives. Right now most of these missions have very low to no propulsion at all and rely on hitchin' a ride with other missions to get to their destinations and have very reduced maneuverability.

Things are changing of course. More and more research and development are going into micro and nanosats, and I expect these types of missions will increase considerably with time.

One last thing, you can probably send 200 probes like the Falcon 9 example you mentioned, but that only makes sense if you NEED the 200 probes, like in some very specific type of experiment or data collection mission, but if you can collect the same data with a single bigger spacecraft, then by far, is more efficient to send only one.

I hope this helps!

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One particular problem of putting miniaturised components into space is the effects of cosmic radiation: Miniature components are more affected by this radiation and that can produce unexpected consequences: Computer resets, memory corruption, noise on instrumentation readouts (especially for imaging satellites).

This research paper, while terrestrial, shows some of the effects of scaling technology:

The author has shown that while the SER of DRAM in a system is relatively unchanged by scaling, SRAM and peripheral logic system SER are increasing rapidly with each new technology node

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    – called2voyage
    May 26, 2016 at 18:00
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It depends on what you are trying to do. For some applications a few grams of DNA are sufficient. No energy, radio or antenna required: Eventually, the DNA will build all of that on destination and call back home.

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