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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? $\endgroup$
    – Organic Marble
    May 23 '16 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 '16 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 '16 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 '16 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 '16 at 9:02
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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 '16 at 11:19
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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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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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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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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage ♦
    May 26 '16 at 18:00

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