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)
- 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?