A femto probe is a probe that weights less than 100 grams. I would expect that it's not much more than:
- Omnidirectional antenna (locating Earth would require relatively heavy equipment, right?)
- Power source (solar cell or an unrechargable battery, depends on destination)
- Micro computer chip
- Super tiny flywheels for rotation
- Thermal control system
- Ultralight sensors (thermistor, photoresistor, could you suggest more?)
- Single scientific device. E.g. I found an of the shelf camera that weighted a little more than 1 gram
Could it do some useful science? If yes, why no space exploration agency has tried sending a bunch of them? Seems to be cheaper, more reliable (redundancy) and can be at many locations at once.