First, remember that temperature rating means that the manufacturer guarantees operation within that range, but the part may very well operate just fine outside that range. There are components with relatively low hard limits (aluminium caps, batteries, etc.) while other components can operate just fine over a much wider range. So I'd say get a temperature chamber and test your product in there.
And if you want to operate your Teensy at a very low temperature, you could just use some a heater. It will use some electricity (might be an issue on a satellite) but it works.
You could also insulate it, but then you may also need to provide a cooler (i.e. Peltier) for high temperature operation. Solid state thermoelectric devices are nice because they can do both cooling and heating.