That's an ugly question! "The bits that keep it on-orbit" are not a discreet set of items. A modern satellite like NPOESS
is comprised of a "bus" and its "payload(s)". The bus contains the thrusters for orbit maintenance, the fuel tanks for those thrusters, the reaction wheels that keep the sensor from tumbling, the solar panels and batteries that keep the sensors working, the heaters that keep sensors from freezing when they're on the dark side of the satellite, and the command antenna that relays information back and forth from the sensors to the ground. Here's a simple list
of all the things a spacecraft bus provides for its payload.
The payload consists of the sensors, any reaction wheels they have internally, any radiators those payloads need to stay cool, and of course, the cables and vibration mounts required to attach them to the bus. One payload on NPOESS is so vibration sensitive that it's actually going to fly "in formation" with the bus -- the spacecraft will actually release it so that it floats a centimeter above the surface of the bus, attached only by its data and power cabling.
Add to all of those concerns that NPOESS is only one type of satellite, and that communications satellites (normally geosynchronous) are sized differently and carry different payloads altogether. To make an analogy, your question is like asking what a modern PC would weigh without the case or power supply -- there's an answer, but calculating it is really tough.
Give me more details -- name the satellite -- and I could probably begin to help you figure out the payload mass, but you're not likely to get an answer that doesn't include fuel or thrusters. Here's a NASA
communications satellite. Notice that they still use pounds and the word "weight", and have a giggle at their expense.
In deference to Ethilrist's
absolutely dead-on pedantry, I'll point out that weight is an earthbound notion, and is often used in America interchangeably with mass. When you work with a satellite, someone saying "weight" is assumed to mean "mass," and kilograms are the unit of choice.