You can now build Mass Drivers on planets that will send mineral packets to other populations in the same system. I will probably change the figures a little over time but the basic mechanics below will remain the same.
A Mass Driver costs 300 BP to build, using 100 tons each of Duranium, Neutronium and Boronide, and is the same size as a factory or mine. You can move them around with freighters so Load and Unload Mass Driver orders have been added. One Mass Driver can launch 5000 tons of minerals per year and catch any amount of minerals per year.
If you have a planet with at least one mass driver then a list of possible destinations will be shown on the Mining tab of the population window. Destinations include any of your populations with at least one mass driver and any detected alien populations regardless of whether they have a mass driver. Each 5-day increment, a mineral packet will be assembled and dispatched toward its destination. The packet is assembled by calculating the amount of minerals that can be sent. This is based on the number of mass drivers * 5000 tons * (Increment Length/Year). A roughly equal amount of each available mineral will be sent. This is done by the program checking each mineral type in turn and adding up to 10 tons to the packet. This process loops until either all available minerals have been added or the packet reaches its maximum size. The speed of a packet of maximum size is 1000 km/s. Smaller packets than the maximum possible will travel more quickly. Half size would be double speed, quarter size would be 4x speed, etc.
When a packet arrives at its destination, the program checks for a mass driver to catch the packet. If one exists then the minerals are added to the population's stockpiles. If no mass driver exists, a planetary bombardment attack is carried out using one tenth of the mineral packet size. So a 300 ton packet hitting a planet would be the same as a missile with a strength-30 warhead.
Attacking planets with mass drivers is therefore possible but easily defended. A single mass driver will defend against all mass driver 'attacks'. However, if you can catch an opponent unawares it could be useful. Any such attack will be rapidly countered so the initial strike would have to be as large as possible. I should also note here that packet sizes are designed for 5-day increments. You could create larger packet sizes by using a 30-day increment, which would create a more devastating attack. That was not my intention so it is up to individual players to decide if they want to take advantage of that exploit to launch an attack. Although perhaps it could be argued that leaving the mass drivers idle for thirty days before they launch that packet just builds up their energy reserves for a huge launch
EDIT: I forgot to mention that mineral packets are invisible to passive sensors because they have no power signature but can be picked up on active sensors as if they were a ship of same tonnage. For example, an 800 ton packet would be detected by active sensors as if it was an 800 ton (size 16) ship but would be completely invisible to all passive sensors.
Steve