On returning from Batatas, Marcus Devani will suggest you return to base, doing so, you will be greeted by an excited Smith who has developed some antimatter weapons for use against the aliens. On launching from the base, Marcus will ask you to go to Christmas L-point and help out there.
1) Head for Christmas L-point, which is shown on your contact list.
2) After arriving, kill any infected ships and aliens coming through the L-point
3) Continue doing so until you are told to retreat, when this happens, flee as fast as you can.
4) Upon returning to the base, you will see a cut scene depicting the aliens arriving at the jump accelerator and you will be put back in your base with an e-mail waiting for you.