The odds of a geomagnetic storm on Dec. 28th are improving with the launch of two CMEs toward Earth in less than 24 hours. NASA's STEREO-B spacecraft photographed this one on Dec. 26th:

261211 CME
© Space Weather
According to a forecast track prepared by analysts at the Goddard Space Weather Lab, the cloud should squarely strike Earth's magnetic field on Dec. 28th at 20:22 UT (+/- 7 hours). Another CME could deliver a glancing blow a few hours earlier on the same date. The double impact is expected to spark mild-to-moderate geomagnetic storms at high latitudes.

Mars is also in the line of fire. The first of the two CMEs is squarely directed toward the Red Planet--estimated time of arrival: Dec. 30th at 1800 UT. Using onboard radiation sensors, NASA's Curiosity rover might be able to sense the CME when it passes the rover's spacecraft en route to Mars.