The Planetary Society has an interesting FAQ on this subject: http://www.planetary.org/programs/projects/innovative_technologies/pioneer_anomaly/update_20050720.html

Also explains why it is seen with Pioneer 10 and 11 and not Voyager 1 or 2 or other more "modern" spacecraft.

From the FAQ: The Pioneers are spin-stabilized spacecraft. The Voyagers are three-axis stabilized craft that fire thrusters to maintain their orientation in space or to slew around and point their instruments. Those thruster firings would introduce uncertainties in the tracking data that would overwhelm any effect as small as that occurring with Pioneer. This difference in the way the spacecraft are stabilized actually is one of the reasons the Pioneer data are so important and unique. Most current spacecraft are three-axis stabilized, not spin stabilized.

Quote: Mars will be even tougher, these models suggest. Some scenarios call for missions that would last 18 months or more. "Right now there's no design solution to stay within safety limits for such a Mars mission," Cucinotta says. "Putting enough radiation shielding around a spacecraft would make it far too heavy to launch, so we need to find better lightweight shielding materials, and we probably need to develop medical techniques to counteract damage to cells caused by cosmic rays." He notes that one of the biggest obstacles to progress in this area is "uncertainty in the types of cell damage deep cosmic ray exposure can cause. We still have a lot to learn."

Source: http://science.nasa.gov/headlines/y2009/27may_phantomtorso.htm?list776758

A Parachute Test Vehicle (PTV) test failed at El Centro, Calif. The PTV was released from a B-52 aircraft at 15,240 meters and the drogue chute programmer was actuated by a static line connected to the aircraft. One drogue chute appeared to fail upon deployment, followed by failure of the second drogue seven seconds later. Disreefing of these drogues normally occurred at 8 seconds after deployment with disconnect at deployment at plus 18 seconds. The main chute programmer deployed and was effective for only 14 out of the expected 40 seconds' duration. This action was followed by normal deployment of one main parachute, which failed, followed by the second main parachute as programmed after four-tenths of a second, which also failed. The main chute failure was observed from the ground and the emergency parachute system deployment was commanded but also failed because of high dynamic pressure, allowing the PTV to impact and be destroyed. Investigation was under way and MSC personnel were en route to El Centro and Northrop-Ventura to determine the cause and to effect a solution. TWX, George M. Low, MSC, to NASA Hq., Attn: Apollo Program Director, Jan. 11, 1968.

Source: http://www.hq.nasa.gov/office/pao/History/SP-4009/v4p2h.htm