A tangle of protein (green) in this scanning electron micrograph of a brain cell of an Alzheimer's patient lies within the cytoplasm (blue) of the cell. The tangle consists of clumps of a toxic form of tau. Thomas J. Deerinck/Corbis hide caption

itoggle caption Thomas J. Deerinck/Corbis

The Rosetta spacecraft, which orbits the comet, captured this series of images of the Philae lander bounding off the surface. The precise spot the lander came to a stop remains unknown. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA hide caption

itoggle caption Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Sherri Erkel and her daughter, Asa, cook dinner in their kitchen in Iowa City, Iowa. The Erkel family is part of an EPA study measuring the amount of food wasted in U.S. homes. Pat Aylward/NET News hide caption

itoggle caption Pat Aylward/NET News

A global positioning receiver on the top of a combine harvester at a farm in Warwick, Md., in June. The equipment uses sensors and computers to help drive the combine along the route where the crops were planted, judge the composition of a crop and generate crop yield reports. Brendan Smialowski/AFP/Getty Images hide caption

itoggle caption Brendan Smialowski/AFP/Getty Images

Lightning strikes near Ben Hill Griffin Stadium at Florida Field in Gainesville, Fla., in August. A new study says a rise in average global temperatures due to climate change will increase the frequency of lightning strikes. Phil Sandlin/AP hide caption

itoggle caption Phil Sandlin/AP

Engineers at the European Space Agency fear that they won't be able to communicate with the Philae lander after Friday. Here, lander manager Stefan Ulamec (left, in foreground) watches as data confirming the comet landing arrived Wednesday. European Space Agency hide caption

itoggle caption European Space Agency