Science feed
~5,730 years
Carbon-14 is a radioactive carbon isotope with a half-life of about 5,730 years. Radiocarbon dating uses its declining abundance in once-living material, together with reference measurements and calibration, to estimate ages.
0.01°C
Water can coexist as ice, liquid and vapor near its triple point, about 0.01 °C and 611 pascals. In a vacuum demonstration, evaporative cooling can make liquid water freeze while it is still boiling.
Low contrast
The Great Wall is generally not discernible with the unaided eye from low Earth orbit, and it is not visible from the Moon. Saying it can never be seen from any altitude called space is too absolute: viewing conditions, equipment and identification all matter.