Figure 8-23 Comet Nuclei (a) This image, taken by the Giotto spacecraft, shows the potato-shaped nucleus of Comet Halley. Its dark nucleus measures 15 km (9 mi) in its longest dimension and about 8 km (5 mi) in its shortest. The numerous bright areas on the nucleus are icy outcroppings that reflect more sunlight than surrounding areas of the comet. Two jets of gas can be seen emanating from the left side of the nucleus. (b) The nucleus of Hartley 2 is 2 km (1.2 mi) long. It spins on its short axis once every 18 hours, while tumbling around its long axis.