The three frames show several-hour time exposures of radar echoes from binary asteroid 1999 KW4. The distance from Earth increases toward the bottom, and speed from Earth increases toward the left. The motion of the secondary (smaller) component about the primary (larger) component in these images is clockwise. Gaps in the trail are due to breaks in the data-taking. The primary appears much wider than the secondary because it is a few times bigger and is rotating much faster. Although the speeds along Earth's line of sight and distances from Earth of both components are the same where their echoes overlap, their positions in space are different. The two components orbit a common center of mass, and each component's average distance from that point is inversely proportional to its mass, just like the distances of two people from the pivot point of a balanced seesaw. The motion of the relatively massive primary is much less obvious than the motion of the secondary, but can be seen in the double appearance of the primary's top edge in the two time exposures that follow the secondary from in front of the primary to behind it. These images were obtained with the Goldstone radar system (8560 MHz, 3.5 cm). These images, which show the distribution of echo power in time delay and Doppler frequency, have overall extents of 37.5 microseconds (5.6 km) by 67 Hz (1.2 m/s).
P-50904
