The greenish-yellow dot in the upper left is the potentially hazardous near-Earth object 1998 KN3 moving past a cloud of dense gas and dust near the Orion nebula in the far, far background. NEOWISE, the asteroid-hunting portion of the Wide-field Infrared Survey Explorer, or WISE, mission, took this infrared picture. Because near-Earth asteroids are warmed by the Sun to roughly room temperature, they glow brightly at the infrared.
Infrared light from asteroids is used to measure their sizes. Combined with visible-light observations, that data can also measure the reflectivity of their surfaces. The WISE data reveal that this asteroid is about 1.1 km in diameter and reflects only about 7 percent of the visible light that falls on its surface. It is relatively dark.
In this image blue denotes shorter infrared wavelengths and red, longer. Hotter objects emit shorter-wavelength light; they appear blue. Stars with temperatures of thousands of degrees are blue. The coldest gas and dust are red. The asteroid appears greenish-yellow in the image because it is about room temperature—cooler than the stars, but warmer than the dust.
Image Credit: NASA