By using Spitzer Space Telescope, their measured the size of an asteroid. Astronomers called the asteroid as 2011 MD(found roughly 20feet in size), structure for asteroid contain a lot of empty space. Spitzer's infrared vision helps to sizing up the asteroid.
Michael Momment said "Spitzer can use its heat-sensitive infrared vision to spy asteroids and get better estimates of their sizes". Indicating the object's true size, infrared light is most useful. The size of 2011MD was roughly known, by using visible light it can be observed.
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