How do we know how big Quaoar is?
We measured the size of Quaoar in two ways: (1) Optical measurements using the Hubble Space Telescope. Using a normal ground based telescope, you can’t see Quaoar’s size directly. You can tell it’s there, but it’s just a pinpoint of light just like any other star. But, the Hubble Space Telescope (HST) has much better “angular resolution” (it can see details a lot better) than a normal telescope because it is outside the Earth’s atmosphere. By very carefully measuring Quaoars size about 10 times over the course of an hour and comparing them to a nearby star, we can see directly that Quaoar is 1250 km in diameter. You can see the HST images here. Note that the vertical elongation is due to object motion during the exposure (“blurring”), and not the size of the object. Also, there is a paper about this technique here. (2) Thermal measurements. Using the IRAM telescope in Spain, we measured the heat coming from Quoar. Optical wavelength light (i.e. what your eye sees and what a “normal” te
