what is that?
i saw that thing in the bottom right corner and thought it is round as an asteroid but does not look like one.
It's a #ffd (flat field defect), a type of artifact that shows up in these images. There is actually a second one in this set on the left side in the middle. You will notice that it moves in the same patter as the larger one in the bottom right corner.
From the wiki here: http://en.wikipedia.org/wiki/Flat-field_correction
Flat-field correction is a technique used to improve quality in digital imaging. The goal is to remove artifacts from 2-D images that are caused by variations in the pixel-to-pixel sensitivity of the detector and/or by distortions in the optical path. It is a standard calibration procedure in everything from pocket digital cameras to giant telescopes.
A ffd is a defect generated by this technique, so while the technique makes the overall quality of the images better by removing the pixel-to-pixel variation, it adds these artifacts.