Are these stars in this MASCOT image of Ryugu?
They seem too bright to be stars (if the foreground is sunlit) and there are similar bright spots in shadowed areas of Ryugu.
Famously there are no stars visible in Apollo Moon photos:
the picture you take is set for bright objects. Stars are faint objects! In the fast exposure, they simply do not have time to register on the film. It has nothing to do with the sky being black or the lack of air, it's just a matter of exposure time. If you were to go outside here on Earth on the darkest night imaginable and take a picture with the exact same camera settings the astronauts used, you won't see any stars!
Phil Plait, Bad Astronomy
Ryugu has half the albedo of the Moon, but that shouldn't make much difference.
@AtmosphericPrisonEscape suggests they may be dust particles as seen on Comet 67P. However, the second video in this article shows that there are also stars in the 67P images. I can't tell whether the comet surface is in direct sunlight or reflected light. Also, I don't know the dimensions of the parts of 67P and Ryugu depicted.
Paul Byrne (@ThePlanetaryGuy, Associate Professor of Planetary Science at @NCState) says "I'm guessing those white dots are hot pixels" and "it looks like the foreground is an area around 40-ish centimeters (~16 inches) across" and "there are four LEDs as part of the camera system to illuminate the asteroid surface during night-time ops".
Physically, these “warm/hot pixels” are mainly caused by charge leakages within the image sensor chip. Although the warm/hot pixels are randomly distributed within the chip, they are in a fixed position. Under normal conditions (shorter exposure times, normal light), many of them are not visible since their contribution to the general noise level is below this level.
pco.knowledge base | warm / hot pixel