> Spotting a 1km moon in orbit around Jupiter is the equivalent of spotting a Rabies virus at the other end of a football field. (1km at 588 million km ~= 185n

> Spotting a 1km moon in orbit around Jupiter is the equivalent of spotting a Rabies virus at the other end of a football field. (1km at 588 million km ~= 185nm at 109m) While a relevant size comparison, this is potentially misleading characterization of our ability to detect these types of satellites. 1 km at 588 million km works out to an angular size of ~6e-6 radians, or 3e-4 arcseconds. The resolution of the telescopes that took the discovery data are typically* 0.3-1 arcseconds (depending on the atmospheric conditions at the site). So the telescope is not even close to being able to directly measure the physical scale of the moon in question. Best case, it's off by ~3 orders of magnitude. (Note of course there are variations in the apparent angular size due to the orbits of Earth and the Moon, but even at the closest the moon is too small to be directly resolved by current ground-based telescopes). The reason we are able to detect these moons is a combination the albedo of the moon (its reflectiveness) and the still somewhat bright sunlight at the distance of Jupiter. So the main requirement here is that the telescope can see faint objects. * - Note that near-infrared telescopes on the ground can achieve higher resolution with corrective "adaptive optics", but that is limited to follow-up observations of objects, not initial survey work.