# Why is Dawn in a much higher observation orbit than Rosetta?

Dawn orbits Ceres at thousands of kilometers periapsis altitude, and doesn't seem to plan to go lower than 375 km. In contrast, Rosetta has been down to about 30 km altitude, and will finally even try to soft-land. And that's at a comet with a coma that might already have caused Rosetta to enter safe mode as its star tracker confused debris around it for stars.

Is it maybe because of gravity, propulsion, instrumentation or spacecraft design? Isn't closer better?

Much, much higher specific orbital energy ($\epsilon$) and Dawn's lack of high impulse thrusters. OK, technically Dawn has 12 x off-axis 0.9 N RCS thrusters, but it only launched with 46 kg of hydrazine for them, so it pretty much depends on its three 90 mN ion thrusters alone. See this answer for more detailed explanation of its propulsive systems and limitations of low thrust propulsion.
As for $\epsilon$, rough calculation for Rosetta's 30 km orbit around 67P/Churyumov–Gerasimenko gives me ~ $-0.01\mathrm{\ m^2s^{-2}}$ (note that 67P is a highly irregular body) and for Dawn in 375 km orbit around Ceres $-37,364\mathrm{\ m^2s^{-2}}$. That's a huge difference of a few orders of magnitude and where the amplitude of required energy to do corrective orbital maneuvers comes from (multiplied with the mass of the orbiter to get required energy in Joules, but Rosetta and Dawn have roughly the same mass by now), should those be necessary, say due to mascons (mass concentrations) and otherwise uneven gravity field or even presence of surface outgassed (or ejected in Ceres' case since it's suspected to be a geothermally active body with cryogeysers of salt water as one possible explanation for those infamous white spots) materials increasing density of the medium it has to orbit through (and thus influencing orbital decay).