# Could aliens observe Curiosity, the rover?

Inspired by:

The color image, taken from a distance of 169 miles, is the combined view of three wavelength bands — red, blue-green and infrared.

Curiosity is not the only Earth visitor that has been spotted. In 2006, the orbiter took this image of the Opportunity rover, which showed not only the rover but also the tracks in the soil and even the shadow of its camera mast.

I'm going to guess no. The image is pretty small and poor resolution and it was taken at a range of 169 miles.

Would a very advanced camera be capable of capturing this small blue dot at a many light-year distance? Or are there limits to the capabilities of physics with regards to cameras?

• There will be some limit to imaging due to the spatial dispersion of emitted/reflected photons from the target object. For an alien around even the nearest star, the angular extent of something like Curiosity is so tiny that only a few photons per second or minute would enter the telescope. An extremely long exposure would be needed, during which time the rover would drive around... not even to mention the spin of Mars. So the very very best they'd get is a slightly discolored latitudinal line on a blurred-beyond-use image of Mars. – Asher Jun 23 '17 at 17:11

In principle, this would be possible, but would require impractically big lenses, at least. The angular resolution of a telescope is limited by diffraction on camera lenses. The Rayleigh criterion states that two points emitting light of wavelength $\lambda$ can be distinguished from each other by a camera with aperture diameter $D$, if the angle, at which these dots appear, is no smaller than
$$\sin \theta = 1.22 \frac{\lambda}{D}.$$
By subsituting all of the lengths (say, the distance is one lightyear, which is $3 \cdot 10^8$ m, the Opportunity rover is about 2 m wide, and the average wavelength of visible light is about 0.5 $\mu$m), we obtain that the lens of such a telescope would have to be at least 75 m wide. We can assume though that the telescope is not optical but UV or something (so that the wavelength is smaller), but still the necessary size scales linearly with distance to the object. This effectively prevents aliens from seeing the rover. And that doesn't take into account positioning of the telescope, stability, etc. etc.