MEPAG recently posted a report from the Next Orbiter Science Analysis Group that describes the need for a new orbiter to launch in 2022 or 2024 to support future Mars robotic and human exploration. Some of the top priorities are to understand active processes and to locate and characterize candidate landing sites and surface exploration regions with HiRISE-class (30-centimeter-per-pixel) or better (10- to 15-centimeter-per-pixel) imaging. They recommend using solar-electric propulsion as well as advanced telecommunications (Ka-band and high-power RF amplifiers, and a laser communication experiment), which together enable a much higher data rate to Earth than past orbiters. Solar-electric propulsion also increases the potential mass of the science instrument payload that can be launched to Mars by a particular rocket. The desire to achieve a spatial resolution better than HiRISE is driven by science objectives such as understanding recurring slope lineae (RSL) and also to support landing site characterization, surface activity planning, and even diagnosis of engineering anomalies (such as how the Beagle-2 lander failed).

A factor of two or three improvement over HiRISE image resolution doesn’t sound very difficult, but it is. Increasing a small camera’s resolution by a factor of two or three is not difficult, but HiRISE -- the largest telescope ever sent to another planet -- is already pushing against several limits. Large telescopes are difficult to accommodate, including the mass, volume, and pointing stability. Resolution is inversely proportional to the diameter of the telescope aperture. HiRISE has a half-meter aperture, so doubling the optical resolution requires a primary mirror about a meter in diameter, which, in turn, enlarges the other mirrors and the structure. So the mass and volume must increase substantially compared to HiRISE. Alternatively, new technologies can reduce the mass of the telescope (like James Webb Space Telescope), but that increases cost and introduces risk, and NASA is very risk-averse.

The challenges don’t stop with simply flying a larger telescope. If the pixel scale is smaller or if more time-delay integration lines are required, then the pointing stability requirement for the entire spacecraft becomes more challenging. This stability challenge may be worsened by flying the large solar arrays needed for solar-electric propulsion.

High-resolution imaging of Mars from orbit is challenging. The atmosphere of Mars causes two problems. First, the orbit cannot be lower than about 250 kilometers (or even 300 kilometers when the air is especially dusty and warm) because of drag on the spacecraft. We have substantial coverage of the Moon at half a meter per pixel from the Lunar Reconnaissance Orbiter Camera (LROC), which is much smaller than the HiRISE camera. The intrinsic resolving power of LROC is about 10 times coarser than HiRISE, but in its prime mission Lunar Reconnaissance Orbiter traveled at an altitude of just 50 kilometers, compared to about 300 for HiRISE. The thin atmosphere of Mars is also surprisingly bright due to suspended dust. As a result, more than half of the brightness of Mars orbital images is from atmospheric scattering; less than half of the light reaching the camera is from direct reflection from the surface that reveals small-scale features. This atmospheric haze can be subtracted from the images to increase contrast, but it still contributes photon noise proportional to the square root of the signal. The solution is to acquire images with a very high signal-to-noise ratio (SNR). The top-of-atmosphere SNR needs to be about 150:1 (a typical value for HiRISE) to insure that the SNR of surface features is at least 50:1 in the majority of images. (Sometimes the dust is so thick that the surface SNR is much worse, approaching zero.)

Another challenge is that the mass of Mars dictates that low circular orbiters must travel at a high velocity of about 3.2 kilometers per second. To image at a scale of 0.3 meters per pixel, this means the line time is only (0.3m)/(3200m/s) = 0.000094 second or 94 microseconds. With HiRISE optics, using that line time as the integration time produces an image with SNR of only 10, rather than 150. The HiRISE solution is use of time-delay integration, imaging each surface patch 128 times and summing the signal on the CCD detector. What if the imaging scale is reduced to 15 centimeters per pixel? Then the integration time is just 47 microseconds and ~256 time-delay integration lines are needed (assuming a 1-meter primary mirror) for adequate SNR, and the columns are half as wide, so the stability must be four times better. The spacecraft pointing stability needs to be very precise to make sure a surface feature is re-imaged in the same TDI column, not jittering or wandering over multiple columns, smearing the image. Mars Reconnaissance Orbiter went to considerable effort (and cost) to achieve high stability for HiRISE. The cost of a factor of two improvement in image resolution is not confined to the cost of the camera; it drives the cost of the entire spacecraft.

Super-Resolution Restoration to the Rescue?

A recent press release from the University College of London presented their Super-Resolution Restoration (SRR) technique, including comparison of regions near Spirit rover (Figure A).