HiRISE is essential to many of these data products. For example, too many boulders are unresolved in bin-2 images to fit a size-frequency distribution down to the scale relevant to landers (Golombek et al., 2012a). In future years HiRISE will provide less full-resolution coverage, due to ADC bit-flips worsening over time (Section 8.9). These points are especially relevant given that HiRISE is the only Mars orbital imager operating or planned and capable of meeting the data requirements for sufficiently characterizing landing sites.

## 12.2. Moving to a later LMST?

Although the battery reconditioning has removed the need to move MRO to a near-terminator orbit to keep solar arrays in the Sun for most of each orbit, the Mars2020 project still prefers to receive the relay data later each Mars day, which facilitates rover planning by increasing the amount of time for operations prior to uplink of critical data to the orbiter for downlink to Earth. The Mars2020 project receives a small fraction of its data through MRO relative to the Mars Atmosphere and Volatile Evolution (MAVEN; Jakosky et al., 2015) and TGO spacecraft. MRO teams evaluated a 4:30 PM LMST and found it significantly worse for science. In the case of HiRISE, the main issues are (1) loss of signal from poorly illuminated surfaces, (2) different illumination angles complicate long-term change detection comparisons with previous data taken at other local times, and (3) shortened observing seasons for features of interest, such as frosted gullies. The later local time would also preclude stereo completion of pre-existing images with matching illumination angles. The later LMST is an advantage for imaging the morphology of subdued (low slope) terrains near the equator.

## 12.3. Simplified operations?

Operations have grown more complicated over the PSP and early mission extensions as the MRO project has balanced various science and engineering issues. Resolving pointing conflicts between experiments is handled with an automated round-robin scheme with multiple iterations, automatically incorporating MCS rules that limit large rolls, spacecraft exclusion zones, and other planning restrictions. As a result, HiRISE planning is indeterminate—we must plan about four times as many observations as are acquired each cycle. This overplanning could be avoided if planning were determinate, i.e., all or most planned observations were actually acquired. The recent termination of observation planning by CRISM has helped: now we only need to over-plan by about a factor of three.

While resources are more limited in each extended mission, the workload involved in observation planning has increased for HiRISE, to manage temperatures to mitigate ADC bit-flips while protecting electronics, and tracking SSR data volume to avoid safing HiRISE. The team has developed new software and procedures to streamline operations, such as the new HiPhoP (Section 7.10), STaR (Section 7.13), CIPP-lite (Section 7.6), and other measures. The latest development efforts were made possible by “standing down” and not acquiring data in some cycles (Table 6). If funding continues to decline and MRO operations remain complicated, we will need to either find further streamlining or use more stand-down cycles. Abandoning the use of CIPPs is one possibility (Section 7.6).

## 12.4. Lessons received for future high-resolution orbital imaging

MEPAG has posted reports from the Next Orbiter Science Analysis Group (NEX-SAG) and other studies that describe the value of a new orbiter to support future Mars robotic and human exploration. The top priorities were understanding active processes and locating and characterizing candidate landing sites and surface exploration regions with HiRISE-class (30-cm-per-pixel) or better (10- to 15-cm-per-pixel) imaging. NEX-SAG proposed using solar-electric propulsion and advanced telecommunications (Ka-band and high-power RF amplifiers and a laser

communication experiment), enabling 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 given launch vehicle. The desire to achieve a spatial resolution better than HiRISE is driven by scientific objectives related to determining process-diagnostic morphology and structure (such as understanding recurring slope lineae (RSL)), to support landing site characterization, aid in surface activity planning, and even diagnosis of engineering anomalies (such as clues to the Beagle-2 lander failure). Accordingly, we discuss challenges and lessons learned for the benefit of future high-resolution imaging science at Mars.

A factor of two or three improvement over HiRISE image resolution doesn't sound very difficult, but it is. Increasing a small camera resolution by a factor of two or three is not challenging. However, HiRISE—the largest telescope ever sent to another planet—is already pushing against several limits. Large telescopes are difficult to accommodate on spacecraft, 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 their support structure. This more capable instrument would significantly increase mass and volume. Alternatively, new technologies can reduce the mass of the telescope (as with James Webb Space Telescope), but that increases cost and risk. However, new launch vehicles may make it more economical to send larger payloads to Mars (Heldmann et al., 2021).

The challenges continue beyond simply flying a larger telescope. If the pixel scale is smaller or more TDI lines are required, the pointing stability requirement for the entire spacecraft becomes even more challenging. The stability challenge may be worsened by flying large solar arrays needed for solar-electric propulsion. Finer pixel scale over the same FOV also increases the data processing load, but advances in detectors and other electronics since 2001 are expected to reduce electronics mass and power requirements compared to HiRISE.

High-resolution imaging of Mars from orbit is also challenging because the atmosphere causes two problems. First, the orbit cannot be lower than about 250 km (or even 300 km when the air is especially dusty and warm) because of drag on the spacecraft (unless specially designed). The MRO project had considered a periapsis at 200 km but opted for the present orbit for precisely this reason (Zurek and Smrekar, 2007). We have substantial coverage of the Moon at half a meter per pixel from the Lunar Reconnaissance Orbiter Camera (LROC; Robinson et al., 2010), which is much smaller than the HiRISE camera, but in its prime mission the Lunar Reconnaissance Orbiter traveled at an altitude of just 50 km, compared to about 300 km for HiRISE.

The thin atmosphere of Mars is also surprisingly bright due to suspended dust. As a result, more than half of the radiance of Mars orbital images is typically 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 ensure that the SNR of surface features is at least 50:1 in most 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 velocity of about 3.2 km per second. Imaging at a scale of 0.3 m per pixel requires a line time of 94 microseconds ((0.3 m)/(3200 m/s) = 0.000094 s) with a requirement of one pixel of smear in the down-track direction. With an integration time of 94 microseconds, HiRISE optics would result in a typical image with a SNR of only 10 rather than 150. HiRISE overcomes the SNR challenge with a TDI sensor, imaging each surface patch up to 128 times and summing the signal on the CCD detector. What if the imaging scale is reduced to 15 cm
per pixel? Then the integration time is just 47 microseconds, and ~ 256 time-delay integration lines are needed for adequate SNR (assuming a 1-m primary mirror), and the pointing stability must be two times better. The spacecraft pointing stability needs to be very precise to ensure a surface feature is re-imaged in the same TDI column, not jittering or wandering over multiple columns, smearing the image. The Mars Reconnaissance Orbiter was designed to achieve high stability for HiRISE (Lee et al., 2003). Improving image resolution by a factor two is not confined to the cost of the camera; this improvement drives the cost of the entire spacecraft.

HiRISE has provided several lessons that may be useful for future instruments. The baseline characteristics of HiRISE (high resolution and signal-to-noise ratio, and color capability) have all proven highly valuable to science analysis, as described previously. We focus here on lessons learned that may be useful for future high-resolution orbital imaging systems at Mars or elsewhere.

* The “People’s Camera” concept of HiRISE has been valuable. Public target suggestions constitute a significant fraction of our target database and ensure that no aspect of Mars science is overlooked, while prioritization of targets by a group of specialist team members has optimized the use of limited coverage. The rapid release of images has benefited the community, especially early-career scientists who are less likely to be on instrument teams.

* Many of the challenges the HiRISE team faces can be summarized as complexity. The instrument has been challenging to calibrate and command, given the 28 separate CCD channels. Requirements developed to accommodate all MRO instruments with different pointing desires also result in complex and labor-intensive operations. Stereo observation planning requiring images on two orbits also adds operational challenges compared with single-pass-stereo used by the High Resolution Stereo Color Camera (HRSC; Jaumann et al., 2007) and CaSSIS. “Keep it simple” is excellent advice.

* The Sun-synchronous 3 PM orbit has generally been a good choice for providing adequate illumination and topographic shading without harsh shadows. The fixed local time of day has been valuable for obtaining repeat images with similar lighting. Future Mars imagers in a similar orbit would benefit from consistency with this existing record. The orbit does impose some limitations, notably the inability to observe phenomena at other times of day (such as morning frost) and limits on the temporal frequency of some repeat observations.

* Although it poses accommodation challenges for some other instruments on MRO, the ability to conduct precision off-nadir targeting has been invaluable for rapid coverage of key events like a new landed asset or ice-exposing impact, stereo coverage, change detection imaging of specific locations, and efficient mosaicking of image coverage.

* Coordinated observations between HiRISE, CRISM, CTX, SHARAD, and CaSSIS have multiplied the value of each. Absence of any one of these instruments would have reduced the science impact.

* HiRISE color has been extremely valuable, but color imaging of only 20% of the image swath is a significant limitation. It forces choices about optimizing placement of the color swath or overall image footprint, and at least 5× as many images (each planned separately) are required if full-color coverage is desired.

* Jitter correction, enabled by the along-track offset of HiRISE CCDs, has been invaluable for DTM production and reduces “wasted” stereo images that require re-acquisition.

* Change detection was originally only a limited part of the planned operations of HiRISE but has grown over time. Planning for such campaigns from the beginning would have influenced early targeting, via an effort to systematically acquire baseline images early in the mission. Change detection would also have influenced tool development, as HiKER observations were a late development and

planning tools such as our target database lack the capability to flag images for annual imaging or for high-quality repeat opportunities.

* Generation of map-projected, radiometrically calibrated RDRs has greatly improved ease-of-use of HiRISE data, and the ability to “stream” images via HiView rather than download complete files has also been beneficial. The JPEG2000 file format, however, is not allowed for standard data products in PDS-4.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgments

We are grateful to have known the following HiRISE team members who have passed away: Ed Bortolini (Ball Aerospace), Kathi Baker (UArizona), Nathan Bridges (Applied Physics Lab), Wojciech Markiewicz (Max Planck Institute), and Bob King (UArizona). This manuscript benefitted from constructive reviews by Mark Robinson (ASU) and Matt Golombek (JPL). The NASA/JPL MRO project supported this research. LLT acknowledges funding and support from the Canadian Space Agency through their Planetary and Astronomy Missions Co-Investigator programme (22EXPCOI3) and the Canadian NSERC Discovery Grant programme (RGPIN 2020-06418). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. All original data used for this paper are available via the PDS.

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