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Why is that? There's been no lack of interest in the sun-quite the opposite. Of all extraterrestrial bodies, the sun has the largest influence on us: It controls the radiation doses that astronauts experience and also affects the electronics in the myriad satellites on which we increasingly rely. Solar storms can even disrupt electric power grids, as famously happened in 1989, when one such storm blacked out the entire province of Quebec and caused ripple effects on electric grids in the United States.

As an end-member of terrestrial planet formation, Mercury holds unique clues about the original distribution of elements in the earliest stages of solar system development and how planets and exoplanets form and evolve in close proximity to their host stars. The importance of in situ measurements via a landed mission to Mercury was recognized by the 2013–2022 Decadal Survey, and again with its selection for a Planetary Mission Concept Study in support of the 2023–2032 Decadal Survey. Mercury is the only inner planet unexplored by a landed spacecraft; landing on Mercury is uniquely challenging due to the large $\Delta\mathrm{V}$ requirements and extreme thermal environment for such a mission. This paper describes the mission concept design that meets those challenges by leveraging recent technology advances including increased launch vehicle performance capability, further development and use of solar electric propulsion (SEP), and Next-Generation radioisotope thermoelectric generator (NextGen RTG) development. These advances enable a New-Frontiers-class mission concept which achieves one full Mercury year (∼88 Earth days) of surface operations with an 11-instrument, high-heritage payload that is delivered to a landing site within Mercury's widely distributed low-reflectance material, and addresses science goals encompassing geochemistry, geophysics, the Mercury space environment, and geology. The mission concept presented here addresses the primary challenges of a Mercury Lander mission with a four-stage design that launches on an expendable Falcon Heavy vehicle. Mass savings are enabled through jettisoning of stages prior to large burns and optimization of propulsion systems for each phase: cruise, orbit, initial descent, and landing. The flight system utilizes an SEP cruise stage to reach Mercury and jettisons this stage prior to the Mercury orbit insertion (MOI) burn. MOI and orbit-lowering maneuvers are executed with a large bipropellant propulsion system on an orbital stage. During the orbital phase, a narrow-angle camera acquires images for final selection of a low-hazard landing zone within the region of interest. The orbital stage is jettisoned just prior to descent. A solid rocket motor (SRM) descent stage executes the braking burn, and final landing is performed following SRM burnout and jettison with the Lander's bipropellant propulsion system and continuous LIDAR operations to support hazard detection. Landing occurs at dusk to meet thermal requirements, permitting ∼30 hours of sunlight for initial observations. The NextGen RTG powered lander continues operations through the Mercury night. Direct-to-Earth (DTE) communication is available for the initial three weeks of landed operations, unavailable for the following six weeks, and resumes for the final month. Thermal conditions exceed lander operating temperatures shortly after sunrise, ending operations. A total of ∼11 GB of data are returned to Earth.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on August 3, 2004, is a NASA spacecraft that will orbit the planet Mercury for a one-year mission. The spacecraft launch mass limitation, combined with the large solar distance variations, impose severe requirements on the spacecraft power and thermal subsystems. The spacecraft is three-axis stabilized. A sunshade protects the spacecraft from the high intensity solar flux. The attitude control maintains the sunshade pointed to the Sun. The solar panels, which are outside the thermal shield, are designed to survive normal Sun incidence at 0.3 AU. The solar panels consist of alternating rows of triple junction cells placed between Optical Solar Reflectors (OSRs). Solar panels thermal control is performed by tilting the panels with increasing solar flux. To minimize the mass of the spacecraft, the structure is made of composite materials. Spacecraft electronic boxes that are high power dissipaters are designed with special thermal vias that conduct the heat directly to diode heat pipes, which transport the heat of the box to thermal radiator panels on the side of the spacecraft behind the sunshade. The MESSENGER spacecraft is on a trajectory to enter Mercury orbit in 2011. The spacecraft is performing as designed.