Abstract. Mars Pathfinder successfully landed at Ares Vailis on July 4, 1997, deployed and navigated a small rover about 100 m clockwise around the lander, and collected data from three science instruments and ten technology experiments. The mission operated for three months and returned 2.3 Gbits of data, including over 16,500 lander and 550 rover images, 16 chemical analyses of rocks and soil, and 8.5 million individual temperature, pressure and wind measurements. Pathfinder is the best known location on Mars, having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high-resolution orbiter images and in inertial space from two-way ranging and Doppler tracking. Tracking of the lander has fixed the spin pole of Mars, determined the precession rate since Viking 20 years ago, and indicates a polar moment of inertia, which constrains a central metallic core to be between 1300 and -2000 km in radius. Dark rocks appear to be high in silica and geochemically similar to anorogenic andesites; lighter rocks are richer in sulfur and lower in silica, consistent with being coated with various amounts of dust. Rover and lander images show rocks with a variety of morphologies, fabrics and textures, suggesting a variety of rock types are present. Rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates (although other explanations are also possible), which almost definitely require liquid water to form and a warmer and wetter past. Airborne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral, interpreted to be most likely maghemite; explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains. Remote sensing data at a scale of a kilometer or greater and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods, which are relatively dust free. The surface appears to have changed little since it formed billions of years ago, with the exception that eolian activity may have deflated the surface by -3-7 cm, sculpted wind tails, collected sand into dunes, and eroded ventifacts (fluted and grooved rocks). Pathfinder found a dusty lower atmosphere, early morning water ice clouds, and morning near-surface air temperatures that changed abruptly with time and height. Small scale vortices, interpreted to be dust devils, were observed repeatedly in the afternoon by the meteorology instruments and have been imaged.

The primary objective of the Mars Path nder mission was to demonstrate an innovative, low-cost, reliable method for placing a science payload on the surface of Mars. The spacecraft performance during entry, descent, and landing is assessed. Analysis of the accelerometer and altimeter ight data obtainedby the Path nder spacecraft during atmospheric ight is provided. Results of an effort to reconstruct the spacecraft trajectory and attitude history are presented. An estimate of the Mars atmosphere pro le encountered during atmospheric ight is given. ½ = atmospheric density, kg/m 3 ! obs = observed vehicle roll rate, rad/s ! z = roll rate about the vehicle Z axis, rad/s

Europa is a premier target for advancing both planetary science and astrobiology, as well as for opening a new window into the burgeoning field of comparative oceanography. The potentially habitable subsurface ocean of Europa may harbor life, and the globally young and comparatively thin ice shell of Europa may contain biosignatures that are readily accessible to a surface lander. Europa’s icy shell also offers the opportunity to study tectonics and geologic cycles across a range of mechanisms and compositions. Here we detail the goals and mission architecture of the Europa Lander mission concept, as developed from 2015 through 2020. The science was developed by the 2016 Europa Lander Science Definition Team (SDT), and the mission architecture was developed by the preproject engineering team, in close collaboration with the SDT. In 2017 and 2018, the mission concept passed its mission concept review and delta-mission concept review, respectively. Since that time, the preproject has been advancing the technologies, and developing the hardware and software, needed to retire risks associated with technology, science, cost, and schedule.

The analysis and design of attitude control systems for spacecraft employing pulse-operated (on-off) thrusters is usually accomplished through a combination of modeling approximations and empirical techniques. A new thruster pulse-modulation theory for pointing and tracking applications is developed from nonlinear control theory. This theory provides the framework for an autopilot suitable for use in digital computers whose performance and robustness properties are characterized analytically, in the design process. Given bounds on the anticipated dynamical modeling errors and sensor errors, it is shown that design specifications can be established and acceptable performance ensured in the presence of these error sources. Spacecraft with time-varying inertia properties can be accommodated, as well as clustered thruster configurations that provide multiple discrete torque levels about one or more spacecraft axes. A realistic application of the theory is illustrated via detailed computer simulation of a digital autopilot designed for midcourse guidance of a hypothetical interplanetary spacecraft.

Performance results are presented for an autonomous navigator using refraction measurements of starlight passing through the upper atmosphere. To obtain a realistic performance indication, a full-state simulation was developed comprising a truth model with an 8 x 8 gravity field, lunar and solar perturbations, atmospheric drag, real star distributions, and the effect of atmospheric density variations on the measurements. Some data on actual observations of refraction by the satellite HEAO-2 are presented to show the variability in density within -+ 20" latitude. The navigation portion of the simulation is capable of handling gravity fields up to degree 4 and order 4, and utilizes the extended Kalman filter to estimate satellite position and velocity.