Enceladus South Pole Explorer

ESPER - Enceladus South Pole Explorer

The Enceladus South Pole Explorer (ESPER) is a Small Sat proposal for an in-situ investigation of Enceladus’s plumes and south polar region from which they emanate. The mission will search for biosignatures in the plume constituents to learn about the possibility of the subsurface ocean supporting a habitable environment. ESPER’s instrument suite consists of a small quadrupole mass spectrometer to observe the molecular composition of the plumes and a high-resolution visual imager to map the Tiger Stripe region of the south pole at high fidelity to support potential future landing sites. This mission will provide the most extensive and high-resolution mass spectrometry and spatial imagery ever obtained for the surface of Enceladus and its plumes, answering one of the highest priority questions in planetary science: whether conditions elsewhere in the solar system could support life and whether life exists there.

This work was completed as a group term project for ASEN 5440 Mission Design and Development at CU Boulder. Team members were Mattie Bowen, Pat Behr, Josh Elliott, Chava Friedman, & Kyle Johnson. My role was spacecraft design, including all CAD modeling and graphics, and developing the budget.

ESPER Mission Overview. The spacecraft orbits Saturn and conducts at least 3 flybys of Enceladus’s south polar region at an altitude of 20-100km, enabling science returns and ensuring spacecraft viability.

Mass Spectrometer example data from Cassini. https://solarsystem.nasa.gov/missions/cassini/mission/spacecraft/cassini-orbiter/ion-and-neutral-mass-spectrometer/

Visual Imager Sample Data from Cassini. https://photojournal.jpl.nasa.gov/jpegMod/PIA17204_modest.jpg

Science Goals & Expected Outcomes

Investigate plume composition and potential biosignatures:

Expand on Cassini's investigation of plume composition with higher resolution mass spectrometry
CO, C2H4, CH2O, NO, C4H8, C5H12, C2H6N2, C2H7NO, and more
Search for signs of biotic processes in subsurface ocean

Map surface topography:

High-resolution visual camera
Gain understanding of surface features
Characterize potential surface sites for future landers

ConOps: ESPER travels to Saturn as a ride share with Dragonfly. Once reaching the system, the spacecraft enters a high-inclination orbit at the radius of Enceladus, enabling two flybys for every 33-hr orbit.

Stowed configuration sized to fit within ESPA ring guidelines, and to not overcrowd Dragonfly in a likely rocket fairing

Spacecraft Design

Size & Structure

The spacecraft is an ESPA-class Small Sat adhering to the SIMPLEx announcement of opportunity. The 56 kg overall mass is below the 180 kg maximum and the total stowed volume fits the ESPA ring requirements. 3mm Aluminum bus panels support all subsystem components, including a 0.4m high-gain antenna and solar arrays with gimbaling capability to ensure optimal array pointing throughout different modes, including minimizing the ram profile during plume flybys.

Subsystems

The mass spectrometer points in the ram direction during flybys and has a 3° pointing error. The visual imager is nadir pointing and has a 0.57° pointing error at a 100 km altitude. ADCS is accomplished with COTS magnetorquers, reaction wheels, and star trackers. Delta-V needed to enter and maintain our desired orbit comes from a high-heritage hydrazine thruster located on the zenith side of the spacecraft.

ESPER’s data budget is 2.56 Gb/day with 1.5:1 lossless compression and the majority of science data coming from the imager. Downlink assumes a Saturn orbiting relay spacecraft linked via the HGA.

Thermal control at Saturn is a combination of MLI blankets and internal active heating elements. The operational range for both instruments is -20-40°C.

Design for Radiation

The expected radiation environment at the orbit of Enceladus, roughly 4 Rs, is similar to that of Earth at GEO. The combination of several factors — a mission duration of days, a high-inclination orbit that spends little time in the radiation belt, a Saturn arrival in 2036 at solar minimum — significantly reduce concerns about material degradation and SEU impacts. Nonetheless, the spacecraft bus is fully surrounded by Aluminum shielding and the CDH, PDU, and PCU are centrally located and nearby high mass elements, such as batteries and propellant storage.

Power

The goal of the power system is to prevent power from limiting the mission scope or duration. An approximately 10-day mission could potentially rely solely on batteries, but would have little margin. The addition of solar arrays adds just enough recharge capacity to extend mission life, helping ensure planetary protection protocols and creating contingency.

Solar power suffers from the 1/r2 falloff in solar energy from the sun, giving only 1.085% of 1 AU. However, 5 m2 of solar panels still produces 16.8W, providing sufficient recharge when not conducting science or downlink. If flown, this would demonstrate using solar arrays at the furthest distance from the sun ever attempted. The mission assumes deployment at Saturn with 100% battery state of charge from Dragonfly’s RTG, but could potentially begin the mission in safe mode to arrive at a full charge.

SOC Simulation. This plot shows battery state of charge over the expected lifetime of the primary science mission, consisting of 10 flybys of Enceladus. Though the recharge mode has only a small positive margin, each flyby only takes approximately an hour with 2 more hours for data downlink, leaving most of the mission time spent in recharge mode. Halfway through the mission, the spacecraft omits science mode and recharges for one full orbit, enabling the completion of the science flybys without completely depleting the batteries.

Following the completion of science operations, a full battery recharge helps ensure the observation of planetary protection and deorbit into Saturn or another moon. This ConOps also creates the option for an extended science mission where propellant becomes the limiting factor.