PERCI - Penetrator for Environmental, Regolith, and Crater Investigation

Overview

A team consisting of fourteen graduate students, including myself, was tasked by University of Michigan's Climate and Space Sciences and Engineering (CLaSP) department to develop a mission concept which could enhance the return of NASA's future UOP mission. This concept developed its requirements and constraints from NASA's SIMPLEx program guidelines as well as the 2023-2032 Planetary Science Decadal Survey, which identified UOP science return as a top priority. While the flagship UOP craft itself focuses primarily on questions relating to Uranus itself, such as its formation, interior, atmosphere, and magnetosphere, less focus is placed on the satellites of the planet, relying on remote sensing for all data pertaining to the Uranian moons.

Under mentorship from NASA's Jet Propulsion Laboratory (JPL), our team developed a full mission concept capable of obtaining in-situ data from the surface of the four largest Uranian satellites: Oberon, Ariel, Titania, and Umbriel. Once deployed from the UOP spacecraft, the probe enters a collision trajectory with its selected moon, and under nominal conditions transmits data before, during, and for a short time after impact.

Systems Engineering

Below are selected excerpts from the systems engineering process developed by our team. Given the scope of the project, only a subset is presented here; additional details can be provided upon request.

Science Traceability Matrix

Measurable science objectives were obtained primarily through findings of the Decadal Survey, which led to specific choices of scientific instruments.

The choice of instruments, as well as other parameters and constraints including compatibility with UOP and NASA protocol led to the development of a complete set of functional requirements:

With a completed STM and set of requirements, we began performing trades on different concepts and their feasibilities. Remote orbiter probes, impactors, penetrators, and even simple fixed instrumentation suites on UOP itself were examined. Given the higher risk tolerance and lower cost of SIMPLEx missions, the potential benefits of a probe which could field a full suite of in-situ instrumentation with minimal complexity outweighed most downsides. Contingency plans were included to ensure the probe would still return significant scientific data from its descent in the event it did not survive post impact.

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Personal Contributions

With an overall architecture selected, additional work was split into subteams such as propulsion, ADCS, thermal analysis, power, structures, and communications. My own personal work included ownership of most electrical power, command/data handling, communication, and telemetry operations, as well as a radiation survivability study.

Electrical Power

Initial impact simulations revealed that the probe would experience 20,000 - 30,000 g of deceleration upon contact with the ice/rock substrates of the selected Uranian moons. After a trade study and consultation from JPL, impact-resistant lithium thionyl chloride primary cells, similar to those used in the Deep Space 2 impactor probe were deemed sufficient to survive both the impact conditions as well as the harsh thermal environment on the surface.

Due to mass and thermal constraints, the probe was designed to survive a best case of only 30 minutes assuming survival on the surface. The above power budget outlines the number of cells required, their configuration, and power draws for each instrument both in transit and post-impact.

Telemetry and Communications

As PERCI only needed to communicate a relatively short (~1000 km) distance between itself and the UOP flagship, a link budget calculation revealed that the communication system was not strongly constrained by power, giving more flexibility in overall link design. An orthogonally-fed patch antenna was able to meet the link requirements, and could be directly integrated into the aft face of the craft, pointing skyward after impact.

Additionally, the orthogonal feedlines can inherently realize circular polarization, reducing orientation mismatch dependence. The proposed system was able to operate at 1-10 kbps uplink data rate depending on the angle between PERCI and the overhead UOP craft with significant margin (30 dB, Eb/N0 ratio of 8). If necessary, the data rate could be pushed significantly higher with a proportional reduction in margin and reliability. The chosen data rate was sufficient for the calculated necessary duty cycles of all onboard instruments.

C&DH and Radiation Studies

In the near-Earth environment, ionizing particle species primarily originate from trapped belt and solar energetic particle (SEP) sources, and total ionizing dosages (TID) can be estimated by software such as SPENVIS. SPENVIS was used for an initial estimation of TID as PERCI left Earth aboard UOP assuming an external mounting location. Background galactic cosmic ray (GCR) flux and probability-based transient SEP models were used to estimate the trip through deep space for the ~12-year journey to the Uranian system. Including an additional 10 krad budget for a likely Jupiter gravity assist maneuver, the TID experienced for the entire journey was estimated to be ~25 krad with a 5 mm aluminum shield. As such, the ATmegaS128 was selected as the primary microcontroller, handling all maneuver, telemetry, communication, and data handling operations. The ATmegaS128 is the space-grade radiation-hardened version of the consumer-grade ATmega128 series, and balances a 30 krad TID limit with a relatively cheap per-unit cost (hundreds of dollars, compared to tens of thousands for other common rad-hardened micros). Given the low cost and power draw, a second backup microcontroller was included for redundancy.

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Concept of Operations

PERCI is deployed from UOP during one of the planned close flybys of the main craft using a redundant spring actuation mechanism, allowing it to safely distance itself from the flagship before engaging its propulsion. The probe is orientated retrograde to the moon, and an array of four small solid rocket boosters executes a maneuver to transition the probe onto an impact trajectory. After the burn, pairs of SRBs are jettisoned at calculated intervals to act as reaction mass, initiating and stopping a 180° flip to re-orient the probe to face the moon's surface. Small internal reaction wheels maintain pointing authority and offset minute gravity-gradient effects.

The probe will be communicating with the flagship during all stages, providing real-time data over the wireless link during its transit. Although the probe is designed to survive impact and up to 30 minutes on the surface, this constant link ensures that imagery and acceleration data is still returned should the probe not survive. Furthermore, the impact itself will provide additional data which remote sensing technologies aboard UOP can utilize, similar to NASA's 2005 Deep Impact mission.

Risk

Risks were identified across all systems and tabulated in a NASA-style risk matrix using FMEA methodology, identifying critical risks, risks which can be mitigated or bought down, and risks which can simply be accepted.

As an example, the most severe risks, circled above in red, are defined below with proposed mitigation plans.

Conclusion

This research project sought to create a secondary mission concept which could enhance the science return of the UOP flagship. The goal is considered accomplished, having:

• Identified a penetrator/impactor as having high likelihood of significant return, and developed its concept
• Determined instruments capable of producing this return, measuring:
• Close-up images of moon surface
• Surface Hardness
• Thermal Properties
• Seismic Data
• Conducted numerous analyses to determine survivability
• Created contingency plan to still return data in event of probe loss upon impact

Our team has continued work on this concept, exploring cost constraints and further developing structural models to increase the likelihood of impact survivability. We hope to soon formally publish this work through AIAA or IEEE.