LEOPARDSat-1

Background

Radiation from cosmic rays and solar energetic particles is one of the most significant challenges to any future manned missions beyond the protection of Earth's magnetic field. Robotic space missions have shown that astronauts will exceed their career radiation limits on a typical mission to Mars lasting roughly 500 days, leading to increased risks of cancer and other serious health issues. In order to undertake such missions, spacecraft will need to adequately mitigate the radiation astronauts recieve during the voyage. However, many materials usually considered for the role of shielding against radiation are typically too massive to be practical (such as lead), or are consumables (such as liquid hydrogen, food, or water) secondarily used as radiation mitigation. Lighter, more permanent materials are needed that can protect astronauts while being more versatile in their application on the spacecraft.

Representations of the three natural forms of radiation encountered in space (Left to Right): Solar energetic particles (pictured: coronal mass ejection, NASA); Galactic Cosmic Rays, or GCRs (pictured: GCR collision with Earth's atmosphere, NSF); Trapped particle radiation (pictured: Earth's Van Allen Belts, NASA)

LEOPARDSat-1 is a CubeSat mission to study the effectiveness of lightweight carbon-carbon composites and polyethylene in shielding against ionizing radiation. To do this, the satellite will measure the difference in radiation dosages between radiation sensors partially protected by varying thicknesses of each material and those which are left unprotected. This data will inform the spaceflight community on the viability of using these materials for manned, deep-space missions. As this is UC Cubecats' first satellite mission, it will also serve as opportunity for our members to gain experience in the design, construction, testing, and launch of space systems.

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Objectives

1. Provide an educational platform for members of UC CubeCats, students at the University of Cincinnati, and students at local high schools to learn about the design, construction, testing, and launch of space systems.
2. Study the effectiveness of carbon-carbon composite materials and polyethylene in shielding ionizing radiation by comparing the shielded dose to an unshielded dose.

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Spacecraft

The LEOPARDSat-1 mission hardware is a 1U CubeSat (10 cm x 10 cm x 10 cm) with a mass of roughly 900 grams. Within the structural frame are five PC104-format boards, with the Payload Assembly mounted on the Z+ (top) face of the satellite.

CAD rendering of LEOPARDSat-1 in its operational configuration

Attitude Determination & Control Subsystem

LEOPARDSat-1 will use a Passive Magnetic Attitude Control System (PMACS) to control its orientation on-orbit, utilizing a permanent magnet to align with Earth's magnetic field and two bars of hysteresis material to dampen out the transient response. For this mission, the PMACS will keep the satellite pointed to within 15° of the local magnetic field vector after a week long period of "detumbling" post-deployment. Although PMACS can not set the satellite to a commanded orientation and is not as precise as an active control system, neither of these are required for the objectives of LEOPARDSat-1, and the passive approach requires no power or computational resources to orient the satellite.

Schematic of the LEOPARDSat-1 Attitude Determination and Control Subsystem (ADCS)

An Inerital Measurement Unit (IMU), containing a 3-axis Gyroscope and 3-axis Magnetometer, will measure the angular rates and local magnetic field, while the power output of the satellites' solar panels will be used to roughly determine the relative pointing direction of the Sun. Although this data is not needed for attitude control, they will be sent down to the ground for processing, in order to ascess the PMACS's performance and contextualize data from the payload sensors.

Communication Subsystem

LEOPARDSat-1 will communicate with the ground station on the amateur band. The subsystem will consists of a transciever and two deployable antennae located at the top of the satellite. Data collected by the onboard computer will be stored in the onboard SD card until the satellite passes over Cincinnati, at which point data will be transmitted to the ground and commands uploaded to the satellite.

On-Board Computing Subsystem

The main computer on board will be an ATmega1284p microprocessor, which is primarily responsible for collecting data from the other subsystems, including radiation data from the payload sensors, solar panel power output, battery usage, ambient magnetic field vector, and satellite angular rates. It will also control radio communications with the ground using the on board tranceiver and antennae. This processor is similar to the ATmega328 used in the Arduino Uno, but with more performance, including 128 KB of flash memory, 4 KB of EEPROM and 16 KB of RAM

Payload Subsystem

Eight Radiation Field Effect Transistors (RADFETs), a type of Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), will be used to measure the radiation dosage on orbit. These are separated into three groups to characterize the effectiveness of the selected composite materials:

• Two unshielded RADFETs act as a reference, monitoring the ionizing radiation that passes through the CubeSat’s aluminum frame.
• Three RADFETs are shielded with carbon-carbon samples of varying thicknesses (3.25 mm, 5 mm, 7 mm)
• Three RADFETs are shielded with polyethylene-coated carbon-carbon samples of varying thicknesses (5.25 mm, 7 mm, 9 mm)

Labeled CAD model of the Payload Subsystem assembly

Power Subsystem

The three main functions of the power subsystem are to provide power generation, power storage, and power distribution. Electrical power will be generated using five 10 cm x 10 cm solar panels, one mounted to each face of the CubeSat other than the Z+ face containing the payload assembly. The panels will be manufactured in-house by UC CubeCats, with each mounting 26 TrisolX Solar Wings. Power generated by these solar panels will be stored in two 18650 Lithium Ion Batteries. The batteries will be mounted to a custom-design power distribution board, which contains battery chargers, a battery monitor to track battery status and health, and DC-DC converters to deliver the required voltage and peak/nominal current draws to each subsystem.

Structure Subsystem

CAD model of the exterior structural frame LEOPARDSat-1's structure will use a custom 1U exterior frame, consisting of four launch rails and two end plates, all made from Aluminum 6061. The rails are smooth, for either forwards or backwards deployment, and contain several deployment specific mechanisms:

• One roller deployment switch on the +X face of one rail, to disable the power subsystem for safety between launch and deployment.
• Two pusher deployment switches on the -Z ends of two rails, also to disable the power subsystem.
• Two separation springs on the -Z ends of the two remaining rails, to provide clean separation from other satellites upon deployment.

To hold the exterior frame together, the top and bottom of the rails will be fastened to the top and bottom plates, respectively. Internally, the structure will utilize standoffs to properly space and hold each of the five PC104 subsystem boards to the exterior frame, while the solar panels and the payload assembly are then mounted direcrtly to these end plates.

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Mission

Operations

Upon deployment from the NRCSD, LEOPARDSat-1 will deploy its antennae and begin detumbling to cancel any residual spining imparted by deployment, eventually aligning with the local magnetic field. After successfully detumbling (a process which will take about one week), the satellite will listen for a signal from the ground and then respond that the satellite is functional. The team will then command the satellite to enter its operational state, in which it will begin collecting data and transmitting it to the ground when in communication range of the ground station (expected to happen roughly twice a day). Radiation dosages from each sensor will be measured at least once a day and battery health measured once an orbit, while data on its attitude state and solar panel output will be recorded at a much higher frequency (0.1667 Hz) in order to determine the satellite's attitude history on the ground. The mission will nominally last four months, but may be extended if the satellite continues to function. Due to the mission's low orbit, reentry into Earth's atmosphere is expected to occur within two years of deployment from the ISS, at which point the satellite will be destroyed.

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In The News

NASA - NASA Announces Ninth Round of Candidates for CubeSat Space Missions, March 2, 2018
UC Magazine - This Cat is Ready to Roar, March 2, 2018
WVXU - These UC Students Want To Help NASA Get To Mars, March 2, 2018
The Cincinnati Enquirer - These UC Bearcats are Heading to Space. (OK, their Experiments are.), March 7, 2018
WCPO-TV - How University of Cincinnati Students are Helping NASA Get to Mars, March 7, 2018
The News Record - CubeCats Satellite is a Go for Launch, March 9, 2018