TLAZÓLTEÓTL

Mission Concept 

The accumulation of space debris in low Earth orbit (LEO) is a threat to space assets and human space activities. According to the latest estimation by the European Space Agency's Space Debris Office, there are around 34,000 debris objects larger than 10 cm, and around 900,000 debris objects larger than 1 cm in LEO. This makes LEO the World’s largest garbage dump. Space debris can cause damage to operational satellites, pose a risk to astronauts, and hinder future space missions. Therefore, effective mitigation strategies are necessary to ensure the safety and sustainability of space activities.  

A possible solution to the issue of space debris in the vicinity of our planet is the use of space tugs to capture decommissioned spacecraft, slow them down, redirect them, and send them on a controlled atmosphere re-entry trajectory. Tlazólteótl is a commercial space junk collection and deorbit concept for LEO. A Capture and Relocation Module (CRM) will undock from the Mother Spacecraft (MSC) to chase the target, acquire it, and redirect it for re-entry. The CRM will use four small robotic arms located at its base to secure the target while it decelerates the debris object. Once the target is successfully controlled, the CRM will move it to a lower orbit and send it on a controlled reentry. Tlazólteótl will operate by contract to deorbit decommissioned spacecraft in LEO. The MSC will carry two spare CRMs that can be deployed in case of failure of the primary CRM.  

Requirements

Requirements Traceability

CONOPS

Mission Geometry and Architecture 

Payload & Instruments

The main objective of the mission is to relocate decommissioned spacecrafts in LEO. To accomplish the mission, there are a variety of instruments and sensors that must be included for the success. The payload can be separated into two categories, the essential payload and the mission payload.  

The essential payload is comprised of non-negotiable instruments. Such components are the minimum necessary for flight and travel and are often part of the main structure. Such instruments include: 

• Attitude Determination and Control System 

• Antennas, transmitters, and receivers 

• Solar panels, batteries 

• Infrared thermometers 

• Global positioning system (GPS) and star trackers  

• Cameras 

• Magnetorquers and reaction wheels 

• Propulsion system 

• Laser rangefinder, and light detection and ranging (Lidar) 

The mission payload is comprised of instruments integral to the execution and success of the mission objectives. The required instruments will more than likely overlap with the essential payload instruments and will include mission specific components such as robotic arms. 

Orbit  

The optimal orbit for the Tlazólteótl mission is the LEO debris ‘hotspot’ at 1000 km altitude and 82 deg inclination, where there’s an estimated 290 littered objects. The orbit will have the same eccentricity as Earth at approximately zero. The orbital period will be between 90 and 120 minutes. Solar radiation pressure (SRP), solar gravity, and lunar gravity would be relatively small in this orbit so close to the earth.   

Ground Stations  

The selected orbit encompasses a significant amount of ground coverage. To establish the locations of the ground stations, the assumption that there is no political conflict with any choice of location was made. One of the ground stations would be in Florida, United States. As the space coast is already a hub for the space industry, it was deemed reasonable to select it as a location, and it will serve as the main domestic site. 

Tlazólteótl will have international partners, as the goal for the module is for it to be contracted by other companies and countries who would like to avoid international orbital fees for not removing property from within the orbits. Other potential locations for additional ground stations include Canada, Japan, Europe, and Russia.  

Space Environment Characteristics  

• Vacuum: The space environment as 1000 km LEO is a vacuum with almost no air molecules or atmospheric pressure (approx. 10e-6 Pa). This has significant impact on thermal management and propulsion.  

• Radiation: In LEO there are high levels of radiation that can damage components such as soar panels, electronics, and sensors. 

• Temperature: The temperature in LEO varies largely from sunny side to dark side, causing cycling thermal expansion and retraction. 

• Orbital Debris: There are significant debris in LEO (hence the mission) that must be avoided in order to successfully deorbit the targeted debris. A collision could be catastrophic and add more debris to LEO than were removed. 

• Charged Particles: LEO carries high levels of charged particles which can damage spacecraft component and electronic systems. 

• Atomic Oxygen: ~4.5 eV Atomic oxygen oxidizes all hydrocarbon based polymers any graphite. LEO atomic oxygen is a highly reactive form of oxygen which can break chemical bonds, change morphology, reduce thickness of these materials. Anti oxidation coatings have been developed in attempts to mitigate these effects. 

Mission Risks  

Below is a sample of potential risks to the mission, severity, and proposed solutions to make them either less severe or less probable via risk mitigation.

Risk Mitigation

Since the risks in green do not need to be mitigated, the mitigation strategies for the risks in yellow and red are: 

[1] Per STR_06 - CRM FS, the CRM structure will have a minimum factor of safety of 1.5, which will reduce the likelihood of damage by micrometeoroid impacts. 

[2] Per SYS_REQ_09 – Target Recognition, a reliable Neural Network algorithm will be used for debris detection, reducing the likelihood of this risk. 

[3] Per SYS_REQ_04 – CRM Redundancy, a backup CRM will replace the primary one, mitigating the consequence of this risk. 

[5] The consequence of this risk would also be mitigated per SYS_REQ_04. 

[6] Per COMMS_10 – Antenna Certification, the antenna will be certified for the mission prior to launch to ensure it can withstand launch loads and will operate at a frequency approved by the FCC to avoid interferences. 

[9] This will be prevented by sharing the spacecraft’s current orbit with the Space Force, who would notify of any possible collisions with active satellites. 

Estimated Cost Budget

Mechanical Subsystems

The configuration of the spacecraft was determined based on hardware and structural requirements. The MSC design was driven by MISS_REQ_03 requiring docking between the CRM’s and MSC, SYS_REQ_04 requiring CRM Redundancy, SYS_REQ_18 requiring 3 cameras, and SYS_REQ_22 requiring solar panels (see Figure 1). The CRM design was driven by SYS_REQ_08 requiring cameras, SYS_REQ_11 requiring berthing between CRM’s and targets and PAY_06 requiring robotic arms (see Figure 2). The robotic arms are all around the body so that a target can be berthed from any side of the craft. The arm extends with an elbow to allow for further degrees of freedom. The docking iris is depicted by an extruded circular lip for simplicity.  

The structural component expected to see the highest mechanical load will be the robotic arms on the CRM’s that will be used to slow down a tumbling target. The highest expected load would be approximately 100 N based on the mission limitations and assuming a maximum target mass of 100 kg. The robotic arm must be sized accordingly by using the yield stress of the material and ensuring it is large enough to withstand the maximum possible load with a safety factor of 1.5 per STR_06 and STR_07. Assuming a 100 kg maximum load and titanium robotic arm with a yield strength of 240 MPa. 

Thermal Control Subsystem  

Cold Case 

The MSC and CRMs will experience the cold case when the spacecraft is in the Earth's shadow, without any direct sunlight (when the spacecraft is in the night side of the orbit), as the temperature is the lowest it can be. In the cold case the environmental conditions will include extremely low temperatures, in the order of -150 °C or lower depending on the altitude and the season. 

Hot Case 

The MSC and CRMs will experience the hot case when the spacecraft is exposed to the maximum solar radiation, which occurs when the spacecraft is at the perigee of its orbit. The environmental conditions in the hot case will include high temperatures, on the order of 100-150 °C or higher depending on the altitude and the season. 

Thermal control hardware  

A combination of passive and active thermal control techniques was selected for the Tlazólteótl mission. The mission will rely on passive techniques, such as insulation and radiative surfaces to minimize heat transfer by radiation and conduction. Active techniques, such as heat pipes and thermoelectric coolers, will be used to manage heat generated by onboard electronics and payloads. 

Specifically, the following thermal control hardware will be used for the Tlazólteótl mission: 

• Multi-layer insulation (MLI) blankets: insulation against thermal radiation. 

• Radiators: reject excess heat generated by spacecraft systems and payloads. The Tlazólteótl mission will use deployable radiators to maximize heat rejection and minimize mass. 

• Heat pipes: manage heat generated by electronics and payloads. 

• Thermoelectric coolers: cool electronics and payloads using the Peltier effect to pump heat from one side of a device to the other. 

C&DH Subsystem 

Telecommunications Subsystem 

A helical antenna is the ideal antenna type for this mission. Due to its compact and lightweight nature, this type is ideal for small spacecraft. These antennas also provide reliable communication with 70% efficiency and a length L=45 cm.

to withstand a mission of long duration. This reliability is partially due to their circularly polarized radiation pattern that can reduce interference from other signals.   

An S-band frequency band would be selected for this mission’s communication because it is a commonly used LEO communications frequency band. This band is highly used and regulated which ensures regulatory requirement compliance and avoids potential interference with other communication systems. The S-band’s low frequency of 2-4 GHz is very reliable and less susceptible to atmospheric interferences, and finally requires low power to transmit data compared to higher frequency bands. Specifically, the selected bandwidth is B=3 GHz.

A bit error level of 10-6 is standard for a mission of this type. In this case there is on average one bit error for every 1 million bits transmitted. Correct data is essential for target recognition and acquisition. Assuming a BPSK modulation scheme, according to the BER vs Eb/No

plot, the required Eb/No would be approximately 10.2 dB. The MSC will downlink data at an altitude h=400 km.

Link Budget

Link Budget Summary: