Extravehicular Activity (EVA), commonly referred to as spacewalking, is a vital part of space exploration that enables astronauts to step outside their spacecraft and operate in the vacuum of space. When an astronaut exits the International Space Station or any other spacecraft, this activity is known as a spacewalk or EVA. These challenging missions demand thorough planning, extensive training, and specialized gear to safeguard the astronauts and ensure mission success. EVA procedures start with depressurizing the airlock or space module and conclude when the airlock or module is repressurized after the astronaut returns. This process also encompasses operations in various pressurized and unpressurized environments, whether for routine tasks or emergencies.

The first spacewalk was accomplished by Russian cosmonaut Alexei Leonov on March 18, 1965, followed by NASA astronaut Ed White during the Gemini 4 mission on June 3, 1965. Astronauts undertake spacewalks for a variety of tasks, such as maintaining the space station, installing new equipment, or conducting scientific experiments. EVAs are crucial for the upkeep and functionality of spacecraft, as well as for collecting important data and samples from outer space. Additionally, spacewalks allow astronauts to witness the incredible beauty of space firsthand, as they float above Earth, surrounded by the boundless universe.

Preparation and Classifications of EVA

Preparing for an Extravehicular Activity (EVA) is a highly detailed and lengthy process that starts well in advance of the actual spacewalk. Astronauts receive comprehensive training on how to operate spacesuits, tools, and equipment, along with mastering the procedures and protocols necessary for conducting EVAs. They also engage in simulations and practice spacewalks to sharpen their skills and become accustomed to the potential challenges they might encounter in space. Furthermore, astronauts must pass thorough physical and psychological assessments to ensure they are capable of handling the demanding conditions associated with spacewalking.

Extravehicular Activity (EVA) offers an efficient way to service, maintain, repair, or replace space equipment without the need to bring it into a pressurized environment, return it to Earth, or discard it.

There are two basic classes of EVA:

• Planned EVA – Performed to complete tasks that are part of the regularly scheduled mission timeline and are necessary to support specific mission objectives.
• Unplanned EVA – Carried out to address tasks not originally included in the mission's timeline, but essential for mission success, enhancement, or to fix or bypass malfunctioning systems.

Apart from these two primary categories, EVA tasks have additional important characteristics that help establish design standards and guide training requirements. Ie, criticality and complexity.

• Criticality: EVA tasks are categorized into three levels of importance:

1. Mission Enhancement – Tasks that contribute to the improvement or increased success of mission goals. These tasks often receive support but are not essential.
2. Mission Success – Tasks that are necessary to achieve the primary objectives of the mission.
3. Safety Critical – Tasks that must be completed to ensure the safety of the spacecraft or the crew.

• Complexity: EVA tasks are also classified by their level of difficulty:

1. Simple EVA Tasks – Tasks that involve the use of standard tools, restraints, or mobility aids, and do not pose significant risks to the crew.
2. Intermediate or Specialized EVA Tasks – Tasks that may require extra tools or equipment but remain straightforward in execution.
3. Complex EVA Tasks – Tasks that demand advanced capabilities, such as specialized tools, challenging access or restraint situations, or extended operations, possibly requiring propulsion maneuvering units for unrestrained movement.

Equipment Used During EVA

The equipment utilized during EVA operations represents some of the most advanced technology in space exploration. During an EVA, astronauts depend on a range of specialized equipment to ensure their safety and enable them to perform tasks effectively. These Extravehicular Activity (EVA) systems include spacesuits, tools, and support systems designed for use in the harsh environment of space. The equipment protects from extreme conditions, along with tools and tethers to assist astronauts in maneuvering and working in the weightlessness of space. Astronauts also use communication systems to maintain contact with mission control and fellow crew members, as well as cameras and sensors to document their activities and collect important data.

The Johnson Space Center (JSC) is equipped with world-class facilities and expertise to support the design, development, testing, and implementation of EVA systems. JSC personnel possess extensive knowledge of the technical challenges related to spacesuit technology, including mobility, sizing, life support, ventilation, hydration, and waste management. Their experience spans the full life cycle of EVA systems, from initial design and development to testing and operational support.

The EVA system consists of three main components:

1. Extravehicular Mobility Unit (EMU)
2. ISS Joint Airlock
3. Equipment & Tools, including the Simplified Aid For EVA Rescue (SAFER).

Space Suit

Spacesuits are meticulously engineered, one-of-a-kind miniature spacecraft designed specifically for each astronaut. They serve as a vital protective shield, providing life support, temperature regulation, and protection from the vacuum of space. Whether astronauts are conducting repairs, performing experiments, or exploring celestial bodies, EVA spacesuits are essential, allowing them to leave the safety of their spacecraft while staying protected and equipped to carry out critical tasks in the hostile environment of space.

Spacesuits are unique in their ability to function as customized spacecraft, offering environmental protection, mobility, and life support during spacewalks. The suit used for spacewalks outside the International Space Station is known as the Extravehicular Mobility Unit (EMU). Since NASA refers to spacewalks as Extravehicular Activities (EVA), this suit is commonly called an EVA suit.

EMU

The spacesuit used for spacewalks outside the International Space Station is known as the Extravehicular Mobility Unit (EMU). The EMU is a self-contained system that offers astronauts environmental protection, mobility, life support, and communication capabilities during Extravehicular Activities (EVA).

The Extravehicular Mobility Unit (EMU) is engineered to equip astronauts with essential resources for a maximum extravehicular activity (EVA) duration of seven hours. This time is segmented into several critical phases: 15 minutes for egress, which is the process of exiting the spacecraft, followed by six hours for the actual work to be done in space. An additional 15 minutes is allocated for ingress, or re-entering the spacecraft, along with a 30-minute reserve as a safety buffer for unexpected situations.

The EMU is a sophisticated system made up of two primary subassemblies. The first is the Space Suit Assembly (SSA), which includes the suit worn by the astronaut, offering protection and mobility in the challenging conditions of space. The second is the Portable Life Support System (PLSS), a vital component that provides oxygen, removes carbon dioxide and helps regulate temperature, ensuring the astronaut can survive and function effectively during the EVA. Together, these systems empower astronauts to undertake complex tasks in the vacuum of space while maintaining safety and support.

Space Suit Assembly Components:

The Space Suit Assembly is a sophisticated system specifically designed for extravehicular activities, incorporating several essential components to ensure astronaut safety and functionality in the extreme conditions of space. At its core are the Hard Upper Torso (HUT) and arms, which provide the primary structure and support for the suit. This is paired with the Lower Torso Assembly (LTA), which ensures a secure and comfortable fit.

Extravehicular gloves are crucial for both dexterity and protection, while the Helmet/Extravehicular Visor Assembly (EVVA) offers visibility and safeguards the astronaut from space hazards. Communication is enabled by the Communications Carrier Assembly (CCA), also known as the Comm Cap, which integrates communication systems seamlessly into the suit.

The Liquid Cooling and Ventilation Garment (LCVG), or Thermal Cooling Under-Garment (TCU), plays a key role in regulating temperature, and the Operational Bioinstrumentation System (EKG) monitors the astronaut's vital signs. Additionally, astronauts use a Disposable In-Suit Drink Bag (DIDB) for hydration, while the Maximum Absorption Garment (MAG) assists with moisture management.

Life support system Components:

The Life Support System is critical for sustaining life during extravehicular activities, supporting the various components of the spacesuit. The Display and Control Module (DCM) acts as the central interface, delivering caution and warning system (CWS) messages, EMU parameters, and control functions directly to the astronaut.

The Portable Life Support Subsystem (PLSS) plays a vital role by providing breathable oxygen, electrical power, and cooling while maintaining the suit's pressure. It effectively circulates oxygen and removes carbon dioxide, humidity, and trace contaminants, ensuring a stable thermal environment. In emergencies, the Secondary Oxygen Package (SOP) automatically activates during EVAs, providing a minimum of 30 minutes of supplemental oxygen in open-loop purge mode.

The Space-to-Space EMU Radio (SSER) and the Early Caution and Warning System (ECWS) are essential for transmitting critical status parameters and biomedical data back to Mission Control, enhancing situational awareness. Power is supplied by a rechargeable EVA Battery Assembly (REBA), while the Contaminant Control Cartridge (CCC)—either a Lithium Hydroxide (LiOH) Cartridge or a Metal Oxide (METOX) Cartridge—efficiently removes carbon dioxide and other trace contaminants from the suit's atmosphere, ensuring astronaut safety throughout their missions.

EVA Systems - ISS Joint Airlock

The ISS Joint Airlock is a crucial part of the International Space Station (ISS) infrastructure, specifically designed to facilitate extravehicular activities (EVAs), commonly known as spacewalks. This airlock serves as the main facility for U.S. EVAs, allowing astronauts to perform essential operations from both the orbiting space shuttle and the ISS itself. Its design addresses the unique challenges of the harsh space environment.

One of the standout features of the EVA Systems - ISS Joint Airlock is its compatibility with Russian Orlan spacesuits. This interoperability promotes international collaboration in space exploration, enabling astronauts from both the U.S. and Russia to work effectively together during joint missions. By ensuring that both U.S. and Russian suits can operate within the same airlock system, the airlock enhances operational flexibility and fosters a unified approach to spacewalk challenges.

The airlock consists of two distinct sections. The Crew Lock is typically maintained at vacuum pressure, providing astronauts with a controlled environment to prepare for their spacewalks. This area is designed to facilitate the donning of Extravehicular Mobility Units (EMUs), allowing astronauts to safely transition from the station's atmosphere to the vacuum of space. By maintaining a vacuum in the Crew Lock, the system mitigates hazards associated with depressurization, ensuring that astronauts can work safely and efficiently during their EVAs.

The second section, known as the Equipment Lock, is essential for the overall functionality of the airlock. This compartment is dedicated to storing, recharging, and servicing EMUs, which are vital for protecting astronauts from the extreme conditions of space. The Equipment Lock allows astronauts to prepare their suits and tools before a spacewalk and secure equipment after their tasks are completed. It also serves as a location for donning and doffing the EMUs, ensuring a smooth transition between the suit and the airlock.

In summary, the EVA Systems - ISS Joint Airlock is an integral component of the ISS that enables spacewalks by providing a safe and efficient environment for astronauts. Its design fosters collaboration between U.S. and Russian space agencies, while its dual-section structure optimizes both the preparation and execution of extravehicular activities. By supporting a wide range of missions and ensuring astronaut safety, the airlock plays a pivotal role in advancing human exploration beyond Earth.

Equipment & Tools (including SAFER)

EVA Equipment and Tools are crucial for ensuring astronaut safety and effectiveness during extravehicular activities (EVAs). A range of equipment is integrated into the Extravehicular Mobility Units (EMUs), enhancing astronauts' capabilities while working in the vacuum of space. Among these are TV cameras for documenting activities, lights for illuminating work areas, and a mini-workstation for organizing tools. Other essential items include waist tethers, an EVA Cuff Checklist to verify that all necessary tools are on hand, a wrist mirror for improved visibility, and a body restraint tether to secure astronauts during their tasks. The Pistol Grip Tool (PGT) is commonly used for various tasks, while an ISS Small Trash Bag is provided for waste management during the EVA.

The Mini Work Station (MWS) is mounted on the front of the EMU, designed to safely carry small tools. Tools are secured using tether rings or bayonet receptacles, ensuring they are both accessible and secure during operations. The MWS also features an end-effector with a retractable tether, providing restraint for the EVA crewmember at the worksite and preventing accidental drift away from the station.

Multiple types of tethers are utilized during EVAs. Safety tethers (available in lengths of 55 and 85 feet) connect the EVA crewmember to the vehicle, while suit tethers (waist and wrist) secure smaller items to the suit, allowing for easy transfer without risk of losing equipment. The Retractable EVA Tether (RET) is particularly useful for securing small items while in use, and the Body Restraint Tether (BRT) attaches to the Mini Work Station (MWS). The end-effector of the BRT provides semi-rigid restraint to the EVA crewmember at their worksite using a handrail, offering greater stability than a Portable Foot Restraint while also requiring less setup time.

It is essential to follow EVA tether protocols, which mandate that both crewmembers and equipment must be tethered at all times. Astronauts are trained to always establish a connection before releasing one, ensuring their safety and preventing equipment loss during critical missions outside the ISS. Collectively, this comprehensive set of tools and equipment significantly enhances the efficiency, safety, and success of EVAs.

SAFER (Simplified Aid for EVA Rescue)

The EVA Systems tools include the SAFER (Simplified Aid for EVA Rescue), an essential self-contained propulsive backpack system designed to enhance astronaut safety during extravehicular activities (EVAs). SAFER equips astronauts with a self-rescue capability when the orbiter is either unavailable or unable to perform an immediate rescue during an EVA. This innovative system is powered by pressurized nitrogen gas, enabling astronauts to maneuver effectively in the vacuum of space.

SAFER is operated using a single-hand controller, simplifying its use and allowing astronauts to concentrate on their tasks while still being prepared to respond swiftly in emergencies. The system is stored in the ISS Airlock and is specifically employed during EVAs conducted from the International Space Station, providing peace of mind to astronauts as they carry out critical operations outside the station.

Designed for a single self-rescue, the SAFER system offers enough propellant and power for approximately 13 minutes of runtime, allowing astronauts to navigate safely back to the ISS in case of an emergency. The system underwent extensive testing, including a demonstration of its self-rescue capabilities during mission STS-64 and further validation during STS-86 when the production model was powered up.

Additionally, tethered test flights of the production model SAFER were conducted on Flights 2A and 3A, ensuring the system’s functionality and reliability for actual EVAs. Overall, the SAFER system marks a significant advancement in EVA safety, giving astronauts the ability to conduct self-rescue if necessary and enhancing their confidence as they face the challenges of space exploration.

EVA Operations Overview

EVA operations can be divided into three phases  

1. Pre-EVA
2. EVA
3. Post-EVA

Pre-EVA:  Preparation & Checkouts

The Pre-EVA Preparation and Checkouts are crucial steps that ensure the success and safety of extravehicular activities (EVAs). This process encompasses several key components, beginning with Equipment Preparation, which readies the airlock and Extravehicular Mobility Units (EMUs) for checkout prior to the EVA. Typically, this preparation occurs a few days before the EVA or alongside the docking of the Orbiter to the International Space Station (ISS).

After the Equipment Preparation, a comprehensive EMU Checkout is conducted, where each system of the EMUs is thoroughly inspected and verified for proper functionality. This step is also performed a few days prior to the EVA or just before the Orbiter docks with the Station, ensuring that all life-support and mobility systems are fully operational.

On the day of the EVA, the EVA Prep involves final procedures leading up to the astronauts exiting the ISS. A critical part of this process is EMU Donning, where astronauts put on their suits and verify a proper fit and seal to protect against the harsh conditions of space. Additionally, astronauts undergo a prebreathe procedure that involves inhaling 100% oxygen to lower nitrogen levels in their bodies, thereby reducing the risk of decompression sickness during their transition from the station’s atmosphere to the vacuum of space.

Overall, these pre-EVA preparation steps are essential for mission safety and success, ensuring that both the equipment and astronauts are fully equipped to handle the challenges they will encounter outside the ISS.

EVA

The night leading up to an extravehicular activity (EVA) is a crucial period for astronauts to get ready for their tasks outside the spacecraft. This preparation commences with a Pre-Sleep routine that spans three hours. It begins with a one-hour Mask Prebreathe session, during which astronauts inhale pure oxygen to help clear nitrogen from their bodies. Following this, the airlock is depressurized to 10.2 psi for 20 minutes, allowing the astronauts to acclimate to the conditions they will face during the EVA. The evening's routine wraps up with a minimum of eight hours and 40 minutes spent in an Overnight Campout at 10.2 psi, ensuring the astronauts are both physically and mentally prepared for the challenges ahead.

On EVA Day, the schedule is meticulously arranged to address every detail necessary for a successful mission. The Post-Sleep routine lasts one hour and 15 minutes, beginning with another Mask Prebreathe session that lasts one hour and 10 minutes. This is followed by an airlock repress, a hygiene break, and other activities to help them transition from sleep. The airlock is then depressurized to 10.2 psi in preparation for the EVA.

The EVA Preparation phase, which takes about one hour and 30 minutes, includes critical steps for putting on the suits. This phase consists of EVA Prep for Donning, lasting approximately 30 minutes, followed by Suit Donning at 10.2 psi, which takes an hour. After the suits are secured, a 12-minute Suit Purge is conducted to ensure they are ready for the EVA. The airlock is then repressurized to 14.7 psi, followed by an In-Suit Prebreathe period of 50 minutes, allowing astronauts to further acclimate their bodies for the transition into the vacuum of space.

Next, the crew lock is depressurized to a vacuum over a 30-minute period, leading up to the EVA tasks, which last for 6 hours and 30 minutes. Once the tasks are completed, the airlock is repressurized for 20 minutes. After the EVA concludes, there is a post-EVA period lasting one hour, during which there is no EMU H2O recharge or METOX regeneration, allowing astronauts to recover from their time outside the International Space Station (ISS). The day ends with a two-hour Pre-Sleep period, ensuring astronauts can rest and recuperate after a demanding day in space. Each of these meticulously planned steps is essential for ensuring the safety, efficiency, and success of the EVA mission.

Overview of EVA Tasks

Post-EVA Operations

Post-EVA operations are essential for facilitating the safe return of astronauts to the International Space Station (ISS) and ensuring that their Extravehicular Mobility Units (EMUs) are adequately prepared for future missions. The process starts with EMU Doffing, where astronauts methodically remove their suits after completing their extravehicular activities. This step is crucial for the comfort and safety of the astronauts as they re-enter the station's environment.

Once the suits are off, a thorough EMU Maintenance and Recharge routine is initiated. This maintenance includes several important tasks:

• Oxygen Tank Recharge: The EMUs require their oxygen tanks to be replenished to ensure they are fully stocked for the next use. This is vital for providing life support to astronauts during upcoming EVAs.
• Battery Recharge: The rechargeable batteries that power various systems within the EMUs also need to be charged. Fully charged batteries are critical for the suit's functionality during future missions.
• Water Tank Refill: The water tanks within the EMUs, which provide hydration, are refilled. This step ensures that astronauts have access to water during their next spacewalk.
• METOX Regeneration and LiOH Swap: The carbon dioxide removal system, which utilizes METOX (metal oxide) canisters, requires regeneration to maintain its efficiency. If needed, LiOH (lithium hydroxide) canisters are replaced to ensure the EMU can effectively remove carbon dioxide from the suit’s atmosphere during subsequent EVAs.
• Suit Cleaning: After each use, the EMUs must be thoroughly cleaned to eliminate any contaminants that may have accumulated during the EVA. This cleaning process is vital for preserving the suits’ integrity and hygiene.
• Suit Resizing: If necessary, the suits can be adjusted to guarantee an appropriate fit for astronauts in upcoming missions.

These post-EVA operations collectively ensure that astronauts can safely and effectively resume their routine aboard the ISS while keeping the EMUs in optimal condition for future extravehicular activities. Each step is integral to the ongoing safety, functionality, and preparedness of both the astronauts and their equipment, contributing significantly to the success of space exploration missions.

General EVA Safety Design Requirements

The following safety protocols must be adhered to in order to ensure the protection of EVA crewmembers:

• Temperatures: The surface temperatures of space module components that require EVA interfaces must align with the touch-temperature limits specified in the design of the pressure suit being utilized.
• Radiation: The design of the EVA system and operational procedures must safeguard the EVA crewmember from radiation exposure throughout the duration of the EVA during the mission.
• Micrometeoroids and Debris: The EVA system must be designed to shield the crewmember from anticipated particles, including sand and dust.
• Chemical Contamination: The EVA system should offer protection against hazardous chemical contamination for the crewmember.
• Edges and Protrusions: Any equipment and structures within the space module that require EVA interfaces must either be constructed to eliminate sharp edges or protrusions, or be covered to protect the crewmember and their essential support equipment.
• Hazardous Equipment: Any potentially dangerous items that could injure EVA crewmembers or damage EVA equipment through entrapment, snagging, tearing, puncturing, cutting, burning, or abrasion must be designed to mitigate or eliminate the associated hazards.
• Ingress/Egress: EVA crewmembers must always have a reliable method to return to the pressurized module.
• Power Sources: Appropriate shielding and/or procedures must be implemented to prevent EVA personnel from approaching a nuclear reactor or radioisotopic generator power source located within the space module, which could lead to increased radiation exposure.
• Transmitters: Procedures should be established to protect crewmembers during EVAs from harmful exposure to non-ionizing radiation emitted by high-power electromagnetic wave transmitters (such as microwave, radar, laser, radio, UV/IR visible lamps) present on or in the space module with exterior antennas or external openings.
• Tethers: EVA crewmembers must always be safely tethered to the space module in microgravity, except when using a free-flying maneuvering unit or when otherwise adequately restrained.
• Ignition Sources: Electrical current-limiting devices must be installed to eliminate potential ignition sources within any oxygen-enriched atmosphere of the life support system and pressure suit.
• Positive Pressure: Measures must be in place to prevent the rupture of the crewmember's pressure envelope due to overpressurization caused by a failure in the pressure supply system.
• Electrical Voltage: EVA crewmembers must be protected from electrical voltage shocks resulting from inadvertent grounding of electric circuits and from electrical discharges caused by static charge accumulation.

Risks, Advantages, Limitations, and Applications of EVA

Conducting an extravehicular activity (EVA) is a high-risk task that subjects astronauts to numerous hazards, including extreme temperatures, micrometeoroids, and the possibility of equipment failures. Furthermore, the weightless environment of space introduces distinct challenges, such as restricted mobility and the necessity to continuously monitor oxygen levels and other critical health indicators. Despite these risks and difficulties, astronauts receive comprehensive training and are thoroughly prepared to manage the demands of spacewalking, thanks to their extensive preparation and ongoing support from mission control.

Applications of EVA

The applications of extravehicular activity (EVA) encompass a variety of tasks, including:

• Payload or Mechanical Override: Manipulating or adjusting payloads and mechanical systems.
• Maintenance and Repositioning: Performing maintenance tasks and repositioning equipment as needed.
• Extravehicular Experimentation: Conducting scientific experiments outside the spacecraft.
• Payload, Equipment, and Personnel Transfer: Moving payloads, equipment, and personnel between different locations.
• Large Space or Planetary Surface Construction: Engaging in construction activities in space or on planetary surfaces.
• Satellite Deployment and Retrieval: Launching and retrieving satellites.
• Servicing and Repair: Performing repairs and servicing of equipment in space.
• Inspection: Conducting inspections of various systems and structures.

Advantages of EVA

The advantages of conducting an EVA include:

• Task Flexibility at the Worksite: EVA crewmembers can undertake a wide range of tasks.
• Dexterous Manipulation: The ability to manipulate tools and equipment with one or both hands at the task site.
• High-Resolution Visual Interpretation: Crewmembers can observe and interpret the task site in high detail.
• Human Cognitive and Interpretive Capability: Astronauts can apply their judgment and understanding during tasks.
• On-Site Decision-Making: Decision-makers are present at the task site to respond immediately.
• Real-Time Problem Solving: Crewmembers can implement alternative and innovative approaches to challenges as they arise.

Limitations of EVA

The limitations of EVA include:

• Sensory Degradation: Reduced sensory perception due to environmental factors.
• Limited Duration: The time astronauts can safely remain outside the spacecraft is constrained.
• Mobility and Dexterity Constraints: Limited mobility, dexterity, force application, and endurance of crewmembers in space.
• Operational Time and Resource Overhead: Additional time and resources may be required to plan and execute EVAs.
• Working Volume and Access Limitations: Constraints on the available space and access to certain areas during an EVA.
• Hazards to the EVA Crewmember: Various risks associated with performing tasks outside the spacecraft.

Extravehicular activities (EVAs) have been instrumental in enhancing our understanding of space and broadening the scope of human exploration. They have allowed astronauts to perform repairs and upgrades on spacecraft, engage in innovative scientific research, and lay the groundwork for future missions to the Moon, Mars, and beyond. Additionally, EVAs have captivated audiences worldwide with stunning images and videos of astronauts navigating the vastness of space, serving as a powerful reminder of the remarkable achievements and possibilities of human space exploration.