Extravehicular activity (EVA) is defined as any activity performed by an astronaut or cosmonaut, outside of the vehicle/habitat, beyond Earth’s atmosphere. This includes both microgravity activities (e.g., outside a vehicle or station while in orbit) and partial gravity activities (e.g., Lunar or Martian surface exploration).
Both microgravity (i.e., on orbit) and partial gravity (i.e., on another planetary body) EVA have been successfully conducted by NASA astronauts, Soviet/Russian cosmonauts, and astronauts from eight other spacefaring partner nations. Here we provide a short summary of the history of EVA operations.
The Apollo program produced the first (and to date, only) instances of planetary EVA, which included tasks such as scientific equipment deployment, rock sample collection, photography, and even a bit of golf. Additionally, three trans-Earth EVAs were conducted as the spacecraft transited between Earth and Lunar orbit. These represent the first successful attempts at deep-space EVA. Following the final Apollo mission, human space activities were once again relegated to Earth orbit, where we remain to this day.
In the wake of the Apollo program, the modern era of EVA firmly took shape with the advent of the modern space station (e.g., Skylab, Salyut, Mir) and the US Space Shuttle Program (offering increased crew and cargo sizes), leading to the penultimate construction of the ISS in 2011. EVA became commonplace on space missions during this era, often including multi-team crews performing science, construction, and maintenance activities. Since the end of the Apollo era, a total of 347 EVA have been conducted – representing 92% of the total EVA performed in the history of the space program (NASA 2008).
Risks and Issues Associated with EVA
EVAs are inherently risky: astronauts must don space suits to protect themselves from the unforgiving vacuum of space (or local planetary atmosphere) and must perform physically demanding tasks to achieve mission goals. Any mishap – from a failure of the onboard life-support system to an astronaut injury or accident – could result in mission failure or even death. In this section we briefly enumerate known risks and issues associated with EVA:
Decompression Sickness (DCS)
Space suits worn by astronauts are gas-pressurized, surrounding the wearer with an artificial atmosphere to provide both breathing gas and full-body counter-pressure (two life-support necessities). This pressurization causes soft suit materials to stiffen, greatly decreasing suit mobility. To combat this effect, suits are designed to operate at low pressure (typically 29.6 kPa/4.3 psi). This requires the use of pure oxygen, rather than air, to ensure sufficient oxygen for respiration. However, space vehicles and habitats are often maintained with air at atmospheric pressure (e.g., the ISS runs air at 101.3 kPa). Consequently, when astronauts transition from high to low pressure as they don their space suits, they are at risk for decompression sickness (DCS) – a condition that arises when tissue inert gas partial pressures exceed ambient pressure, resulting in gas bubble formation that can have serious health implications (Buckey 2006).
To prevent DCS onset in space, astronauts are required to perform prebreathe (PB) protocols in advance of an upcoming EVA. These protocols typically require the astronauts to breathe gases with elevated oxygen content to purge excess nitrogen from their body. Specific protocols include 4-h, in-suit pure oxygen PB; overnight air lock “camp-out” PB using 26.5% O2 gas at 10.2 psi; and PB protocols that combine exercise with increased oxygen content exposure. To date, no DCS events have been recorded in space (Conkin et al. 2015).
Workload, Injury, Metabolism, and Temperature Control
Due to the physical nature of EVA tasks (and the extra work required to manipulate the stiff space suit), astronauts are at significant risk for both overheating and exhaustion/injury during EVA operations. To keep astronauts cool, undergarments are typically worn that contain a network of small tubes to circulate chilled water over the body – known as liquid cooling and ventilation garments (LCVGs). The current space suit used by US astronauts, the extravehicular mobility unit (EMU), can handle a continuous thermal load of 252 kcal/h. Astronaut metabolic rates for Shuttle EVAs averaged 195 kcal/h, well within the capability of the LCVG (Buckey 2006). This represents a significant improvement over initial suit designs, where overheating and visor fogging were common experiences (e.g., nearly leading to catastrophe on Leonov’s historic first EVA) (Buckey 2006).
Astronaut injury during EVA, especially during long duration missions where both musculoskeletal deconditioning is expected and frequent EVAs are required, can be debilitating. In particular, soft-tissue damage and joint injuries are common, as is onycholysis (fingernail delamination) (Opperman et al. 2010). These issues stem from a variety of issues, including poor suit sizing, poor task selection, and overexertion/repeat exposure. To combat EVA injury, proper suit sizing is critical, as well as astronaut conditioning, and thoughtful consideration of EVA task selection and frequency.
Thermal, Radiation, and Debris Protection
In addition to the hazard posed by vacuum exposure, the space environment includes additional hazards associated with thermal loading, radiation, and debris collision. For example, temperature on the Lunar surface can vary between 100 K and almost 400 K between day and night phases (Larson and Pranke 2000), imposing significant thermal loads on astronauts when outside of the protection of their vehicle/habitat. Space debris – both naturally occurring, like micrometeorites, and artificial, like space junk or remnants of other spacecraft/satellites – can travel at speeds in excess of 10 km/s, presenting significant hazard to an astronaut on EVA (Larson and Pranke 2000). And radiation hazards – both from solar particle events (SPEs) and from galactic cosmic rays (GCRs) – pose serious risk to astronaut health beyond the protective influence of Earth’s magnetic field (Larson and Pranke 2000; Buckey 2006).
Space Suit System Failure
The space suit worn by an astronaut on EVA represents the first and only defense against the hazardous space environment (as previously discussed), as well as the sole life-support system keeping the astronaut alive (including oxygen delivery, carbon dioxide scrubbing, humidity control, thermal control, food/water delivery, and waste removal) (Larson and Pranke 2000). Trade-offs exist between system robustness, system redundancy, simplicity, and mass. A failure in life-support or suit structure – such as a suit puncture or a fluid leak – could have catastrophic consequences.
This exact scenario manifested on July 16, 2013, on ISS EVA-23. Italian astronaut Luca Parmitano was forced to abort the EVA due to a fluid leak in his helmet. His visibility and breathing were both impaired, with water covering his eyes, nose, and ears, requiring him to navigate back to the air lock using the manual feel of his safety tether cable. The source of the leak was ultimately attributed to a plugged water separator – a very minor hardware mishap – that led to an urgent, emergency situation on orbit (NASA 2013).
Effect of Space Suit Design on EVA Performance
As previously mentioned, suit architecture decisions significantly affect EVA safety and performance. In addition to potential DCS hazards associated with low suit operating pressures, suit mass and mobility play large roles in astronaut health, effectiveness, and efficiency. Further, microgravity EVA differs considerably from partial gravity (surface) EVA in terms of task allocation and operational requirements: microgravity EVA requires extensive arm and hand manipulations (and benefits from stiff lower body joints used for anchoring/leverage), whereas surface EVA requires full-body mobility and low mass to enable locomotion and exploration in the presence of partial gravity.
An advanced US suit concept – the Mark III – is under development for future planetary exploration missions. In anticipation of full-body EVA mobility requirements, the Mark III suit incorporates a hard lower torso complete with rotational hip bearings designed to enable a wide range of hip mobility. Further, the Mark III is significantly lower in mass (59 kg) than the EMU, which is more suitable for use in partial gravity environments. The Mark III is depicted in Fig. 4 (NASA 2015b).
Finally, an alternate space suit concept known as a mechanical counter-pressure (MCP) suit promises radical improvements in EVA mobility and reductions in suit mass compared to traditional gas-pressurized suits. MCP suits provide pressure to the wearer using tight-fitting materials rather than pressurized gas, providing necessary pressurization for life support without mobility limitations. MCP suits were demonstrated over four decades ago (Annis and Webb 1971), but work continues today to produce a flight-ready prototype (Newman et al. 2004). An MCP suit concept – the MIT BioSuit™– is depicted in Fig. 4 (Newman 2015).
EVA is a highly complex, potentially hazardous, yet critically important component of human space exploration. Mitigating risk during EVA requires strategic suit design, optimized task analysis, conscientious planning, and advanced life support and protective technologies. Failures during EVA can result in loss of mission and, worse, loss of life; however, successes during EVA can result in hugely important scientific breakthroughs and iconic human achievements.
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