The temperature in space during a spacewalk is often misunderstood, with many people assuming the environment is simply a uniform, absolute cold. The reality is far more complex than a single number, as the thermal experience of an astronaut performing an Extravehicular Activity (EVA) is dynamic and constantly fluctuating. Conditions outside the International Space Station (ISS) create an environment of extreme thermal contrast. The temperature an object experiences depends entirely on its exposure to the Sun and the physics governing heat transfer in a vacuum. Understanding this environment requires separating the theoretical temperature of the vacuum itself from the intense thermal load placed upon an object, such as the Extravehicular Mobility Unit (EMU) spacesuit.
Defining Temperature in a Vacuum
Temperature relies on the average kinetic energy of atoms and molecules. In the near-perfect vacuum of space, the density of gas molecules is incredibly low, meaning there is almost no medium for heat conduction or convection. The kinetic temperature of the few existing particles is near absolute zero, registering about \(\text{-455}^\circ\text{F}\) (\(\text{2.7}\) Kelvin) due to the Cosmic Microwave Background radiation.
This kinetic temperature has almost no effect on a solid object like a spacesuit. Heat transfer in space is dominated by thermal radiation, which is the movement of energy via electromagnetic waves. All objects with a temperature above absolute zero constantly radiate heat away and absorb radiant energy from sources like the Sun. Without an atmosphere to scatter or absorb this energy, an astronaut is subjected to the full, unfiltered intensity of solar radiation.
The spacesuit’s thermal experience is defined by the balance between the radiant energy it absorbs and the energy it radiates away into the cold void. Since conduction and convection are absent, the thermal properties of the suit’s surface materials determine its temperature. This reliance on radiation creates the thermal swings characterizing a spacewalk.
The Extreme Thermal Range
An object in Low Earth Orbit (LEO), such as a spacesuit, cycles rapidly between scorching heat and deep cold as it orbits the Earth every \(\text{90}\) minutes. Exposed to direct, unfiltered sunlight, the external surface can quickly heat up to temperatures as high as \(\text{+250}^\circ\text{F}\) or higher. This rapid heating occurs because there is no atmosphere to absorb the intense solar energy, allowing it to be fully absorbed by the suit’s outer layers.
Conversely, when the astronaut moves into the Earth’s shadow or a shaded area, the absorbed heat quickly radiates away into deep space. With no ambient air to slow this process, the temperature on the shaded side can plummet to \(\text{-250}^\circ\text{F}\) or lower. This stark difference means an astronaut could simultaneously experience a \(\text{500}^\circ\text{F}\) temperature gradient across the suit.
This thermal cycling poses a significant challenge to the astronaut and their equipment. The sophisticated design of the spacesuit must manage this constant, extreme thermal stress to prevent materials from expanding and contracting excessively, which could cause mechanical failure or structural damage. The environment demands effective insulation and heat rejection mechanisms to maintain a habitable internal temperature.
Thermal Control Systems in the Spacesuit
The astronaut remains safe and functional during a spacewalk thanks to the sophisticated engineering of the Extravehicular Mobility Unit (EMU), which acts as a miniature, self-contained spacecraft. The thermal control system within the EMU is divided into passive and active components to manage the extremes of the space environment. Passive thermal control is handled by the suit’s outer layers, which are constructed from multiple layers of specialized insulating materials, often including aluminized Mylar.
The white, highly reflective outer layer, known as the Thermal Micrometeoroid Garment, reflects incoming solar radiation to minimize heat absorption on the sunlit side. This multi-layered insulation also works to trap the astronaut’s internally generated body heat, providing protection from the severe cold experienced in the shadow. These layers buffer the astronaut from external thermal fluctuations.
The active thermal control system centers on the Liquid Cooling and Ventilation Garment (LCVG), worn directly against the astronaut’s skin. This mesh suit is embedded with a network of thin plastic tubes that circulate cool water. The circulating water absorbs the metabolic heat produced by the astronaut’s body, which is a major factor since the physical exertion of a spacewalk can quickly lead to overheating.
This warmed water is pumped to the Primary Life Support System (PLSS) backpack, where heat is rejected to space using a device called a sublimator. The sublimator works by allowing a small amount of water to freeze and then sublimate (turn directly from solid ice to water vapor) into the vacuum of space. This continuous process draws heat energy from the circulating water, keeping the astronaut’s core temperature stable, typically maintaining an internal environment of \(\text{70}^\circ\text{F}\) to \(\text{75}^\circ\text{F}\).