Space is often imagined as a boundless, icy wasteland where a human would freeze instantly without protection. However, from a physics perspective, this is not entirely accurate: outer space itself has no temperature in the conventional sense. Temperature is a measure of the motion speed of particles, while heat is the energy these particles possess. In a completely empty space, devoid of particles and radiation, there would be no temperature. But the universe is not entirely empty: it contains stars, gas, particles, and various forms of radiation. Therefore, the question of how cold space actually is turns out to be much more complex than it seems.

Why space becomes "warmer" near stars

The primary sources of heat in the universe are stars. Thermonuclear reactions occur within their cores, releasing vast amounts of energy. When a star's radiation reaches a region of space containing particles of matter, it transfers energy to them. This is precisely why Earth is significantly warmer than the surrounding space: solar energy heats the atmosphere, and air molecules transfer heat to one another. However, proximity to a star does not guarantee a high temperature. 

For instance, Mercury—the closest planet to the Sun—scorches during the day, yet its surface cools to approximately −178 degrees Celsius at night. The reason is that the planet has virtually no atmosphere to retain heat. Uranus is even colder: its surface temperature can drop to around −224 degrees Celsius, which is lower than that of the more distant Neptune. Scientists attribute this to its unique history: it is believed that in the distant past, Uranus collided with an object roughly the size of Earth, causing its axis to tilt severely and depriving the planet of its ability to retain internal heat effectively.

The coldest places in the universe

Far from stars lies the interstellar medium—the space between cosmic objects. There, particles of matter are so sparse that heat transfer through collisions is nearly impossible. In the coldest dense molecular clouds, temperatures can drop to about 10 Kelvin—around −263 degrees Celsius. In less dense clouds, the temperature is higher—around 100 Kelvin or −173 degrees Celsius. Yet, this is still not the lowest temperature known in the universe.

The "coldest" temperature in space: 2.7 Kelvin

Despite the vast diversity of temperatures in the universe—ranging from millions of degrees inside stars to near-absolute cold in the void—there is a radiation that permeates all of space. This is the cosmic microwave background (CMB) radiation, the remnant heat from the early universe. Its temperature is approximately 2.725 Kelvin, or about −270 degrees Celsius. This is merely 2.7 degrees above absolute zero—the temperature at which all particle motion ceases. 

This radiation originated approximately 400,000 years after the Big Bang, when the universe became transparent to light. Before that, electrons moved freely through space, obstructing the propagation of photons. After the first hydrogen atoms formed, light gained the freedom to travel. Originally, this radiation was much hotter—around 3,000 Kelvin. But over 13.8 billion years, the expansion of the universe stretched its wavelengths, lowering its energy and temperature. As the universe continues to expand, space becomes progressively colder.

What would happen to a human without a spacesuit in space?

Movies often depict a person exposed to open space turning into an ice statue instantly. In reality, that is not what happens. In physics, there are three primary modes of heat transfer: conduction (energy transfer via direct contact), convection (heat transfer via fluid or gas currents), and radiation (energy loss in the form of electromagnetic radiation). In the vacuum of space, the first two mechanisms are practically impossible due to the lack of sufficient matter. A human can lose heat almost exclusively through radiation, which is a comparatively slow process. 

The primary danger of entering open space is not instantaneous freezing, but the total absence of pressure. Without a spacesuit, the body would face rapid decompression: water in tissues and blood would begin to vaporize, the lungs would fail to function properly, and consciousness would be lost rapidly. Thus, space is indeed extremely cold, but not because it freezes everything around it instantly. In truth, the cosmic void barely transfers heat at all, and its "coldness" is primarily due to the absence of matter and an extremely low energy density. Based on materials from Space.com.