# What was the average temperature of space near the Spitzer Satellite Telescope?

The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), is an infrared space telescope launched in 2003, to be retired in January 2020.

The temperature of space near earth is about as 10 degrees Celsius, but much cooler in interstellar space:

The average temperature of outer space near Earth is 283.32 kelvins (10.17 degrees Celsius or 50.3 degrees Fahrenheit). In empty, interstellar space, the temperature is just 3 kelvins, not much above absolute zero, which is the coldest anything can ever get.

So what is the temperature near this telescope?

• Yikes! It's a good question, and good that you asked here rather than took your source's word for it! – uhoh Jul 18 at 11:21
• This archived fact sheet may be useful. It's been in deep space, far from Earth for quite a while now, in it's own ~1AU circular orbit around the Sun. – uhoh Jul 18 at 15:48
• @Hobbes I think it's Earth-centered inertial (non-rotating) and probably correct. Over years it lags farther and farther behind Earth in a heliocentric orbit at slightly greater than 1 AU. If you keep the Earth at the origin but don't let your frame rotate, that's what it should look like. – uhoh Jul 18 at 16:09
• @GittingGud, the scope has an orbit, but not around the earth. Cool animated plot, though. – Camille Goudeseune Jul 18 at 20:38
• Here is a less misleading graph of Spitzer's orbit: cfa.harvard.edu/news/su201909 – Hobbes Jul 19 at 13:45

The temperature of space near earth is about as 10 degrees Celsius, but much cooler in interstellar space.

It depends. A better value is 5.2 degrees Celsius, and that assumes a macroscopic perfect blackbody that rotates rather quickly. The temperature of a sub-microscopic object in near Earth space would more likely be in the tens of thousands of degrees. Compare those values with the Spitzer Space Telescope, whose ambient temperature is about 30 kelvins.

The reason the temperature of a sub-microscopic object is so high is because interactions with other sub-microscopic objects bring those tiny objects to thermal equilibrium with the warm interplanetary medium. There are regions of intergalactic space where the very rare material that fills that space is in the millions of kelvins.

Despite being surrounded by material whose temperature is in the millions of kelvins, a macroscopic object in intergalactic space would cool to 2.7 kelvins because the intergalactic medium is so very, very sparse that thermal radiation dominates many over against the tiny amount of warming that results from contact with that material.

The reason a macroscopic blackbody is about 5.2°C (278.3 kelvins) is because such an object effectively absorbs sunlight on one quarter of its surface (halved because half of the surface is in darkness, and halved again because the surface area of a hemisphere is twice its cross section), but emits thermal radiation from all of its surface. This, combined with the Stefan-Boltzmann law yields an effective blackbody temperature of 278.3 kelvins at one astronomical unit.

The reason the Spitzer Space Telescope has a much lower ambient temperature is because it is not a blackbody. The Spitzer is essentially tidally locked, with one part always facing the Sun. The parts of the Spitzer that are exposed to sunlight are coated with a highly reflective surface, making the Spitzer absorbs very little incoming sunlight. The parts of the Spitzer that are self-shaded from sunlight are coated with a very black surface, making those parts radiate very efficiently into empty space. The very sparse nature of the warm interplanetary medium essentially has no effect on the Spitzer's temperature.

The end result is that passive thermal techniques alone keep the Spitzer a rather cool 30 kelvins. That's too warm for some of Spitzer's more sensitive instruments. The operation of those instruments required active thermal cooling techniques. Those instruments were powered down when the liquid helium used for that active cooling ran out. The less sensitive instruments continue running to this day.

• So 30 Kelvin is -243.15C – hawkeye Jul 19 at 13:09

In its orbit around the sun, it's far enough from Earth, a good fraction of Earth's orbit's radius, that we wouldn't learn anything from plots of temperature w.r.t. altitude, that is, distance from Earth.

The "temperature of space," aka the black-body temperature of something sitting there, for something at Earth's distance from the sun, is indeed about 10 degrees C, mostly from solar flux.