THE TEMPERATURE OF SPACE
XMM will be in space and "space is cold" so will XMM also be cold?
We tend to talk of space being cold but we need to be clear what we are talking about. The Earth is in space but the Earth is not cold; the surface of the Earth is at a temperature of about 20oC. Why is this? Does the Earth have an internal supply of heat which keeps it warm or is it warmed by something else. Space is only cold in places far removed from stars. Obviously the Earth is warmed by the Sun. Although the Earth's core is very hot it is not effective in heating the surface.
Why is the Earth at about 20 Degrees C (293 Degrees K)?
As discussed above, the temperature of a body is determined by a consideration of the inflow and outflow of heat. The Earth essentially receives heat only from the Sun (i.e. from a disk in the sky which is 1/2o in diameter and at a temperature of 6000 Degrees K) and looses heat in all directions to cold space (which can take as almost Degrees K). To explore this further click here to read about the Cooling of the Earth.
Why then is the Earth only slightly cooler at night when it is not illuminated by the Sun?
Because the heat is very effectively transported from the hot side of the Earth to the cold side by the atmosphere. When this motion is coupled with the motion of the spinning Earth we get a circulation of air which we call the weather. Hot air rises and thus vertically transports energy by convection, and the heating and cooling of the atmosphere sets up regions of high and low pressure which then generates the winds. hence distributing the hot air over the surface. Air is not a good conductor of heat (hence double glazing). Similarly water is not either. Water transports heat in a similar way to air and we get vertical and horizontal flows of water in the oceans which we call currents. These are also very important in equalising temperatures on Earth and also affect the weather (ex. Elminio) Another reason is because the Earth spins about its axis (once in 24 hours) and there is insufficient time for the temperature on the surface to reach a steady value (i.e. even thermal steady state is not reached). So the temperature has a cyclical, diurnal variation If this diurnal variation is averaged out, can we then say we have thermal steady state?. The answer is essentially yes but you might like to think about this a bit more; if so please click here.
Is the temperature of the Moon very different from that of the Earth?
The Moon is essentially at the same distance from the Sun as is the Earth so should it have the same average temperature? The answer would be "yes" were it not that the Earth's atmosphere acts as a thermal blanket which keeps the surface of the Earth warm. When there are clouds the atmosphere is an even better blanket (remember that it is colder on a clear night because heat is also lost from the surface of the Earth by radiation). However the variation of the temperature over the Moon is much greater than that of the Earth because it has no atmosphere or oceans. Also it spins about its axis much more slowly (i.e. once a month).The temperature on the hot side of the Moon is about +110oC and on the cold side about -180oC.
What has all this got to do with spacecraft in general and XMM in particular?
A spacecraft in low Earth orbit is also at a similar distance from the Sun as is the Earth and the Moon and could therefore be expected to be at much the same temperatures. It is perhaps a bit more similar to the Moon because it does not have an atmospheric blanket (indeed convection as a mode of heat transfer does not feature significantly in the thermal design of spacecraft). One difference however is the presence of the Earth itself which acts as an additional heat source. It reflects some of the Sun's radiation from the tops of the clouds back into space on the day side but it is also a warm body at about 20oC in its own right and radiates this energy into space both on the day and night side. Some of this radiation is intercepted by the spacecraft in addition to that which it receives directly from the Sun. We can therefore surmise that the thermal environment of a satellite overall is about the same or slightly warmer than that of the Earth itself. Hence there is not generally a problem with the overall power budget.
What then is the problem?
As indicated earlier in the discussion about the Moon, it is the variation of temperature which is the problem not the mean temperature. That is, how do we make the Moon look a bit more like the Earth bearing in mind that we cannot introduce an 'atmosphere' which transfers heat by convection. This leaves us with conduction and radiation as means to equalise the temperatures.
