Thursday, 21 May 2015

How cold is it in Space?

How cold is it in Space? 

Well the answer is pretty cold.  With no atmosphere to protect it, the spacecraft will experience an "outside" temperature of around -140degC.  When the sun shines on a surface of the spacecraft it can change the external temperature that the spacecraft sees to around +140degC. 

As you can imagine, these changes in temperature aren't great for electronics on board the spacecraft which generally prefer to be kept at a nice ambient temperature of around 22degC.  A bit like a mobile phone, if the spacecraft gets too cold, the systems onboard will eventually begin to fail.  Equally the same will happen if they become too hot. 

So, how can ambient temperature be maintained out in the hostile space environment?

The answer is with a thermal control system.

A thermal control system can consist of two main elements:
- passive control elements which make use of the environment and,
- active control elements which actively seek to change the spacecraft internal environment.

The usual ways of transporting energy (in this case heat) around are:
- Conduction
- Convection
- Radiation
In space we can generally only harness two of these three physical phenomena. Due to the vaccum conditions, convection is not possible in a space environment, unless a specific fluid environment to support convection is located within the spacecraft.

Examples of active elements are things like heaters.  LPF has a great number of heaters all over it.  Most are fitted to electronic units in order to control their temperature and some are also fitted to the actual structure of the spacecraft.  Each heater can be turned on and off by sending a command to it to switch it on and off to acheive the desired temperature.  In addition some spacecraft also essentially have their own central heating system. Using a series of pipes filled with a voltaile substance which easilly changes state from liquid to gas, heat can be transported around the spacecraft via convection currents, moving the warmth from hotter areas to cooler ones.  

Passive elements include items such as blankets or painted finishes.  Blankets are wrapped around particular units and more routinely, the outside of the spacecraft is covered by them - they're the shiny gold foil you see in the photograph below.  These work by insulating the spacecraft from the cold space environment.  On the Picture below you can also see some areas of white uncovered by blankets, these are painted areas, which use Special paint finishes to provide a reflective surface such that when the sun shines on this surface the radiation is reflected away.  This can be used to make sure that certain parts of the spacecraft don't get too hot when facing direct sunlight. 

LPF inside the TVAC chamber
Pictures are property of Airbus Defence & Space Ltd and reproduced here with permission

The types of finishes and blankets used vary depending on the specific aspects of the Space Mission.  Factors such as the orbit, the size of the spacecraft and the type of equipment onboard all Impact on the overall thermal system design.

Creating Space, on Earth...

The usual engineering process of design, build, and test applies to a spacecraft the same as it applies to a car or aircraft.  However, unlike a car or an aircraft, testing the item you've built in a realistic and representative environment is a little bit more tricky, after all, to do such a test, we literally need to recreate Space here on Earth!

How do you create Space on Earth?

To do this we use something called a Thermal Vacuum chamber.  This device once the spacecraft is sealed inside of it, is first used to create a vacuum environment by evacuating all of the air from the chamber.  For our recent test, we "pumped down" the chamber to around 1x10-6mbar of pressure (recall that atmospheric pressure is 1bar, so this is an air pressure of 0.0000000001bar!) once this vacuum condition is reached, the chamber is cooled to a chilly -140deg using liquid nitrogen.  Prior to cooling the chamber, the spacecraft is powered on and the test team begin to activate the various heaters and monitor the temperatures of all the units to ensure they remain within their operating limits. 

The TVAC (Thermal Vacuum test) was the first test that LISA Pathfinder under went on arrival in Germany.

LPF Installed in the TVAC Chamber ahead of the test
Pictures are property of Airbus Defence and Space Ltd, reproduced here with permission

LPF fully installed in the TVAC chamber
Pictures are property of Airbus Defence and Space Ltd, reproduced here with permission

One final look before closing the door for the 8 day test
Pictures are property of Airbus Defence and Space Ltd, reproduced here with permission

The door is closed and Space Simulation can begin
Pictures are property of Airbus Defence and Space Ltd, reproduced here with permission

LPF is removed from the chamber 8 days later following the test, time for a Team photoshoot!
Pictures are property of Airbus Defence and Space Ltd, reproduced here with permission