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Messenger Spacecraft Prepared for Mercury
An anonymous reader writes "NASA's first orbiter to the planet Mercury is shown today in cut-away, revealing the parasol design that will protect it from intense heat. Twenty layers of aluminized Kapton will be its sunshade. Curiously since the innermost planet is so close to the Sun, the Mercury mission itself will look for (cometary) water-ice preserved on the less baked north pole."
Being the closest planet to the Sun you would expect Mercury to be the hottest but this is not true. Mercurys maximum temperature falls 50C short of that of Venus. The reason for this is that Mercury has very little atmosphere so there is no 'greenhouse' effect on the environment. The 430C daytime temperature is dictated purely by the Suns radiation. The Mercurian day is 176 terrestrial days long, the night is 88 terrestrial days long with a minimum temperature of -180C.
Be very, very careful what you put into that head, because you will never, ever get it out. -Thomas Cardinal Wolsey
How can you keep the spacecraft at room temperature if everything around it is at least 212F? I need to get some of those fans for my computer.
It would be amazing if it was true that everything around the spacecraft was at 100C. But the side which doesn't face the sun A) doesn't need the sun shields, and B) sees the cold vacuum of space, a great place to passivly radiate unwanted heat to.
Check out this page from the MESSENGER site showing the sun shields only on the side facing the sun.
--Xandu
Here's the key concepts: Stefan's Law states that the net power of radiated heat absorbed is proportional to the surface area, the emmissivity (black = 1, white = 0), and the temperature difference to the fourth power (T_you ^ 4 - T_them ^ 4). Since them is the Sun in one case, you aren't going to win so you put up the Parasol to block off the Sun. In the second case, them is just space, so all you have to do is adjust your emmissivity to have the power created by your electronic components equal the power radiated into space.
You're thinking of a solar sail. Ion drives derive thrust directly from the force of the escaping gas (lightweight but high energy), generally xenon.
Trying to ride the 'wind' from your own ion drive is very similar to trying to windsurf by blowing into your own sail -- or, to use a more familiar analogy, pulling one's self up by one's own bootstraps.
Conduction and convection are not going to work in a vacuum, but radiation works just fine. This is electromagnetic radiation, like light and radio waves, so it does not need a medium.
Mea navis aericumbens anguillis abundat
For comparison purposes the best Isp from a chemical rocket system in use is pretty much Lox/H2 which gives you an Ispvac in the 460s range.
More info here: http://nmp.jpl.nasa.gov/ds1/tech/ionpropfaq.html
And yes, I am a rocket scientist.
It's worth mentioning that although it's true that Mercury is tidal-locked with the sun, it's in a 3:2 lock, not 1:1.
This means that it does rotate relative to the sun, so there's no permanent "dark side".
(For comparison, the moon is tidal-locked 1:1 with Earth, so we never see the far side.)
Actually, there is no heatsink . . . space may be cold, but it also has no heat capacity. Heatsinks rely on conduction (which requires heat capacity). Stefan's law states that radiating heat in the form of electromagnetic radiation has nothing to do with the ambient temperature, only the temperature of the radiant object.
Let me define a few things real quick
It=total Impulse=Thrust*Time [N-s]
F=Thrust [N]
t= time [s]
Mp=Propellant Mass [kg]
dMp/dt=Propllant mass flow rate [kg/s]
Wp=Propellant weight [N]
Isp=It/Wp=F*t/(Mp*g)
which if you solve for F in terms of Isp you get:
F=Isp*Mp*g/t
Then, you have Newton's law: F=dP/dt=d(mv)/dt which for a constat exit velocity you get:
F=Ve*dMp/dt
which for a constant mass flow can be written as:
F=Ve*Mp/t
Setting the two equations for thrust together you get:
F=Isp*Mp*g/t=Ve*Mp/t
which if you cancel out the Mp/t on both sides of the equation you get that:
Ve=Isp*g.
One of the real nice things about using Isp is that it's one of the few things that is the same in both EE and metric because it's units are seconds. For more info on this I recomend Chapter 2 from 'Rocket Propulsion Elements' by George Sutton and for a more advanced look at this stuff check out 'Space Propulsion Analysis and Design' By Humble, Henry and Larson. Both books can be purchased through Amazon or other large book sellers.