Hello everyone, I am new to here.

I have a question:
I wonder why MRO's HGA is wrapped with a layer of metal (aluminium?), whcih is the same as Mars Odyssey's. I can't find the answer on MRO official site, wikipedia or google. Could anyone help?

This is the so-called "radome" that protects the antenna from heating during aerobraking. MGS had one too. It's made out of aluminized kapton.

Close, but not quite. It's *germanium* coated kapton (aluminum would degrade the RF signal, but germanium is RF-transparent, at least at X-band). Also, the radome isn't there for aerobraking per se; during aerobraking, the heating was on the back side of the HGA (while the radome is on the front). The radome primarily exists to keep the HGA at an even temperature across the dish. Otherwise, since the HGA is normally pointed at the Earth (vs. the Sun), uneven solar illumination would cause the dish to warp a bit, messing up its shape and reducing the HGA's performance.

It's my understanding that this level of thermal control is really just needed for big dishes...

Lorne

On the first one, fair enough. On the second, what's your source? On MGS it was definitely for aerobraking, because the MGS HGA was just the spare one from Mars Observer, and the MO one didn't have a radome. See http://mars.jpl.nasa.gov/mgs/movpics/atlo_...al/arrival.html -- "The radome over the high gain antenna is to protect it from the aerobraking environment."

I concur with Lorne Ipsum. The backside was use for aerobraking. The front is covered for solar heating

On MGS, the main issue as I recall was loss of attitude reference during aerobraking, so that the spacecraft couldn't tell which direction the aeroflux would be from and needed the radome to avoid heating. I'd be surprised if the MRO situation were any different, given the amount of thermal analysis and design we had to do for this case for CTX.