[AR] Re: Cubesats orbital grenades ?
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 11 Dec 2017 12:23:56 -0500 (EST)
On Mon, 11 Dec 2017, Galejs, Robert - 1007 - MITLL wrote:
One obvious counter-example: If you are trying to get high resolution
ground images from space, you need optical apertures larger than
cubesats can handle . From 300 km altitude at nadir, you'd need a 2 ft
diameter circular aperture for 1 ft resolution at 550 nm. That's hard
to fit on a cube sat...
In principle that can be overcome using formation flying and imaging
interferometry, but that is *MUCH* easier said than done, especially in
visible wavelengths. (The astronomers have enough trouble making it work
in near-IR -- where the longer wavelengths make everything easier -- with
all the telescopes bolted firmly to solid rock.)
And then square and cube that if you're also trying to look at something
relatively faint, i.e. looking up instead of down. Imaging interferometry
needs enough photons per millisecond to form detectable interference
fringes, so it pretty much works only on bright sources. For dim stuff,
you've got to gather photons the old-fashioned way if you want results.
Especially if you want to do spectroscopy or zillion-pixel imaging: the
finer you slice the photons, the more of them you need.
Our BRITE astronomy nanosats do good astrophysics, but the small apertures
do limit them to working only on fairly bright stars; our first satellite,
MOST, did better but at the cost of being a 53kg microsat. (We could make
it smaller now, but I don't know if it could go into a 12U cubesat, and
just its telescope alone is rather bigger than a 3U.)
Sometimes there is no substitute for aperture. And that applies to radio
too -- when working with faint signals or long distances or high data
rates, you quickly find yourself needing dishes or long helixes or
whatever on the satellite, not just little omni antennas. And those are
sized by the radio wavelength and can't shrink without losing performance.
Henry
Other related posts:
