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Pluto Probe Delayed
setirw writes "Due to high wind conditions at the launching site, the launch of the NASA's probe to Pluto has been delayed for 24 hours. "The wind limit at the pad is 33 knots [and] we have exceeded that limit several times today," said NASA spokesman Bruce Buckingham." From the article: "Glitches with an Atlas 5 vent valve, a ground tracking station in Antigua and NASA's Deep Space Network also led to launch delays, though the wind concerns were omnipresent throughout those issues."
Wind limits on rocket lauches are a combination of several things, just as most complex engineering problems.
The structural forces placed on the structure from side winds are negligible when compared to the acceleration forces due to lauch.
For the most part, it's not the final trajectory of the payload that sets them, as secondary burns and mid-course corrections are more than adequate to correct any small variation in the launch vector.
More importantly is stablity of the rocket under side forces. Because of a rocket's tall slender build, the center of mass is far away from point of thrust (the engine nozzles). Any small horizontal motion of the center of mass with respect to the point of thrust can quickly lead to tumbling. It's this control problem that really determines the launch limits. A rocket is inherently unstable and requires dynamic control, typically small engines around the periphery of the main nozzle that can swivel to provide righting moment. As with any real control system there are limits to the perturbation it can handle, and this is translated into wind speed limits.
They want it to explore the Kuiper belt afterwards, so they would have to enter orbit and then leave it and build up the speed again, which would take a lot of fuel.
It will be in the vicinity of Pluto for longer than a day - it won't be very close for that time, but it will be more than close enough for the work they want to do.
The launch has been scrubbed for Wednesday as well, because of a power outage in Maryland at the New Horizons control center
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(see http://www.floridatoday.com/floridatoday/blogs/pl
Quoth the parent:
In brief, yes.
(Warning: back-of-the-envelope calculations follow)
In order to reach Pluto in a reasonable number of years, the probe must move very fast. Let's see ...
very roughly, it goes 40 AU in nine years. That's about
4.5 AU per year, or a cruising speed of about 21,000 m/s.
If you wanted to put it in orbit around Pluto, you'd have to decrease its speed to the orbital speed of Pluto, which would be a few hundred m/s. That means you'd have to decrease the speed by roughly 20,000 m/s ... or, to a good
approximation, you'd have to remove all the velocity you
had added to the probe in the first place. The mass of
the probe is roughly 1,000 kg, so its momentum must
be decreased from about (20,000 m/s) * (1,000 kg) = about
20 million kg*m/s to zero.
To reduce the momentum, you fire an engine pointing backwards: the engine throws exhaust products forward at some speed, and the momentum they carry away reduces the remaining momentum of the probe. Chemical rockets have typical exhaust speeds of around 2,000 m/s, so to remove 20 million kg*m/s, you'd have to throw around 10,000 kg of mass out of your engine. (Yes, yes, it's more complicated than this, but for the purpose of illustration, it's close enough).
But, wait a minute: the probe's mass is only 1,000 kg. It can't carry 10,000 kg of fuel and oxidizer, too. So it cannot slow itself down enough to enter orbit around Pluto. If you wanted to design a probe which could enter orbit, you'd have to make it carry huge amounts of fuel for this burn when it reaches Pluto ... but then
you'd need an enormous rocket to accelerate the
fuel and probe to 20,000 m/s in the first place.
It just isn't practical. Sorry.
Michael Richmond "This is the heart that broke my finger."
mwrsps@rit.edu http://stupendous.rit.edu