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Brad Fitzpatrick

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More Mars Craziness [Jan. 4th, 2004|11:43 pm]
Brad Fitzpatrick
To land in a precise location on Mars after traveling over 300 million miles, navigators at the Jet Propulsion Laboratory (JPL) had to overcome the head-spinning challenges of calculating the exact speeds of a rotating Earth, a rotating Mars, and a rotating spacecraft, while they all simultaneously are spinning in their own radical orbits around the Sun.


, but even the seemingly insignificant solar radiation pressure and thermal radiation forces acting on the spacecraft to a level equal to less than a billionth of the acceleration of gravity one feels on the Earth need to be taken into account. Without knowing the acceleration error to that degree, the spacecraft would have moved off course by 3.7 km (2.3 miles) over 10 days.


"We had to know everything from how the iron molten lava in the center of the Earth was churning to how plate tectonic movements were affecting the wobble of the Earth to how the plasma in the atmosphere delayed the radio signals to and from the Deep Space Network stations,


Three DSN sites are roughly equally spread around Earth's globe at 120-degree intervals, so that antennas are pointed toward Mars at any given time as the Earth turns. If the exact location of any of these antennas is incorrect by just 5 centimeters (2 inches) on the surface of Earth, that math error builds over the 150 million kilometers (90 million miles) distance between Earth and Mars, creating a 1500-foot (0.3-mile) location error at Mars.


The navigation team also used a tongue-tying tracking technique called spacecraft-quasar delta differential one-way range or DDOR (pronounced "Delta Door"), which utilized their knowledge of locations of quasars to a few billionths of a degree to help locate the spacecraft's motion in the "up or down" direction in the sky.

[User Picture]From: jwz
2004-01-05 07:52 am (UTC)
Wow, that's insane!

I suppose there's a reason (and I suppose it's fuel), but wouldn't you assume it'd be easier to just get it going in the general direction of mars, and correct along the way?
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[User Picture]From: brad
2004-01-05 07:59 am (UTC)
They did correct a few times during its flight. And they skipped the last two scheduled corrections because their previous ones were so good.

Even if fuel wasn't an issue (and it might not be -- they generate a lot of power enroute), I think the larger problem is that course corrections aren't deterministic. (which is why the AI community now realizes a computer beating a human at chess isn't interesting)

Even if they corrected often, they can only really correct if they know where the damn thing is at to begin with, and the further away it gets, the lesser accurate their readings get. Hence the DSN antennas being off by inches, and plasma in the atmosphere and magma in the earth: they all interfere with knowing where it's at.

And I suppose to a lesser degree latency makes frequent course corrections difficult. You can't just "drive" it real-time.
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[User Picture]From: d4b
2004-01-05 08:11 am (UTC)
All true, if the pilots are on the Earth. But wouldn't it make sense to put some of the decision-making power on the ship directly? I mean, a couple miles off one way or the other shouldn't be a huge problem if the ship has a goal-seeking algorithm along the lines of, "You see that big red ball straight in front of you...?"

Sorry if I'm being over-simplistic. But we have been navigating in space for over four decades, (from long before being able to measure some of those subtleties); it's not rocket science. (Sorry; I just couldn't resist.)
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[User Picture]From: brad
2004-01-05 09:21 am (UTC)
Reads like rocket science to me.

I'm sure "follow the big red dot" has field problems you've not anticipated.
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[User Picture]From: joshc
2004-01-05 07:53 am (UTC)
I'm glad that someone else finds this Mars mission exciting.
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[User Picture]From: matte
2004-01-05 08:07 am (UTC)
it is actual rocket science!

thank you for the link earlier today. i have found myself re-loading all day to get more info. even found myself cursing my cable provider and all 200+ channels tonite because none of them carried the press conference at 9pm pst tonite!
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[User Picture]From: ydna
2004-01-05 08:24 am (UTC)
I love the mixture of imperial and metric measures. Those rocket scientists are always flirting with disaster.
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[User Picture]From: d4b
2004-01-05 10:26 am (UTC)
Flirting? Isn't that what caused the total loss of one of the earlier missions?
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From: jon
2004-01-05 08:33 am (UTC)
"Measure twice, cut once."
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[User Picture]From: rivulet
2004-01-05 02:38 pm (UTC)
NOVA was on OPB last night and talked about their trials and errors with the airbags and the parachute and stuff - pretty interesting stuff. They showed footange from 1/3 as well, when it landed. They should have another show about it on Tuesday.
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[User Picture]From: bostonsteamer
2004-01-05 05:41 pm (UTC)

thought you'd enjoy

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From: (Anonymous)
2004-01-08 03:41 am (UTC)

Quasar Queeziness

Prof. H.C. Arp says quasars are closer by (http://www.mpa-garching.mpg.de/HIGHLIGHT/2001/highlight0108_e.html) than NASA thinks,so using them for "up and down" navigation in space could conceivably throw one or both probes off-course if they relied on them.

In his book "Seeing Red" (Apeiron Press) Prof. Arp tells his story of how he was thrown out of the U.S. observatory system for sticking to his guns for many years saying this. Prof Geoff Burbidge and mom Margaret tell other astronomers they need to listen to him but so far their advice has fallen on deaf ears. Maybe NASA needs to listen , too, before they lose another probe. Did Beagle 2 use the "Delta -Dor" system too? I dunno, but ...?

Regards from the NSS A FOP (National Space Society Atlanta Far-Out Physics) Commmittee
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