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New Horizons’ first hi-res imagery of Ultima Thule

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Re: New Horizons’ first hi-res imagery of Ultima Thule

#141

Post by lisper »

kamaal wrote: post #130
lisper wrote: post #44

Earlier quoted context omitted.

No, that's actually not true. Hitting Ultima-Thule is much harder. For starters, just figuring out where U-T actually is is very difficult. The observations we have only constrain two of its three degrees of orbital freedom. The third has to be inferred via orbital mechanics. Likewise, figuring out where the spacecraft is is also non-trivial. By way of contrast, figuring out where you are relative to New York to with…

>>I know these things because I used to work at JPL. There are entire teams dedicated to spacecraft navigation. The stuff they do will blow your mind. How do we get to work with these teams, if even possible? (Though it should be impossible for me(given I'm an Indian), still how do someone who could work there go about joining them)

Start here:

https://www.jpl.nasa.gov/opportunities/

(BTW, in this day and age if you can't figure out the answer to a question like that on your own, you're not ready.)

Re: New Horizons’ first hi-res imagery of Ultima Thule

#142

Post by vidarh »

karmakaze wrote: post #98
dekhn wrote: post #25

Earlier quoted context omitted.

the development of double precision floating point, and the kalman algorithm, were two major improvements. Basically there is a computer at JPL that, given a location in the solar system and a current location of a ship, will give you instructions on when to burn. After you burn fuel to move, you update based on visual locations of known stars, fit a model, and do incremental burns until convergence. The book Digital…

> the development of double precision floating point So refreshing to hear that a double is good enough for JPL when repeatedly told it's not good enough for mere dollars and cents.

The problem with floating point for money is that simple calculations can give different answers than you would like because of how numbers are represented. E.g in a JS console:

    > 0.1 * 0.1
This matters whenever you do things where the error might be allowed to accumulate and you're not careful to control for it. The general advice to avoid floating point for money is not because it's impossible to do correctly, but because it's very easy to get wrong in ways that are hard to discover with testing, and doing it with money is one of those areas where it's easy to get it wrong in ways that people will care about (because you're suddenly paying the wrong amount of tax, for example).

I don't know about you, but I don't trust myself to get this right, much less developers who often don't understand the issues involved.

Re: New Horizons’ first hi-res imagery of Ultima Thule

#143

Post by garmaine »

lisper wrote: post #140
garmaine wrote: post #93

Earlier quoted context omitted.

We have one--stellar navigation. It's what they use.

Not exactly. You can use star location to determine a spacecraft's orientation (and they do), but its location is determined by measuring the doppler shift of the carrier signal sent from the spacecraft back to earth and running those results through some pretty hairy math. GPS works by having a network of beacons in known locations transmitting signals with known timing. I just learned from a sibling comment in this…

Parallax. (And not stars, but other solar system objects.)

Re: New Horizons’ first hi-res imagery of Ultima Thule

#144

Post by lisper »

dwc wrote: post #136
lisper wrote: post #33

Earlier quoted context omitted.

> photos of a 20 mile object whilst flying past at ~8 miles/sec Not to detract at all from the astonishing technical achievement, but that's the wrong comparison. You can see the ISS with your naked eye, and it's a Actually, the right comparison is of the closest approach distance (~2000 miles) with the distance from earth (4,000,000,000 miles). That's like launching a missile from Los Angeles to New York and hitting…

> The right comparison is with the distance at which this photo was taken: 18,000 miles, or about half an hour from closest approach. Implicit in this statement: the camera was pointing close to the direction of travel and the target wasn't moving much within the field of view. Hugely easier than trying to image from the side at closest approach, which at best would have given a smeared image and at worst a complete…

> Hugely easier than trying to image from the side at closest approach

True that. AFAIK the closest-approach images have not yet been downloaded.

Re: New Horizons’ first hi-res imagery of Ultima Thule

#145

Post by PaulHoule »

lisper wrote: post #139
rbanffy wrote: post #134

Earlier quoted context omitted.

They plan to get within 1km, which is quite impressive for one system alone. Thanks for the pointer (after reading it, I remember having read about NICER, but didn't remember the precision they got)

> quite impressive Especially when you consider how far away the "beacons" are.

The far away beacons make the math easier. Also those beacons are awfully bright.

Re: New Horizons’ first hi-res imagery of Ultima Thule

#146

Post by lisper »

garmaine wrote: post #143
lisper wrote: post #140

Earlier quoted context omitted.

Not exactly. You can use star location to determine a spacecraft's orientation (and they do), but its location is determined by measuring the doppler shift of the carrier signal sent from the spacecraft back to earth and running those results through some pretty hairy math. GPS works by having a network of beacons in known locations transmitting signals with known timing. I just learned from a sibling comment in this…

Parallax. (And not stars, but other solar system objects.)

You'd think, but no, that's not actually how it's done. See e.g.:

http://dnc.tamu.edu/drjunkins/yearwise/2000/conference/AIAA_...

https://core.ac.uk/download/pdf/9069467.pdf

for some state-of-the-art research on this topic.

Re: New Horizons’ first hi-res imagery of Ultima Thule

#148

Post by dekhn »

karmakaze wrote: post #98
dekhn wrote: post #25

Earlier quoted context omitted.

the development of double precision floating point, and the kalman algorithm, were two major improvements. Basically there is a computer at JPL that, given a location in the solar system and a current location of a ship, will give you instructions on when to burn. After you burn fuel to move, you update based on visual locations of known stars, fit a model, and do incremental burns until convergence. The book Digital…

> the development of double precision floating point So refreshing to hear that a double is good enough for JPL when repeatedly told it's not good enough for mere dollars and cents.

floating point was designed for scientific and numerical/math applications (written by people with numerical analysis skills), not money. There are decimal data types which are much better for money management.

Re: New Horizons’ first hi-res imagery of Ultima Thule

#149

Post by dekhn »

perl4ever wrote: post #101
karmakaze wrote: post #98

Earlier quoted context omitted.

> the development of double precision floating point So refreshing to hear that a double is good enough for JPL when repeatedly told it's not good enough for mere dollars and cents.

New Horizons is a mere few billion miles/km away, while financial matters these days sometimes involve trillions . Plus extra digits for cents and fractions thereof, of course. “There are 10^11 stars in the galaxy. That used to be a huge number. But it’s only a hundred billion. It’s less than the national deficit! We used to call them astronomical numbers. Now we should call them economical numbers.” - Richard Feynma…

i know you're joking but it's really the precision at large numbers that makes doubles so valuable. Typically when somebody is steering a spacecraft to a remote location, the large distance as well as the fine precision needs to be represented. I am not aware of people working with very large sums of money who also need penny-level precision (and even then, floats aren't the right solution).

Re: New Horizons’ first hi-res imagery of Ultima Thule

#150

Post by garmaine »

lisper wrote: post #146
garmaine wrote: post #143

Earlier quoted context omitted.

Parallax. (And not stars, but other solar system objects.)

You'd think, but no, that's not actually how it's done. See e.g.: http://dnc.tamu.edu/drjunkins/yearwise/2000/conference/AIAA_... https://core.ac.uk/download/pdf/9069467.pdf for some state-of-the-art research on this topic.

Actually, in the case of New Horizons, it is. Mission navigators use LORRI images of known objects relative to the background stars to calibrate their position.
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