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Comment by lisper | Hacker News Reader

> 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 <1 mile object flying past at 4.8 miles/sec. The right comparison is with the distance at which this photo was taken: 18,000 miles, or about half an hour from closest approach.

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 your target to within a meter.

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To be fair, while that's an accurate representation of the distances, it's not accurate of the difficulty. They don't have to deal with unpredictable atmosphere. There's reasons laser-guided missiles are about 1-2 meters-ish of accuracy with constant guidance, while we can basically fire-and-forget spacecraft to 4 billion miles away.

Spacecraft also do some corse corrections. You can also measure where your going very accurately averaging over a few hours.

They do have to deal with accurately determining the target destination, especially so in this case.

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 within a meter of accuracy is, nowadays, a simple matter of switching on your GPS receiver. (Even without GPS, all you need is inertial guidance that is stable for half an hour or so. That's much easier to achieve.)

Finally, there are unpredictable orbital disturbances. N-H uses thrusters for attitude control, whose effect on the trajectory is not entirely predictable. (c.f. https://en.wikipedia.org/wiki/Pioneer_anomaly, which was only detectable because, although Pioneer had thrusters, they were turned off for long periods of time).

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.

I wonder how much would it cost to build a GPS-like network for the whole solar-system...

The good thing is that, for such a thing to be really useful, we'd have to invent better propulsion systems which would, in turn, make the network much cheaper to build.

Id guess it could be done for not that much, assuming accuracy can go from a few meters to few thousands of miles. (Someone more versed might be able to give a better oom on accuracy). Just need to launch 4(+) satalites, 2 up and 2 down relative to the orbital plane. So long as a ship isn't hiding behind a planet/sun it should have LOS to all 4 for triangulation.

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 thread that they're working on using pulsars as the beacons. But building an artificial GPS system for the solar system would require sending out a network of satellites into solar orbit. That might be technically feasible, but not economically feasible.

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.

OK, I stand corrected.

Pulsars can be used to arrive at celestial positioning accurate to ±5 km. There is a test rig on the ISS that has validated this. No need to build human made beacons when the universe has so graciously provided us with them.

https://en.m.wikipedia.org/wiki/X-ray_pulsar-based_navigatio...

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.

> two of its three degrees of orbital freedom

Three?

> I know these things because I used to work at JPL

Ok, I don't. What are the three DoFs and how do they relate to 6DoFs or six Keplerian elements? Some background-fixing?

Brain wedgie. I meant six. And direct observations give you four of the six. I guess I was just thinking about 3-d location and just forgot about the time derivatives. Sorry.

Thanks for the link, didn't know the pioneer anomaly had a conclusion.

>>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.)

"They don't have to deal with unpredictable atmosphere"

Well, no, but they do have to deal with an atmosphere that has, until they get to it, only been theorized about, never observed - which is not the case for the atmosphere above New York.

There have been precisely four spacecraft that have gone anywhere near so far out as New Horizons, and they are between them the source for the majority of data we have about the atmosphere in the outer solar system. One of them discovered an anomalous acceleration effect that confused scientists for decades (and which, while it turns out not to have been externally caused, certainly left scientists wondering for a while whether their model for the solar wind was accurate). Of those that we believe have passed through the termination shock of the solar atmosphere, none have yet returned much accurate information, so we actually don't know much about the conditions there. I don't mean to say that these effects have meaningfully impacted navigation for deep space probes, but more these are genuine voyages of discovery - where they're going is predictable but until they get there we really don't necessarily know whether our predictions will be accurate.

I believe New Horizons did course corrections, even fairly late.

> 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 miss.

It's still impressive and awesome. But always try to skew the odds of success in your favor when dealing with stuff like this. You have one pass and then the opportunity is gone.

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

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

>>That's like launching a missile from Los Angeles to New York and hitting your target to within a meter.

As long you have navigation(GPS) and fuel(to make adjustments), that shouldn't be a problem?

But of course you don't have a map in space, so how do you navigate, and course correct etc, is another big puzzle in itself.

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