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Comment by aklein | Hacker News Reader

The numerical accuracy and calculations needed for getting the spacecraft so close to Pluto must be pretty awesome. Does anyone know what the precision is on calculations like these?

Also, anyone know why the spacecraft has to do a flyby, as opposed to, say, going into orbit around Pluto? Is it because the fuel involved in slowing down the spacecraft would be forbiddingly heavy?

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Nobody is quite answering your question about going into orbit. A simple hohmann transfer would be able to get into orbit without expending very much fuel at all at Pluto. This talk of it going too fast to slow down is specifically because they didn't do a hohmann transfer.

Pluto is really far away, and the way a hohmann transfer works, the craft ends up moving very slow relative to the destination when it gets there. I haven't done the math myself but read it would take somewhere around 35 years to do it. But the thing is, it could have been done without being very expensive. That is, unless the nuclear generator doesn't last long enough. And nobody wanted to wait 35 years so instead they got going really fast and got an additional boost from Jupiter.

So the real answer is, they're going too fast because they wanted to get there in "only" a decade.

(edited down to 35 years)

Yes, basically slowing down enough to orbit Pluto would have required too much extra launch weight, although the difference isn't enormous. The best candidate, a nuclear-electric propulsion system, would have approximately doubled the spacecraft's mass. This Stack Exchange question has more information: http://space.stackexchange.com/questions/9851/requirements-t...

Nice discussion, thanks for posting.

Additionally, from the linked paper [1], it would have required a 15 year transfer phase and a more capable launcher than the Atlas V.

The report proposed using a then-unavailable Ariane 5 variant, which would have delayed the launch to 2016 (to line up the gravity assist with Jupiter) and put the final encounter with Pluto at June 2033.

[1] http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Plut...

So basically, if we can build a new probe within the next year and a half, and if this new Arianne variant is now available, we might have a chance of sending an orbiter to reach Pluto by 2033? That sounds like an opportunity that's too good to pass up if we can pull it off.

I mean, the flyby's awesome, and it's bound to give us all sorts of new insights on the formation of the Solar System, but having a permanent orbiter there would let us continually study Pluto (and its moons), giving us that degree of information on a continuous basis.

>if we can build a new probe within the next year and a half

The amount of cost and level of precision and robustness required tends to mean that these probes take years to plan, design, build, and test. It's also far from clear that the value of the science of sending an orbiter to Pluto would outweigh other potential missions (that also wouldn't have the same timing constraints.)

Sure, though it sounds like the ESA's been planning such a probe for quite some time already. The linked report even has specifics on instrumentation payload and the mass thereof, and includes a rendering and parts list for the proposed "POP" probe. We'd just need to do the "build" and "test" parts, and while that likely would normally take a couple years, I reckon we could cram that into a year and a half with enough effort.

I am wondering if you could use some sort of atmosphere breaking? The equivalent of a large parachute?

Until this mission (and perhaps even now) we did not/do not have enough information about the atmosphere of Pluto to design the parachute.

I think we knew there was an atmosphere on pluto before this, but I was more thinking about a new mission.

Knowing there is an atmosphere is not the same as knowing the exact makeup, in particular density and pressure.

Looks like a great proposal, but I don't see the point of preparing such a mission before we get all the data back from New Horizons. Once we have the latest data, then we'll have a much better idea about the kinds of questions that we would like an orbiter probe to answer. Already even with just about 2% of the NH data back, it turns out Pluto is radically different than most planetary scientists predicted. On the basis of the full data set, we'll have a much better idea of the types of instruments we'd need an orbiter to have in order to help solve these mysteries. So while the linked paper is a very useful proof of concept for an orbiter mission, and was in no way a waste of effort, actually building and committing to it and a launch in 2016 would have been a mistake.

> The numerical accuracy and calculations needed for getting the spacecraft so close to Pluto must be pretty awesome.

You can actually get by with relatively mediocre precision since the craft can be remotely piloted and its trajectory adjusted as needed. NASA was sending corrections to New Horizons just hours before its periapsis to Pluto.

It would be very different if we launched a satellite and then couldn't alter its trajectory in any way (and it would probably impossible to get it to do anything interesting since even the presence of just three bodies can lead to a chaotic system).

Source on that? Even the Tajectory Correction Maneuvers (TCMs) are planned in advance, and it's just the details of the TCM that are finalized in the lead-up to those events.

As far as the final approach goes, the core sequence started July 7[1], and no adjustments were made to it. They even had a TCM opportunity before that in case of a possible collision, that I heard was skipped.

[1] http://www.spaceflight101.com/new-horizons-encounter-timelin...

Good question about the precision; we only hear little snippets about hitting virtual windows in space within 100km or so. New Horizons is running a MIPS-1 R3000 processor[0], that has double-precision floating point available but presumably fixed-point arithmetic could be more suitable here. But I don't know that there would be so much accumulated error, there were a few course corrections between Earth and Pluto. Maybe the precision is not so much of an issue?

As for the fly-by, well. Speed is a problem. At the point of closest approach to Pluto it was going at 13.78 km/s, and it's about the mass of a grand piano[1] so that's about 45TJ of kinetic energy. Or some large number anyway. Let's just carry on with Newtonian stuff and assume the numbers get worse if you do it properly. At current speed its orbital radius[2] would be about ... 4600 metres. Three miles!

That's somewhere deep within the icy core, I'm guessing.

In order to orbit at the distance of the photos we're getting now, it would need to slow right down to 264m/s, shedding basically all of that kinetic energy. Back of the envelope numbers suggest that would require about the equivalent of a 90 megaton explosion! Of course, if we strapped the Tsar Bomba[3] onto New Horizons, there would be more mass, so we'd need more stopping power ... and so on.

Other reasons for doing a flyby, I suspect, is that we get to see more stuff this way. There are a few moons, and an uncharted Kuiper Belt. Seems a shame to come all this way and only see the one thing. Not to mention the allure of the tantalising glimpse ... always leave them wanting more! This mission started with Pluto being nothing but a blurry dot occupying a dozen pixels. Imagine what the next mission can do, based on what we're already learning! Iterate and improve.

[0] http://blog.imgtec.com/mips-processors/mips-goes-to-pluto [1] https://what-if.xkcd.com/137/ [2] http://www.physicsclassroom.com/class/circles/Lesson-4/Mathe... [3] http://nuclearweaponarchive.org/Russia/TsarBomba.html

I assume the spacecraft carried navigation sensors to let it control its route in a closed loop. That's how you can go however far you want without requiring unbounded precision in your initial calculations.

I'll repost an answer I gave elsewhere[1]:

> See the "Guidance and Control" and "Communications" section of the NH > Spacecraft Systems page for a detailed answer. > > The short version is that it uses a combination of star trackers and IMUs > (Inertial Measurement Units). The star trackers analyze pictures of the > surrounding star field to determine how it is pointing instantaneously, and > the IMUs track how it is rotating in between each of those instants. This > determines the attitude (which way it is pointing). > > For position determination, "ranging" tones are sent from the earth and > echoed back by the craft. This combined with the angle that the dish is > pointing at to get the strongest signal tells the operators where the craft > is in space. This information is fed back to the craft, which has an on- > board physics simulation, and predicts where it will be until the next > ranging event. > > Now, you might have noticed that I didn't mention Pluto once. That is > because this system (minus the exact details) is used by pretty much every > spacecraft, from those around Earth, to New Horizons, and beyond.

[1] https://www.reddit.com/r/space/comments/3cz66a/new_horizons_...

>it would need to slow right down to 264m/s, shedding basically all of that kinetic energy. Back of the envelope numbers suggest that would require about the equivalent of a 90 megaton explosion!

seems like you're several orders of magnitude off. Delta-v of 14km/s for a piano would require on the order of 90 tons of fuel. Delivering of those 90 tons to Pluto (ie. delta-v of 14km/s for 90 tons - that's 9000 tons, still far from Tzar bomba, though close to Fat Man/ Little Boy)

Yes. To get there in less than ten years, it had to be low mass, since a high mass spacecraft would require a launch vehicle we don't have. Since it is low mass, it can't take enough fuel to slow down. Thus, a flyby. Getting the Cassini spacecraft into orbit around Saturn was quite a feat. Pluto is even harder.

> Is it because the fuel involved in slowing down the spacecraft would be forbiddingly heavy?

Yep. It was traveling about 31,000 miles an hour relative to Pluto when it flew by. You'd have to decelerate to about 3,000 miles an hour to get under Pluto's escape velocity - that's a lot of speed to bleed off.

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