What-if 0082: "Hitting a Comet"

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What-if 0082: "Hitting a Comet"

Postby ucim » Thu Feb 06, 2014 2:46 am UTC

Hitting a comet
what-if wrote:Would you rather bet a million dollars on a spacecraft landing engineer's ability to successfully perform eye surgery, or an eye surgeon's ability to land a probe on a comet?

I'm a pilot, and my friend is a sailor. We have this idea that he'd sail to Montauk Point, I'd fly there, and then we'd trade keys.

Bets on who makes it home first, and who dies.

Jose
Last edited by ucim on Thu Feb 06, 2014 3:17 am UTC, edited 1 time in total.
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Re: What-if 0082: Hitting a Comet

Postby Jeffrey3527 » Thu Feb 06, 2014 3:09 am UTC

It's pretty obvious an eye surgeon has a better chance to land a probe on a comet. While the eye surgeon has Kerbal Space Program to practice and simulate his experience, all a spacecraft engineer would have at hand is Surgeon Simulator. Sure, KSP doesn't have comets to land on (Yet) and SS doesn't have Laser Eye Surgery, but what he learns in KSP is far more usable than what SS can provide.

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Re: What-if 0082: "Hitting a Comet"

Postby rjsteg » Thu Feb 06, 2014 3:54 am UTC

OK. I'll bite.

Why is it impossible to throw (launch) a baseball from NYC to San Fran?

Is it because it will burn up from reentry?

Curious,
Randy

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Re: What-if 0082: "Hitting a Comet"

Postby ucim » Thu Feb 06, 2014 4:40 am UTC

rjsteg wrote:Is it because it will burn up from reentry?
No, it will burn up as soon as it leaves the pitcher's hand. And it needs to leave the pitcher's hand with enough velocity to overcome air friction, as unlike a rocket, it stops being propelled right away.

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Re: What-if 0082: "Hitting a Comet"

Postby taemyr » Thu Feb 06, 2014 4:44 am UTC

rjsteg wrote:OK. I'll bite.

Why is it impossible to throw (launch) a baseball from NYC to San Fran?

Is it because it will burn up from reentry?

Curious,
Randy


Randal has described a method of estimating how far an object can potentially travel trough any given medium, originally given by Newton iirc. Unfortunately I don't remember where.

The gist of it is; for an object traveling faster than the speed of sound in a medium, in order to penetrate into the medium it will have to push the particles of the medium forward. This transfers momentum from the object into the medium. And, when the object have travelled a distance equal to it's own length, multiplied by the density of the object, divided by the density of the medium the momentum transfered will be equal to the momentum the object used to have.

The above breaks down when an object travels slower than the speed of sound, but it means that there is an upper limit to how far an object can travel if it's not acellerating in flight.

Edit: Wikipedia link: http://en.wikipedia.org/wiki/Impact_depth

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Re: What-if 0082: "Hitting a Comet"

Postby mcdigman » Thu Feb 06, 2014 5:17 am UTC

Well, there are more ophthalmologists in the US than nasa employees, so its probably more likely that a random person in the US can successfully do eye surgery than land a probe on a comet.

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Re: What-if 0082: "Hitting a Comet"

Postby Cal Engime » Thu Feb 06, 2014 5:18 am UTC

Image

I'm pretty sure this was the origin story of Freakazoid.

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Re: What-if 0082: "Hitting a Comet"

Postby Eebster the Great » Thu Feb 06, 2014 5:30 am UTC

Surgery requires not only extensive knowledge and practice but also hand-eye coordination. Mapping a trajectory for a space probe requires only extensive knowledge and math. I think the odds of either going successfully are quite small unless the participants are given years to hone their skills, but if absolutely forced to I would put my money on the surgeon.

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Re: What-if 0082: "Hitting a Comet"

Postby Jamaican Castle » Thu Feb 06, 2014 5:49 am UTC

Why does the target switch from SF to LA for a paragraph or two? (Admittedly, it's not like one is an easier shot than the other, from New York).

Also, does the probe really take such a circuitous path? Is it, indeed, possible to slingshot around oneself (my every instinct says "no", with a side of "but that never stopped NASA" and a bit of "but it works in Kerbal Space Program").

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Re: What-if 0082: "Hitting a Comet"

Postby miles60 » Thu Feb 06, 2014 5:53 am UTC

So, when you mouseover the solar system map that shows the path that Rosetta has taken, the alt-text is:

"Rosetta got gravitational assists from several flybys of Earth and one of Mars, flew past two asteroids, and - in a maneuver that still has some at ESA confused - became the first spacecraft to do a flyby of itself."

Is Randall just completely pulling that last part out of his ass or is there something to this (the last part)? Add me to the list of confused people if its true. I'd love to hear this one explained.

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Re: What-if 0082: "Hitting a Comet"

Postby Klear » Thu Feb 06, 2014 6:29 am UTC

I'm disappointed by the lack of sbemail reference in the third picture. :(

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Re: What-if 0082: "Hitting a Comet"

Postby nayhem » Thu Feb 06, 2014 6:32 am UTC

Image

This seems rather like a neat little Flash game idea. You're given control of the launch vector of a spacecraft, some limited number of course corrections, and a target to match. Simply launching straight at the object would get you stranded, so you have to make use of gravitational wells.

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Re: What-if 0082: "Hitting a Comet"

Postby ucim » Thu Feb 06, 2014 6:36 am UTC

Jamaican Castle wrote:Why does the target switch from SF to LA for a paragraph or two?
Quantum mechanics. :)

Jamaican Castle wrote:Also, does the probe really take such a circuitous path? Is it, indeed, possible to slingshot around oneself?
Yes, in the same sense that a rocket's exhaust pushes against the air. Remember that once the rocket is launched, it's no longer "part" of the earth, so it doesn't have to "slingshot around itself" in order to slingshot around the earth. All it has to do is almost-meet the earth again. As I understand it, they did a bunch of slingshots around inner planets, robbing momentum from them each time, and one of those inner planets happened to be the earth. Once it was whizzing around fast enough, they sent it out on its mission to meet Rosetta.

It was a clever ploy, especially since the alternative would be to wait 14000 years or so.

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Oh wait - the spacecraft is Rosetta. Quantum mechanics again!
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Re: What-if 0082: "Hitting a Comet"

Postby paha arkkitehti » Thu Feb 06, 2014 7:33 am UTC

So the next question is how much did the length of year change because of Rosetta stealing Earth's momentum in the flybys?

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Re: What-if 0082: "Hitting a Comet"

Postby CharlieP » Thu Feb 06, 2014 9:14 am UTC

ucim wrote:
Jamaican Castle wrote:Oh wait - the spacecraft is Rosetta. Quantum mechanics again!


I was just about to write a snarkless reply involving time travel and meeting oneself coming back, but you corrected yourself in time. :)
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Re: What-if 0082: "Hitting a Comet"

Postby Mikeski » Thu Feb 06, 2014 9:51 am UTC

paha arkkitehti wrote:So the next question is how much did the length of year change because of Rosetta stealing Earth's momentum in the flybys?

Without going into the math, it'll change our year about as noticeably as this girl will change our day.

With some very approximate math...

Momentum robbed from earth is added to the orbiter. (Assume losslessly, because spherical cows rock.)

mass of orbiter = 3000 kg at launch (it's probably burned some fuel since then, but whatever.)
mass of earth = 5.97e24 kg
velocity of earth = 108000 km/hour

So how fast do we want to go? Our comet target is going 135000 km/hour, so that's a reasonable speed to think about. If we were to get all of it from slingshotting around the earth:

Keeping everything in kg, km, and hours, we steal (135000 x 3000) = 4.05e8 of momentum from earth, slowing it down by (4.05e8 / 5.97e24) = 6.78e-17 km/hour.

Since the length of our year is proportional to our speed around the sun, that's a change of 6.78e-17/108000 = 6.28e-22 of a year (or 6.28e-20 percent of a year, if you like percentages.)

A year is about 365.25 days or 31557600 seconds, so we've added about (31557600 x 6.28e-22) = 1.98e-14 seconds, or about 20 femtoseconds seconds to a year to do this.

I'd say having to adjust for this by subtracting a leap-second once every 50 trillion years counts as "meh", since the sun will go red-giant and burn all our pin-up calendars a mere 5 billion years from now.

(Disclaimer: even basic arithmetic performed at 4AM is likely to be miscalculated.)

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Re: What-if 0082: "Hitting a Comet"

Postby speising » Thu Feb 06, 2014 10:16 am UTC

TODAY'S WHAT-IF ARTICLE WILL BE POSTED WEDNESDAY INSTEAD OF TUESDAY. SORRY FOR THE DELAY!


hm, is there some timey-wimey thing going on?

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Re: What-if 0082: "Hitting a Comet"

Postby Hel-G » Thu Feb 06, 2014 12:32 pm UTC

After reading the last question, I can't believe noone has posted a link to http://www.youtube.com/watch?v=THNPmhBl-8I.

Well, it's not exactly eye surgery, but it's pretty close!

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Re: What-if 0082: "Hitting a Comet"

Postby cellocgw » Thu Feb 06, 2014 12:47 pm UTC

Maybe I found GLR's "lost interview." Quote from a Matt Fordahl article in 1999, "NASA's most expensive unmanned spacecraft made its closest approach to Earth late Tuesday, arriving within three miles of its target and six-tenths of a second late -- all within an acceptable margin of error."
http://www.greatdreams.com/cassini.htm

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Re: What-if 0082: "Hitting a Comet"

Postby cellocgw » Thu Feb 06, 2014 12:54 pm UTC

Mikeski wrote:
paha arkkitehti wrote:So the next question is how much did the length of year change because of Rosetta stealing Earth's momentum in the flybys?

Without going into the math, it'll change our year about as noticeably as this girl will change our day.

With some very approximate math...
[snip...]

(Disclaimer: even basic arithmetic performed at 4AM is likely to be miscalculated.)


Well, you could have used this slingshot calculator :mrgreen:
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Re: What-if 0082: "Hitting a Comet"

Postby rjsteg » Thu Feb 06, 2014 1:23 pm UTC

taemyr wrote:
The gist of it is; for an object traveling faster than the speed of sound in a medium, in order to penetrate into the medium it will have to push the particles of the medium forward. This transfers momentum from the object into the medium. And, when the object have travelled a distance equal to it's own length, multiplied by the density of the object, divided by the density of the medium the momentum transfered will be equal to the momentum the object used to have.

The above breaks down when an object travels slower than the speed of sound, but it means that there is an upper limit to how far an object can travel if it's not acellerating in flight.

Edit: Wikipedia link: http://en.wikipedia.org/wiki/Impact_depth


According to WolframAlpha, this is approximately 130 feet. This isn't quite right or is this what you mean by "breaks down"? :-(

http://www.wolframalpha.com/input/?i=diameter+of+baseball+*+density+of+baseball+%2F+density+of+air

Still curious,
Randy

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Re: What-if 0082: "Hitting a Comet"

Postby speising » Thu Feb 06, 2014 1:34 pm UTC

rjsteg wrote:
taemyr wrote:
The gist of it is; for an object traveling faster than the speed of sound in a medium, in order to penetrate into the medium it will have to push the particles of the medium forward. This transfers momentum from the object into the medium. And, when the object have travelled a distance equal to it's own length, multiplied by the density of the object, divided by the density of the medium the momentum transfered will be equal to the momentum the object used to have.

The above breaks down when an object travels slower than the speed of sound, but it means that there is an upper limit to how far an object can travel if it's not acellerating in flight.

Edit: Wikipedia link: http://en.wikipedia.org/wiki/Impact_depth


According to WolframAlpha, this is approximately 130 feet. This isn't quite right or is this what you mean by "breaks down"? :-(

http://www.wolframalpha.com/input/?i=diameter+of+baseball+*+density+of+baseball+%2F+density+of+air

Still curious,
Randy



you know now that a supersonic baseball will only get 130 feet.

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Re: What-if 0082: "Hitting a Comet"

Postby J L » Thu Feb 06, 2014 1:40 pm UTC

In any case, it would be a real bummer if an eye surgeon messed up the imperial and the metric system during operation.

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Re: What-if 0082: "Hitting a Comet"

Postby gazer649 » Thu Feb 06, 2014 2:54 pm UTC

Thanks to my Liberal Arts education I would choose to bet the million dollars of someone else's money on the eye surgeon landing the probe on a comet - nobody gets hurt.

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Re: What-if 0082: "Hitting a Comet"

Postby taemyr » Thu Feb 06, 2014 3:33 pm UTC

rjsteg wrote:
taemyr wrote:
The gist of it is; for an object traveling faster than the speed of sound in a medium, in order to penetrate into the medium it will have to push the particles of the medium forward. This transfers momentum from the object into the medium. And, when the object have travelled a distance equal to it's own length, multiplied by the density of the object, divided by the density of the medium the momentum transfered will be equal to the momentum the object used to have.

The above breaks down when an object travels slower than the speed of sound, but it means that there is an upper limit to how far an object can travel if it's not acellerating in flight.

Edit: Wikipedia link: http://en.wikipedia.org/wiki/Impact_depth


According to WolframAlpha, this is approximately 130 feet. This isn't quite right or is this what you mean by "breaks down"? :-(

http://www.wolframalpha.com/input/?i=diameter+of+baseball+*+density+of+baseball+%2F+density+of+air

Still curious,
Randy


Yes, it's what I meant by "breaks down". :)

I am sure that there are people on this forum that can give better answers than the following. In particular aerodynamics is weird, and something I know far too little about.

As I said the approximation assumes that the objects penetrates the medium by pushing the particles in the medium ahead of itself. When the medium is allowed to flow past the object the object can penetrate deeper, because transfering moment is no longer the only way the object can penetrate.

Also, and I have no idea how big a factor this is, a baseball is under acceleration due to gravity during the flight. This enables a trade of velocity against gravitational momentum while the ball is ascending, and trading the opposite way while the ball is descending.
When the ball is travelling at subsonic speeds the air is allowed to fly

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Re: What-if 0082: "Hitting a Comet"

Postby taemyr » Thu Feb 06, 2014 3:36 pm UTC

speising wrote:you know now that a supersonic baseball will only get 130 feet.


Well no because the approximation doesn't hold for subsonic speeds.

So what you know is that a supersonic baseball will be subsonic after 130 feet. - Giving an upper bound for the length of a throw as 130 feet + how ever far a baseball trown at the speed of sound would travel.

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Re: What-if 0082: "Hitting a Comet"

Postby Spectrum » Thu Feb 06, 2014 5:11 pm UTC

Hitting the comet is harder than it appears from the path length that the spacecraft has travelled. That's because of the three slingshot events; those amplify any directional deviation beyond what the geometry alone implies. That is, several trajectories coming in to the gravity well that deviate from each other only slightly will deviate from each other a great deal more as they exit the gravity well. Worse, the amplifications due to the three slingshot events multiply.

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Re: What-if 0082: "Hitting a Comet"

Postby Spectrum » Thu Feb 06, 2014 5:13 pm UTC

Would you rather bet a million dollars on a spacecraft landing engineer's ability to successfully perform eye surgery, or an eye surgeon's ability to land a probe on a comet?


I'd go for the landing engineer. Those people are conservative and into careful planning, so he'd go out and get trained in eye surgery before attempting it. Surgeons, however, are famous for over-estimating their competence in non-surgical activities.

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Re: What-if 0082: "Hitting a Comet"

Postby speising » Thu Feb 06, 2014 5:33 pm UTC

Spectrum wrote:Hitting the comet is harder than it appears from the path length that the spacecraft has travelled. That's because of the three slingshot events; those amplify any directional deviation beyond what the geometry alone implies. That is, several trajectories coming in to the gravity well that deviate from each other only slightly will deviate from each other a great deal more as they exit the gravity well. Worse, the amplifications due to the three slingshot events multiply.

but the spacecraft isn't a one shot thing. it can correct it's course.

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Re: What-if 0082: "Hitting a Comet"

Postby rmsgrey » Thu Feb 06, 2014 5:42 pm UTC

ucim wrote:Hitting a comet
what-if wrote:Would you rather bet a million dollars on a spacecraft landing engineer's ability to successfully perform eye surgery, or an eye surgeon's ability to land a probe on a comet?

I'm a pilot, and my friend is a sailor. We have this idea that he'd sail to Montauk Point, I'd fly there, and then we'd trade keys.

Bets on who makes it home first, and who dies.

Jose


My money's on the guy taking the boat being the one to survive - though I don't know enough about the local topography and/or climate to judge how that affects the outcome.

It also depends on the details of the vehicles...

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Re: What-if 0082: "Hitting a Comet"

Postby ack403 » Thu Feb 06, 2014 6:34 pm UTC

Seems like the comet would be way harder than the baseball in actual practice.

There's enough Earth infrastructure at this point that it's pretty trivial to hit most things with a baseball--simply throw it to a friend in New York, then have that friend travel to wherever with the baseball and throw the ball at the target from a couple yards away. If it's a high window, your friend could even hit the window from the inside. This is easy, because we're pretty good at moving things around on or near the Earth's surface; I posit that the difference between

1) catching a baseball and getting on a plane, and
2) a gas-jet course correction on a space probe

is more or less semantic.

Without Earth infrastructure, course correction gets a lot harder, your initial course needs to be more accurate, and your corrections also need to be more accurate (a plane leaving two hours late is OK, but firing a course correction 2 hours past your window is not...).

QED: Given the same initial conditions (a bunch of political will and a big pile of money), the baseball goal is way easier than the comet goal.

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Re: What-if 0082: "Hitting a Comet"

Postby kedji » Thu Feb 06, 2014 7:11 pm UTC

In the orbital diagram it looks like Rosetta is "catching up" to Kevin as they both travel counter-clockwise around the sun. Shouldn't Rosetta be in front with the faster-moving Kevin asteroid closing in from behind?

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Re: What-if 0082: "Hitting a Comet"

Postby Flumble » Thu Feb 06, 2014 7:36 pm UTC

rmsgrey wrote:My money's on the guy taking the boat being the one to survive - though I don't know enough about the local topography and/or climate to judge how that affects the outcome.

It also depends on the details of the vehicles...

I'd bet on the guy flying the plane, since you need a flight certificate in virtually any country, whereas any fool may steer* a boat in many places.


*what does one do to control a boat in English? steering? driving? piloting?

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Re: What-if 0082: "Hitting a Comet"

Postby eran_rathan » Thu Feb 06, 2014 8:45 pm UTC

Flumble wrote:
rmsgrey wrote:My money's on the guy taking the boat being the one to survive - though I don't know enough about the local topography and/or climate to judge how that affects the outcome.

It also depends on the details of the vehicles...

I'd bet on the guy flying the plane, since you need a flight certificate in virtually any country, whereas any fool may steer* a boat in many places.


*what does one do to control a boat in English? steering? driving? piloting?


Usually its 'piloting'.
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Re: What-if 0082: "Hitting a Comet"

Postby AMarquez » Thu Feb 06, 2014 9:05 pm UTC

I don't have a lot of experience, but I think Asteroids are way less likely to sue, so I think an engineer is gonna be more nervous operating machinery on the face of other person than a surgeon is gonna be operating machinery on the face of other celestial body.

Also, hi xkcd people!

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Re: What-if 0082: "Hitting a Comet"

Postby AMarquez » Thu Feb 06, 2014 9:10 pm UTC

-----Here it says nothing-----
Last edited by AMarquez on Thu Feb 06, 2014 9:50 pm UTC, edited 1 time in total.

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Re: What-if 0082: "Hitting a Comet"

Postby ucim » Thu Feb 06, 2014 9:29 pm UTC

speising wrote:you know now that a supersonic baseball will only get 130 feet.
No, think of the 130 feet as a "half life" of sorts for velocity (assuming that the air that you push in front of you also has to push the other air in front of it too...) So you need to impart a total momentum to the baseball equal to (or greater than) the momentum of the baseball plus a cylinder of air that goes all the way out into space in the direction of the baseball's travel... when the whole shebang is moving at whatever speed you want it to achieve in orbit. For a baseball that's 10 inches in cross section, traveling straight up, you'd end up accelerating 140 pounds or so of air. If the baseball weighs half a pound, you'd need a starting momentum of 280 times whatever you'd otherwise need. To get into space you'd probably need something like five miles per second; this implies an initial velocity of 700 miles per second to achieve the required momentum.

I don't think the baseball will actually last very long.

rmsgrey wrote:My money's on the guy taking the boat being the one to survive - though I don't know enough about the local topography and/or climate to judge how that affects the outcome.

It also depends on the details of the vehicles...
Single engine piston airplane (two to four seats) like a Cessna 172, and a small sailboat (26 feet?) that bunks four.

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Re: What-if 0082: "Hitting a Comet"

Postby drummerpatch » Fri Feb 07, 2014 12:59 am UTC

eran_rathan wrote:
Flumble wrote:
rmsgrey wrote:My money's on the guy taking the boat being the one to survive - though I don't know enough about the local topography and/or climate to judge how that affects the outcome.

It also depends on the details of the vehicles...

I'd bet on the guy flying the plane, since you need a flight certificate in virtually any country, whereas any fool may steer* a boat in many places.


*what does one do to control a boat in English? steering? driving? piloting?


Usually its 'piloting'.


Unless it's a sailboat. Then it's usually "sailing".
Although "sailing" is usually used in slang to mean maneuvering any kind of boat, but if it's not a sailboat the proper term is technically "piloting".

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Re: What-if 0082: "Hitting a Comet"

Postby kriel » Fri Feb 07, 2014 3:51 am UTC

I'd bet on the engineer doing the surgery.

I can very easily see a doctor trying to manuever the spacecraft without having an intuitive sense or feel for saving fuel, or overshooting, etc. I can just see him wiggling it back and forth to see how responsive the controls ar--

Hm. I suppose that doesn't matter as much if there's no real-time controls. I'd imagine there's a significant delay between sending a command and having it executed.. and then getting feedback.

Whereas, an engineer would have controls (I'm imagining a joystick-like interface?) that have.. what.. a few hundred ms of latency? Maybe a full second, if it's crappy internet and/or hardware. Hell, a run of the mill gamer could probably do a decent job. Though, the engineer would probably read the documentation. =p

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Pfhorrest
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Re: What-if 0082: "Hitting a Comet"

Postby Pfhorrest » Fri Feb 07, 2014 5:18 am UTC

ucim wrote:Hitting a comet
what-if wrote:Would you rather bet a million dollars on a spacecraft landing engineer's ability to successfully perform eye surgery, or an eye surgeon's ability to land a probe on a comet?

I'm a pilot, and my friend is a sailor. We have this idea that he'd sail to Montauk Point, I'd fly there, and then we'd trade keys.

Bets on who makes it home first, and who dies.

Jose

From my very limited experience piloting a small sailboat and a small plane, both in very amateur circumstances (with professional supervision thank god), I would put all money on you, assuming you are both inexperienced with each others' vehicles and we're talking generally clear conditions in both the air and sea and neglecting legal complications like your friend not having a pilot's license vs you not needing a boating license.

Figuring out how to operate the rigging on the boat may be trickier than figuring out the controls of the plane at first, but once you've each figured that out and are on your respective ways, making the boat go where you want it is largely a matter of understanding the same basic physics you should also already know to pilot a plane (what happens when wind hits a surface at an angle, that kind of thing), and if you mess it up a little, you've got a lot of time to make corrections, at any point in the trip, unless you do something radically wrong like flip the thing upside down, which would be just as bad in the plane. A plane in the middle of the air has decent time and space to make corrections to small mistakes too, but ohhhhhhmygod takeoff and landing scares the shit out of me.

Giving your friend a little benefit of the doubt, I'd expect him to take off before you really make way on the boat, and for you to eventually make your way back home to discover that he died attempting to land.
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