New Horizons

[Ikaneko]

How much ∆v did Clyde Tombaugh's ashes cost NH? I really like that Jupiter-Io pic btw.

Put it in the rocket equation to find out!!

[GigaG]

I wonder how much delta-V would be required for braking a hypothetical future Pluto orbital mission (assuming it followed a fast trajectory like NH, or maybe just a bit slower would be fine.) If Pluto is awesome, I'd love to see that! (Of course, it would probably be easier to make a Uranus/Neptune probe, but I can dream, right?) On another note, this looks like a job for the SLS...

[Bill Phil]

Or any other heavy launcher.

[Frida Space]

I wonder how much delta-V would be required for braking a hypothetical future Pluto orbital mission (assuming it followed a fast trajectory like NH, or maybe just a bit slower would be fine.) If Pluto is awesome, I'd love to see that! (Of course, it would probably be easier to make a Uranus/Neptune probe, but I can dream, right?) On another note, this looks like a job for the SLS...

New Horizons is travelling at 43 000 km/h (27 000 mph). At that speed, an orbit insertion burn would require New Horizons to reduce its speed by over 90%, which would require more than 1,000 times the fuel that New Horizons can carry.

[Streetwind]

New Horizons is flying an extremely accelerated transfer, though. It took the "point at planet and floor it until out of fuel" approach. Take a few years longer to fly a less energetic transfer, and you can vastly reduce the capture burn required.

I'm fairly sure electric propulsion can do it. ESA tested something with 20,000 Isp in the lab and it worked just fine. Problem is, Pluto receives so little sunlight that it can safely be called "none at all" in a first approximation. So such a mission would require an onboard nuclear reactor to drive its ion engines.

[Frida Space]

True. New Horizons didn't use a Hohmann transfer, so things could be done more efficiently, although it would take more than 9 years.

At that point, the main problem would be surviving a cruise phase that long.

Edited January 6, 2015 by Frida Space

[NERVAfan]

I wonder if you could use aerobraking (once we know how high the mountains, if any, on Pluto are). Pluto's atmosphere is really thin, but if you came barreling in at that speed, it might slow you enough...

[lajoswinkler]

I wonder if you could use aerobraking (once we know how high the mountains, if any, on Pluto are). Pluto's atmosphere is really thin, but if you came barreling in at that speed, it might slow you enough...

Nope. Pluto can't have anything more than a whispy atmosphere and that's during the "summer". Pluto is very small and thus it can't hold it. Also, very cold so most stuff is just frozen on the ground.

If its atmosphere was of any use for aerobraking, we'd probably detect it by star occultations. What Pluto must have is something similar to Triton. Few pascals at most, during summer. You can't aerobrake a passing probe with that. It would take a long time before a low orbiting probe would decay let alone an object with tremendous speed.

Edited January 7, 2015 by lajoswinkler

[NERVAfan]

Nope. Pluto can't have anything more than a whispy atmosphere and that's during the "summer". Pluto is very small and thus it can't hold it. Also, very cold so most stuff is just frozen on the ground. If its atmosphere was of any use for aerobraking, we'd probably detect it by star occultations.

It has been; that's how we know Pluto has an atmosphere in the first place.

What Pluto must have is something similar to Triton. Few pascals at most, during summer.

It's probably somewhere in the general order of magnitude of 0.1-1 Pa, yeah, maybe a bit more at certain times... but it actually seems to be thickening now as Pluto moves out of its summer. http://www.planetary.org/blogs/emily-lakdawalla/2013/09051420-plutos-atmosphere-does-not-collapse.html

You can't aerobrake a passing probe with that. It would take a long time before a low orbiting probe would decay let alone an object with tremendous speed.

I don't think that's true. That's about what you'd get at 80-90 km altitude on Earth, at or just above the mesopause, and IIRC you can't even get close to completing one orbit at that altitude. And a faster object has more drag.

[Starwaster]

As a quick note, do not expect anything special on the day of the flyby. Probably for a few days after that, too. Bandwidth available at that distance is very low; the plan is for NH to store all data from the flyby on internal storage, then send it down over a period of a year or more.

Now see, that's just not true. I've seen the Exede commercials. Internet from Space is fast!

Brian won't let us down.

[lajoswinkler]

It has been; that's how we know Pluto has an atmosphere in the first place.

What I meant to say was that if the atmosphere was thick enough to allow such aerobraking, we'd figure it out by star occultations. We know it does have a very tenuous atmosphere, yes.

It's probably somewhere in the general order of magnitude of 0.1-1 Pa, yeah, maybe a bit more at certain times... but it actually seems to be thickening now as Pluto moves out of its summer. http://www.planetary.org/blogs/emily-lakdawalla/2013/09051420-plutos-atmosphere-does-not-collapse.html

Hopefully we'll see more nitrogen clouds than on Triton.

I don't think that's true. That's about what you'd get at 80-90 km altitude on Earth, at or just above the mesopause, and IIRC you can't even get close to completing one orbit at that altitude. And a faster object has more drag.

More like a bit above 90 km. There is no way a probe going as fast as New Horizons could get caught in such small planet's orbit using such tenuous atmosphere. It would certainly deflect its trajectory by a small amount and it would bleed off a fraction of its relative speed, but that's it. We have no means of stopping the probe, unfortunately.

[Nikolai]

What I meant to say was that if the atmosphere was thick enough to allow such aerobraking, we'd figure it out by star occultations. We know it does have a very tenuous atmosphere, yes.

And he's saying that it was. Stellar occultations in 2002, observed and analysed by teams led by Bruno Sicardy of the Paris Observatory, James L. Elliot of MIT, and Jay Pasachoff of Williams College yielded an atmsopheric pressure of 0.3 Pascal.

That density, combined with the estimated speed of New Horizons with respect to Pluto at closest encounter (roughly 11 km/s), yield a maximum dynamic pressure of approximately 18 megapascals. I'd have to calculate the cross-sectional area of the probe to find the total force precisely, but it's commonly compared to a grand piano, so you can multiply that by two or three to get the total force on a go-for-broke aerobrake. Let's say two for 36 meganewtons of force.

Combine that with the probe mass (487 kg at launch) and you have a maximum acceleration of 74,000 m/s. That's enough to bring New Horizons -- or whatever survives of it -- to a dead stop in only about a millimeter.

EDIT: I failed to account for the force correctly, since it would decrease as the probe slowed down; an accurate accounting would involve integration to account for this. But this BOTE calculation still seems to support the idea that New Horizons could potentially aerobrake to orbit if they really wanted it to and if the probe could withstand the mechanical stresses.

Also, I forgot to divide by the final velocity squared when coming up with that distance.

Edited January 7, 2015 by Nikolai
Added qualification to distance needed to stop

[Frida Space]

I'm no expert so I can't support what I'm saying with calculations, but I do know the project scientist of the mission and she told me there was absolutely no way of bringing New Horizons into orbit, and that they would have preferred to get a global coverage of Pluto than to see half of its surface plus another Kuiper belt object... So if NASA says it I guess aerobraking around Pluto isn't a thing. Sure, they might have not wanted to risk it since we don't know the planet so well yet, but from what I understood they never even took that idea under consideration.

[PakledHostage]

I'm no expert so I can't support what I'm saying with calculations, but I do know the project scientist of the mission and she told me there was absolutely no way of bringing New Horizons into orbit...

By my read, NERVAfan was speaking about a hypothetical future mission, not about New Horizons. I understood that his point was made in response to GigaG's comment about the viability of doing so on some hypothetical future mission.

IIRC, New Horizons has already been re-targeted to flyby Pluto at a greater distance than first planned because they are worried about the amount of debris in orbit about Pluto. They don't want to hit any of it and destroy the probe on their way by.

[lajoswinkler]

And he's saying that it was. Stellar occultations in 2002, observed and analysed by teams led by Bruno Sicardy of the Paris Observatory, James L. Elliot of MIT, and Jay Pasachoff of Williams College yielded an atmsopheric pressure of 0.3 Pascal.

That density, combined with the estimated speed of New Horizons with respect to Pluto at closest encounter (roughly 11 km/s), yield a maximum dynamic pressure of approximately 18 megapascals. I'd have to calculate the cross-sectional area of the probe to find the total force precisely, but it's commonly compared to a grand piano, so you can multiply that by two or three to get the total force on a go-for-broke aerobrake. Let's say two for 36 meganewtons of force.

Combine that with the probe mass (487 kg at launch) and you have a maximum acceleration of 74,000 m/s. That's enough to bring New Horizons -- or whatever survives of it -- to a dead stop in only about a millimeter.

EDIT: I failed to account for the force correctly, since it would decrease as the probe slowed down; an accurate accounting would involve integration to account for this. But this BOTE calculation still seems to support the idea that New Horizons could potentially aerobrake to orbit if they really wanted it to and if the probe could withstand the mechanical stresses.

Also, I forgot to divide by the final velocity squared when coming up with that distance.

You've forgot that any atmosphere gradually dissolves with height. 0.3 Pa on average, yes. That plus the integration approach would show that what NASA says is correct. There can be no stopping with New Horizons even if it had a shield.

[Frida Space]

By my read, NERVAfan was speaking about a hypothetical future mission, not about New Horizons. I understood that his point was made in response to GigaG's comment about the viability of doing so on some hypothetical future mission.

IIRC, New Horizons has already been re-targeted to flyby Pluto at a greater distance than first planned because they are worried about the amount of debris in orbit about Pluto. They don't want to hit any of it and destroy the probe on their way by.

english is not my native language as you have probably noticed by now, so thanks for clearing that up for me and yes, the current target distance is such that there should be no stable orbits for small particles of debris to be in.

[Nikolai]

You've forgot that any atmosphere gradually dissolves with height. 0.3 Pa on average, yes. That plus the integration approach would show that what NASA says is correct. There can be no stopping with New Horizons even if it had a shield.

No, I hadn't forgotten that. It's kind of irrelevant even if my BOTE calculation is off by several orders of magnitude; the thickness of the atmosphere won't change appreciably over a few tens of meters. (Unless you can demonstrate that the integration approach yields remarkably different results.)

What I think is the limiting factor here is the amount of stress the spacecraft would have to endure if it were to slow down to orbital velocity in the space that Pluto's atmosphere offers. It has to slow from 11 km/s to 0.8 km/s in about 960 km(*), meaning an average acceleration of some 6.5 gees -- which would naturally be much higher near the front end of the deceleration.

---

(*) EDIT: Assuming the atmosphere is as deep as Earth's (100 km), that gives us a maximum of about 480 km to aerobrake through.

Of course, there's a much easier way to think about this. What happens when things going 11 km/s -- roughly Earth's escape velocity -- hit the upper atmosphere, long before the pressure gets to 0.3 Pascal? That's the pressure at approximately 89 km above sea level, way below where meteors like the Perseids burn up.

Edited January 7, 2015 by Nikolai

[Superfluous J]

That's enough to bring New Horizons -- or whatever survives of it -- to a dead stop in only about a millimeter.

Oh. My. Jool.

Pluto is KERBIN!

[Frida Space]

Slightly updated the layout of the main thread. Hope it looks a bit better despite those humongous images still being there.

[Starwaster]

You've forgot that any atmosphere gradually dissolves with height. 0.3 Pa on average, yes. That plus the integration approach would show that what NASA says is correct. There can be no stopping with New Horizons even if it had a shield.

No, I hadn't forgotten that. It's kind of irrelevant even if my BOTE calculation is off by several orders of magnitude; the thickness of the atmosphere won't change appreciably over a few tens of meters. (Unless you can demonstrate that the integration approach yields remarkably different results.)

What I think is the limiting factor here is the amount of stress the spacecraft would have to endure if it were to slow down to orbital velocity in the space that Pluto's atmosphere offers. It has to slow from 11 km/s to 0.8 km/s in about 960 km(*), meaning an average acceleration of some 6.5 gees -- which would naturally be much higher near the front end of the deceleration.

---

(*) EDIT: Assuming the atmosphere is as deep as Earth's (100 km), that gives us a maximum of about 480 km to aerobrake through.

Of course, there's a much easier way to think about this. What happens when things going 11 km/s -- roughly Earth's escape velocity -- hit the upper atmosphere, long before the pressure gets to 0.3 Pascal? That's the pressure at approximately 89 km above sea level, way below where meteors like the Perseids burn up.

0.3 pascals starting at the surface. It has a scale height of 19km. Its atmosphere is more like the tail of a comet and so stretches out away from it, blown by the solar wind. I'd say its atmosphere can't be treated like an ordinary planet except the side facing the sun or out to the sides somewhat. Taken like that, the pressure would fall off considerably, having a pressure of

0.000000000000001 atm (

1.01325e-10 pascals) at an altitude of 418 km. I don't think you're going to do any aerobraking there.

The tail is another matter and falls off less gradually. The tail has higher pressures at higher altitudes than the rest of Pluto. Maybe you could do something with the tail, but I still doubt it.

On the subject of aerobraking, hypothetically one would assume that you had a craft there that was designed for it (New Horizons is not, and will be steering clear of any potential hazards if it can)

It would be something small like Stardust or Galileo both of which had to withstand enormous g-forces. So, assuming you could find an opportunity for aerobraking at Pluto, it would still be within survivable limits of what we can send.

[KerikBalm]

http://www.engineeringtoolbox.com/dynamic-pressure-d_1037.html

11,000 m/s velocity

0.01 kg/m^3 (I only found pressure, not density, and one source saying its density was similar to mars, which is about 1% of Earth's)

605 kilopascals....

How did you get 18 mega pascals?

*edit* Ok, so regarding the density of Triton's atmosphere.

Earth's atmosphere at sea level is roughly 1.2 kg/m^3

Lets assume Triton's atmospheric pressure is 1 pascal, and Earth's is 100 kilopascals. Earths pressure is roughly 10^5th higher than Tritons. Both atmospheres are mainly nitrogen, so that makes them easier to compare. Earth's atmosphere at sea level is roughly 300k, whereas Triton's atmosphere ranges from 40 to 100k. Lets assume 40k. So for the same pressure, colder gas is denser... proportional to temperature.

lets just take 300/40 * 10^-5 * 1.2 kg/m^3 = 0.000075 kg/m^3

Plugging these new numbers in... 4538 Pascals.

4.5 kilopascals vs your estimate of 18 megapascals... I think your numbers are off by orders of magnitude, without checking your other calculations, it already seems that your first number is 4,000 times too high.

*edit* I don't know why I was using Triton asa standin for Pluto... lets go with Pluto... 0.3 pascals @43k for pluto , and use 293k and 101 kPa for earth.

0.3/101,000 * 293/43 = 0.00002024. That *1.2 for earths sea level atmosphere density at 293k = 0.0000243 kg/m^3

Plugging that into the dynamic pressure equation, with 11km/sec as the velocity, we get 1470 pascals.

1.47 kPa vs your estimate of 18,000 kPa

I really don't trust your math.

Lets assume it has a cross section area of 3 square meters... so that would allow for a force of 4.41 kilonewtons of force (assuming it is very blunt, I'm not sure how the aerodynamics work here). On a craft that weighs 0.478 tons, that is an acceleration of 9.22 m/s

Therefore, assuming that constant acceleration, and a change in velocity of roughly 10km/s is needed to establish orbit, it would take 1084 seconds of that acceleration to capture. The distance is then given by 1/2 at^2 -> 4.61 m/s^2 * (1084 seconds)^2 -> 5,417,000 meters....

It would need to aerobrake for at least 5,417 km to capture, the planets radius is about 1185 km....

A very flat trajectory that is roughly tangent to the surface is going to have a path through the thick part of the atmosphere much much less than the planet's radius.

Its not going to happen...

And of course, thats not "integrative", when we consider that when it has bled off half its speed, the force will be only 1/4th....

There is no way that a probe is going to aerocapture on Pluto.

Edited January 8, 2015 by KerikBalm

[Nikolai]

How did you get 18 mega pascals?

Dynamic pressure. Q = (rho)(v^2)/2, where rho is the density of the medium and v is velocity.

... And now I'm realizing that I used pressure instead of density. <dope slaps self> Well, that changes everything. My apologies.

I really don't trust your math.

Yeah, you really shouldn't have. Thanks for checking.

[KerikBalm]

So I think a better way to do these calculations, is to use this http://en.wikipedia.org/wiki/Impact_depth

Basically, we'll get a conservative estimate (granted its for a full stop, which is about 1km/s more than what we want) if we just figure the length of the path through the atmosphere that would be needed at which point the mass of displaced atmosphere is equal to the probe's.

478 kg probe, 0.0000243 kg/m^3 atmosphere density, (complete guess) 3 m^2 cross section.

19,670,781 cubic meters of atmosphere would need to be displaced, which would be a path length of 6,556,927 meters if it has a 3 sq meter cross section.

Thats 6,550 km... Which would be about equal to circumnavigating the entire planet at ground level...but the incombing trajectory is about a straight line.

When I get home, I intend to do the integration to determine the "effective" length of a path tangent to the planet's surface, assuming the atmosphere starts at 100km (giving you a path of 960 km) - ie what path lenght along the surface that corresponds to.

At any rate, its clear that its simply not possible to aerobrake that probe at Pluto

[*Aqua*]

How long can the probe observe Pluto and Charon? I'm hoping it's long enough to photograph the complete surface.

It would suck a lot if we get only parts of it like we had of Mercury and Venus some time ago.

[KerikBalm]

Well, it takes 6 days to rotate once, so if we get 3 days of good observation, we should get the whole surface.

It won't be all at the same level of detail though.