Wikipedia:Reference desk/Archives/Science/2019 February 8

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February 8

Voyager

Animation of Voyager 1's trajectory from 5 September 1977 to 30 December 1981
  Voyager 1 ·   Earth ·   Jupiter ·   Saturn ·   Sun

Animation of Voyager 2's trajectory from 20 August 1977 to 30 December 2000
  Voyager 2 ·   Earth ·   Jupiter ·   Saturn ·   Uranus ·   Neptune ·   Sun

I cannot seem to find this in the Voyager program article:

How did they get it to go really fast? Did the rocket that sent it on its way just launch from Earth and then go really fast into space in the right direction and then open the pod bay doors (hal) and then let it drift out? Is that how it works? And if that's right, what became of the rocket? Wouldn't it follow along behind it the whole way?

Anna Frodesiak (talk) 03:12, 8 February 2019 (UTC)

Gravity assist is how they got so fast. As for the final stage, see List of artificial objects leaving the Solar System#Propulsion stages. --Guy Macon (talk) 03:51, 8 February 2019 (UTC)

Hi, Guy Macon. But what about leaving Earth? Did that happen the way I said? Best, Anna Frodesiak (talk) 04:51, 8 February 2019 (UTC)

I don't know about Voyager specifically, but the way this works in general is through staging. The majority of the hardware you see leaving the pad is jettisoned early in the flight and either falls back to Earth, ends up in Earth orbit or, for the upper stages of interplanetary missions, perhaps heliocentric orbit. The last stage of propulsion is provided by a relatively small rocket that forms part of the spacecraft itself and is used for later trajectory correction manoeuvers, so it's only this that goes all the way, rather than the earlier stages of the rocket drifting behind the spacecraft forever: those stages don't achieve the velocity to do this. The 'pod bay doors' you mention are the payload fairing, which is jettisoned as soon as the spacecraft is out of the atmosphere even if there is still a lot more accelerating to do, since they just become useless dead weight at this point. Beorhtwulf (talk) 05:55, 8 February 2019 (UTC)

See this video of the launch of InSight, on an interplanetary trajectory not in principle too different from how Voyager would have been sent on its way (minus the later gravity assists). At about 44:28 in the video, less than five minutes into the flight, we switch to an computer-generated view based on telemetry and you can see payload fairing separation. By this time you'll see the overall hardware looks a lot smaller than what was launched, because the Atlas V lower stage has already separated. At this point we are left with the spacecraft itself and the Centaur upper stage, which separates a bit later on leaving the cruise stage to take InSight all the way to Mars. In Voyager's case it does appear, according to List_of_artificial_objects_leaving_the_Solar_System#Propulsion_stages, that the Centaurs are in heliocentric orbit - so they accompanied Voyager until it was fast enough to have escaped Earth orbit, but didn't get as fast as the probes themselves, which are not in heliocentric orbit but on their way out of the solar system. There were however third stages to Voyager that are presumed to be on an escape trajectory. Beorhtwulf (talk) 06:14, 8 February 2019 (UTC)

Thank you, Beorhtwulf!! Did you just write that? I mean, if that is original, you should add it to an article. I totally understand what you wrote, and that's a big compliment, because I don't fully understand tic-tac-toe. I am very grateful. By the way, I could not view the youtube video because that site is blocked where I am. Thank you again. Oh, and I got all interested in this since yesterday when I watched The Farthest 2017, which was wonderful. Anna Frodesiak (talk) 07:32, 8 February 2019 (UTC)

Thanks, it is original and I just wrote it from having a general interest in space exploration and following missions over the last few years. There will be people around who have a more detailed technical knowledge of the specific rockets used for Voyager and other missions. I am not sure that text could go in an article as written, and I think much of the information is already present in Wikipedia but spread over several articles. It is possible some articles could be improved to state more explicitly how these missions work and I may have a go at this one day. This is certainly a fascinating and inspiring topic, especially Voyager, which might just be the greatest uncrewed mission of all time due to its scope, achievement and longevity. Interplanetary probes get less coverage and attention than crewed spaceflight but are cheaper, more frequent, and a constant source of new discoveries. If you are interested in missions that have been a hive of activity just in the last few weeks there's New Horizons, Chang'e 4, OSIRIS-REx, Hayabusa2, InSight for starters. And of course both Voyagers are still returning science data, more than 40 years after launch! Beorhtwulf (talk) 08:12, 8 February 2019 (UTC)

Also see Oberth effect for very effective acceleration during slingshots. --Kharon (talk) 19:24, 8 February 2019 (UTC)

Hi, Kharon. Thank you. I will read that. Best, Anna Frodesiak (talk) 09:55, 9 February 2019 (UTC)

Hi, Beorhtwulf. Well, I hope people find a way to add some of that content here and there. I agree about Voyager maybe being the greatest. I will check out the links you provide to those other missions. Thank you again so much. :) Anna Frodesiak (talk) 09:55, 9 February 2019 (UTC)

What has not been mentioned here is that Earth itself has orbital velocity - 30 km/s. So any velocity that is achieved by action of an engine can be added to it. Simple example. Suppose a spacecraft is accelerated to 14 km/s, which is only slightly faster than the escape speed of 11.2 km/s. Then the final speed relative to Earth is: $v_{final}={\sqrt {14^{2}-11.2^{2}}}=8.7$ km/s, which when added to 30 km/s is close to that necessary to reach Jupiter. Ruslik_Zero 14:48, 9 February 2019 (UTC)

Thank you, Ruslik. That is very interesting and helpful. Anna Frodesiak (talk) 14:59, 9 February 2019 (UTC)

Aerographite balloon

If you wrap aerographite in a plastic bag and pump all the air out, would it rise? If you then constructed an aircraft to Francesco de Lana's design (but using evacuated plastic-wrapped aerographite-filled spheres of the same size instead of the hollow copper ones the original design had), how much payload could it lift? 2601:646:8A00:A0B3:64EB:2E36:2529:F033 (talk) 11:16, 8 February 2019 (UTC)

Yes (at least potentially, you'd have to check the precise numbers). Try vacuum balloon for more. Andy Dingley (talk) 11:28, 8 February 2019 (UTC)

Aerographite has a density of 180 g/m3, air has a density at sea level of about 1.225 kg/m3, so your bag would fly and could lift up to about 1 kg per m3 (minus the weight of the bag). But I suppose it could not resist air pressure and it would be crushed long before the bag is empty (Young's modulus is ca. 15 kPa at 0.2 mg/cm3, if this helps). 89.204.137.105 (talk) 15:57, 8 February 2019 (UTC) Marco Pagliero Berlin

No, the plastic skin would be sucked into the foam until it ripps. Only solid containments, the lightest possible constructed in ball or bulb-form, can hold a vacuum. These are usually way to heavy, to prevent any chance of implosion, for the missing air to make up the total weight. --Kharon (talk) 19:43, 8 February 2019 (UTC)

I don't think that the above is correct. Assuming that the Aerographite is strong enough to avoid crushing and that the "holes" (gaps? Places-where-aerographite-isn't?) are small enough, I see no theoretical reason why this would not work. The question is whether the Aerographite is strong enough to avoid crushing. --Guy Macon (talk) 17:24, 9 February 2019 (UTC)

Aerographite is not strong enough unless its density is boosted. The article states that the strength is 160 kPa at 8.5 mg/cm³. Air pressure is 100kPa, but that density needs to be down to to 1 mg/cm³ for it to be as light as air. So the answer is still no. If you made it light enough to float it would be crushed by atmospheric pressure. Graeme Bartlett (talk) 11:13, 10 February 2019 (UTC)

For a moment there my befuddled brain wondered if it would float on mars (The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi; 6.0 mbar), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi; 1.013 bar). --Atmosphere of Mars.) But then I realized that less density not only means less crushing force, it means less lift, leaving us in the same "anything strong enough is too heavy" boat. But what about Jupiter? We know that at 1G a dense enough "atmosphere" can make many, many things float upwards. We call them "boats". :) But could a vacuum balloon float in the deeper, denser layers of Jupiter?

There was a mythbuster episode where they made a lead balloon float. a really big lead baloon.[1] Alas, I don't think larger size would help an aerographite-vaccum balloon. The lead ballon worked because as size increases the lead weight is squared while the helium lift is cubed. With aerographite-vaccum the weight of the aerographite in cubed.

Hmmm. Thinking further, what if we vary the density of the aerographite? Could it be that the stuff near the middle gets less crushing force that the stuff near the surface? --Guy Macon (talk) 01:41, 11 February 2019 (UTC)

So it would be crushed, just like de Lana's original design? Thanks for the info! 2601:646:8A00:A0B3:DD98:2DB1:2850:1E54 (talk) 09:59, 15 February 2019 (UTC)

Nucleation theory of Liesegang rings

Has this theory been discredited ?

1) The solvent (such as water) in a chemical solution evaporates until the point at which the concentration of the solute rises high enough to allow deposition to occur.

2) The deposition occurs at the top of the solution, perhaps because the precipitate is less dense than the solution.

3) The previous deposits act as nucleation sites to encourage additional deposition, thus reducing the concentration below that which is required for further deposition.

4) Deposition stops, and evaporation of the solvent continues.

The cycle continues at step 1. SinisterLefty (talk) 17:41, 8 February 2019 (UTC)