Van Allen on Space Radiation.

[bknight]

Was Jupiter a "solar twin" that didn't get enough hydrogen to start fusion?

[Grashtel]

Was Jupiter a "solar twin" that didn't get enough hydrogen to start fusion?

Not by a very long way, even a brown dwarf is something like ten times Jupiter's mass

[ka9q]

The "surface" of a gas (actually fluid) giant is variously considered to be at the cloud tops, the 1-bar pressure level, or the level where the optical depth reaches unity. These aren't the same, but it doesn't really matter much for our purposes since the "surface" gravity is probably very close to the maximum gravity you'd experience as you descend into a planet. Remember that inside a spherically symmetric planet the gravity of the shell of material above you cancels out so you only see that gravity exerted by all the material closer to the planet's center than you are.

It can still increase somewhat with depth (as it does on earth) when the center of the planet is much more massive than the outer layers.

[bknight]

There was a comment at CosmoQuest considering what the core will be, if any.

[nomuse]

And if I recall correctly, the density/pressure profile is not your friend. By the time you "floated" on a liquid surface, atmospheric pressure will have crushed you.

That nice relationship of mass versus distance from (putative) center of mass also leads to the paradox of small planets (like Mars) having a higher surface gravity than one might intuit.

[ka9q]

Well, being a terrestrial planet Mars is a lot denser than your typical gas giant. 3.9 vs 1.3 for Jupiter, and (famously) 0.687 for Saturn.

Given such a low average density, you'd have to sink pretty far into Jupiter's atmosphere before you'd float. And that's assuming your own density doesn't increase as well, which it almost certainly would.

A while ago I got curious about whether a gas balloon (as opposed to a hot-air balloon) could work on Jupiter. Although most of its atmosphere is hydrogen, a substantial fraction (24%) of its mass is helium, so a balloon containing pure hydrogen could still float. I found that for reasonable balloons and payloads you'd have to operate well below the cloud decks at the 5-6 bar level (IIRC), and that wouldn't be very scientifically interesting. It would also be pretty dark.

The hot air balloons that have been proposed could operate above the clouds in the <1 bar region.

[raven]

With no significant free oxygen as far as combustion is concerned, you could have a hot hydrogen balloon on Jupiter and other gas giants as well as on Venus, an absolutely terrifying concept on Earth but practical there.

[Kiwi]

A little late now but still worthy of note, a very early article in "Time" magazine about the Van Allen Belts (12 May 1958) which has a final comment that some hoax-believers seem to have missed:
"The crew of an outbound spaceship need not worry about the radiation belt. If moving fast enough to leave the earth, they would pass through it in about 20 minutes."

Time,  Monday 12 May 1958
Science: Radiation Belt

   Space near the earth is not as beset with micrometeorites as some space pessimists have feared. During last week's Washington meeting of the American Physical Society, Drs. Edward Manring and Maurice Dubin of the Air Force Cambridge Research Center told about the experiences of the Army's satellite Explorer I, which carries two meteorite detectors. One of them, a microphone that picks up the slight vibrations in the satellite's shell that are caused by the smallest dust particles, registered only seven hits during the 120 minutes that the transmitter could be heard. The other detector, a set of delicate coils designed to be damaged electrically by meteorites at least 10 microns (1/2500 in.) in diameter, showed no more than one hit (possibly none) in 32 days. The satellite was not damaged, and Manring and Dubin conclude that only long exposure to this concentration of micrometeorites would do it any harm.

Choked Tubes
   Less reassuring news came from a team of cosmic ray experts at the State University of Iowa headed by Dr. James A. Van Allen. Both Explorer I and Explorer III, said Van Allen, ran into a belt of intense radiation at about 600 miles elevation. Each of the satellites carries a single Geiger tube to count cosmic rays. The radio transmitter of Explorer I sends a signal whenever the tube has made 128 counts. Explorer III has a magnetic tape that records the tube's counts during each circuit of the earth and reports to a ground station.
   When the Explorers' orbits were carrying them near the earth, they both reported reasonable numbers of cosmic rays, around 30 per second, but as they climbed up toward their apogees the count came faster. At 1,100 kilometers (684 miles) the tubes registered as high as 140 counts per second. Then a strange thing happened. As the satellites climbed even higher, the transmitters reported no rays at all. During orbit after orbit the counter of Explorer III was silent for 15 minutes. When the satellites swung down again to lower levels, they resumed reporting reasonable numbers of cosmic rays.
   Van Allen was sure that no ray-free belt could exist between the earth and space. The only reasonable explanation, he decided, was that the silenced Geiger tubes had been knocked out temporarily by radiation too intense for them to handle. So he subjected a spare tube to X-ray bombardment in the laboratory. After studying its behavior, he decided that the tubes carried by the satellites must have passed through radiation equivalent to 35,000 counts per second, but were so choked up that they could not report their experience.

Plasma from the Sun
The radiation belt, Van Allen conjectured, is probably a "plasma" made of disassociated hydrogen atoms (protons and electrons) that came originally from the sun and are held high above the earth by the earth's magnetic field. The belt may extend outward for two earth radii (8,000) miles before it disappears. Van Allen suspects that the supply of plasma fluctuates a good deal; the particles tend to leak down to the earth's atmosphere and are replenished from time to time by fresh particles shot into space by disturbances on the sun.
   The radiation zone is by no means a "death belt" that will keep humans from reaching space, but it might do some damage to men who live for a long time in a satellite. Van Allen figured that the radiation level inside the satellite might reach about 0.06 roentgens per hour. At this rate a man would receive in five hours his maximum weekly permissible dose of 0.3 roentgens. A small amount of lead shielding would reduce the dose to a supportable level. The crew of an outbound spaceship need not worry about the radiation belt. If moving fast enough to leave the earth, they would pass through it in about 20 minutes.

[bknight]

A little late now but still worthy of note, a very early article in "Time" magazine about the Van Allen Belts (12 May 1958) which has a final comment that some hoax-believers seem to have missed:
"The crew of an outbound spaceship need not worry about the radiation belt. If moving fast enough to leave the earth, they would pass through it in about 20 minutes."

Time,  Monday 12 May 1958
Science: Radiation Belt

...
Plasma from the Sun
The radiation belt, Van Allen conjectured, is probably a "plasma" made of disassociated hydrogen atoms (protons and electrons) that came originally from the sun and are held high above the earth by the earth's magnetic field. The belt may extend outward for two earth radii (8,000) miles before it disappears. Van Allen suspects that the supply of plasma fluctuates a good deal; the particles tend to leak down to the earth's atmosphere and are replenished from time to time by fresh particles shot into space by disturbances on the sun.
   The radiation zone is by no means a "death belt" that will keep humans from reaching space, but it might do some damage to men who live for a long time in a satellite. Van Allen figured that the radiation level inside the satellite might reach about 0.06 roentgens per hour. At this rate a man would receive in five hours his maximum weekly permissible dose of 0.3 roentgens. A small amount of lead shielding would reduce the dose to a supportable level. The crew of an outbound spaceship need not worry about the radiation belt. If moving fast enough to leave the earth, they would pass through it in about 20 minutes.

Which as you know that Apollo traveled these trajectories.   Too bad the hoaxers don't really study their claims very well.  Very nice information.  It is never too late for good information.

[ka9q]

Ah, so "they" simply "got to" Van Allen even earlier than we thought!

[ka9q]

With no significant free oxygen as far as combustion is concerned, you could have a hot hydrogen balloon on Jupiter and other gas giants as well as on Venus, an absolutely terrifying concept on Earth but practical there.

The problem with a pure hydrogen balloon on Jupiter is that you'd either have to carry the hydrogen all the way from earth, which would be expensive when you consider how hard it is to contain in a small volume for long periods of time, or you'd have to figure out how to extract it from the helium and other things it's mixed with in the Jovian atmosphere.

There's nothing wrong with even hot hydrogen in balloons on Earth as long as you keep it from mixing with (much) oxygen. We use hydrogen to fly high altitude balloon payloads, and I've never heard of anybody having an accident with it. It's even mixed with oxygen in some deep sea diving work, below depths where the O₂ fraction can be kept below about 4% without suffocating the diver. That's the explosive limit.

[bknight]

Ah, so "they" simply "got to" Van Allen even earlier than we thought!

The long arm of NASA's money and power wins again.  Everyone is a shill.

[gwiz]

Ah, so "they" simply "got to" Van Allen even earlier than we thought!

The long arm of NASA's money and power wins again.  Everyone is a shill.

NASA must have time travel, that piece pre-dates them.

[bknight]

All scientists "lie" concerning NASA, from fear of losing money, reputation, job, accreditation, and life, or so many hoaxers respond when asked to cite any that have written a report that would cast dispersions on the Apollo program.

[Cat Not Included]

Ah, so "they" simply "got to" Van Allen even earlier than we thought!

The long arm of NASA's money and power wins again.  Everyone is a shill.

NASA must have time travel, that piece pre-dates them.

Of course. Remember, NASA can do ANYTHING.
Except put a man on the moon.