
Jupiter has 10 more moons we didn't know about - oedmarap
https://www.nature.com/articles/d41586-018-05725-6
======
krylon
Is it time to have a Pluto moment for moons? In the end, all such labels are
arbitrary, but an calling an object of 2 or 3 km radius a moon feels wrong to
me, somehow.

Plus, without a lower boundary, pretty much every single chunk of ice in
Saturn's rings would count as a moon, so having a formalized definition might
come in handy. ;-)

~~~
autokad
well, I certainly think pluto is more of a planet than these are moons

~~~
dmoy
I think they mean to make a formal definition of moon that would exclude some
of the new Jupiter moons, similar to the new formal definition of planet
excluding Pluto from being a planet.

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cup-of-tea
I wonder if discovering lots of tiny moons will lead to a new definition for
moon like what happened with the planets.

~~~
JFFalcon
Yes, perhaps these will be reclassified as satellites and moons will have to
be large enough to form spheres under their own gravity. That would mean that
Mars would have no moons though.

~~~
fvdessen
Can you expand a bit on why a satellite would need the planet's gravity to
keep a spherical shape ?

~~~
mnw21cam
No, we're talking about a satellite forming into a sphere under its _own_
gravity, not the planet's. Given a large enough lump of matter, gravity will
overcome structural strength and force the material to flow into a roughly
spherical shape. On Earth, this happens with the maximum mountain height being
a few kilometers, but on Mars' moons, which are much smaller, the gravity is
far too weak to overcome the rock's structure, and they are basically strange
shaped rock things.

~~~
gonvaled
Given enough time [1], all solid objects become perfect spheres, no matter
their size.

[1] 10^65
[https://en.wikipedia.org/wiki/Timeline_of_the_far_future](https://en.wikipedia.org/wiki/Timeline_of_the_far_future)

~~~
jerf
I am comfortable with a definition of "moon" that does not involve
extrapolating 10^65 years into the future.

~~~
justusthane
Well, then you'd need to define an arbitrary time limit for how long it would
take to form a sphere. In which case, you might as well have just defined a
size/mass limit in the first place.

~~~
TheCoreh
I think the definition would be that they're presently a sphere, that was
formed under its gravity. That way objects could possibly become moons in the
future, but the timescales are so long it's mostly irrelevant

------
joshe
The eighth moon sounds exciting... "it travels in the same region of space as
the retrograde moons, but in the opposite direction... That means it could
easily smash into the retrograde moons, pulverising itself into oblivion."

Setting up science nerds for thousands of years of suspense!

~~~
MrLeap
Wouldn't it be a spooky experience to stand on one moon, and have another pass
close enough for you to touch it? If they were of similar mass my intuition is
that you'd go weightless while it was happening. Might be best to be harnessed
during something like that.

While I'm thinking about spooky space stories. Jupiter creeps me right the
hell out. Thinking about being on one of its moons where Jupiter takes up a
HUGE percentage of your visible sky and is just screaming radiation at you.
Personify the gas giant a little and you've got a decent proxy for hell.
[https://solarsystem.nasa.gov/resources/184/simulated-view-
fr...](https://solarsystem.nasa.gov/resources/184/simulated-view-from-europas-
surface-artists-concept/?category=planets_jupiter)

~~~
joshe
That would be amazing, it looks like Europa travels at 14 km/s or 31,000 mph.
With no atmosphere there would be no sound or pressure wave. And the bodies
are probably small enough to not shatter because of the gravity interaction.
Escape velocity is about 36,000 mph. So you could see a huge Manhattan size
chunk of rock flying at you at rocket speeds. Would be amazing to see it pass
by 100 feet above you.

~~~
ars
> Would be amazing to see it pass by 100 feet above you.

At those speeds, and the necessary angle, I'm not convinced you would be able
to see it at all.

The actual encounter would last for too short of a time to see (like trying to
see a bullet in flight), and their motion would take them out of your field of
view, unless the planet you were on was rotating very fast.

~~~
joshe
Cool question/objection.

The two moons are going in opposite directions, so the speed guess is really
28 km/s.

For a full second it would be 2km across and 14km or less away (.5 seconds
before and .5 seconds after it's overhead). That's 16 degrees of visual angle.
The moon is .5 degrees of visual angle, so you get a full second of something
32 times bigger than our moon.

It would be moon sized about 230 km away, which is over 8 seconds away. So
you'd get to see a moon grow quite large over 8 seconds.

So you would see it, but would it be awesome?

Lets say our eyes can see things in the 100 ms range (really a bit better).
For that 100 ms, the object would be < 1.4km from you and subtend fully 71
degrees of your vision. That's about 2 basketballs width at arms length.

Which sounds cool though maybe not awesome. But hey you are looking at a moon
rushing at your face like a basketball, I would totally go.

[https://wolfe4e.sinauer.com/wa03.01.html#Calculator](https://wolfe4e.sinauer.com/wa03.01.html#Calculator)

~~~
ars
You have to check the angles. Think of the X I posted earlier. Imagine your
goal is to stand at the point looking "up" (because that's were they will
meet), both moons were traveling bottom to top, and you were on the moon that
started bottom left. (So your gaze is at an angle to direction of travel.)

For most of the time the moon will be behind you to your right out of sight.
After the interaction, if the other moon was larger than yours, you might be
able to see it.

But if it was smaller, then your own moon's horizon would block your view of
it.

But it's worse than that - even if you could see it, you would have to turn
your head to the left to see it, but it's moving so fast you just don't have
enough time to physically move your head before it's far far away.

i.e. It would whiz by _across_ your field of view, not toward it. Which makes
it really hard to see.

Edit: I'm tying to visualize how it would look, but I may have missed some
detail. It would be cool if someone could program a visualization of this.

~~~
joshe
In the spirit of "could this be awesome" rather than is it likely...

So pretend we stand on the side of the moon facing toward Jupiter so that
Jupiter is straight overhead. And pretend the other moon will pass just inside
our orbit 100 ft above our head.

Imagine 2 quarters overlapping, representing the orbits. Pushing the top one
up a bit keeps the inner moon completely above the horizon for the whole
approach. You can also push the top quarter to the left a smidge so that the
inner moon orbit is coming from high and to the right. This would be a moon
sized object popping up high and right in the sky and then disappearing in a
blink below the horizon to your left.

Speed isn't an issue because for the 8 full seconds before it's right overhead
it will be our moon sized and bigger. Up to a 2 "arm's length basketballs"
when right overhead. You could definitely track the moon across the half the
sky over 8 seconds.

~~~
MrLeap
I was thinking a bit more on this today. Specifically If a moon did this past
earth and assuming it was made out of something that wouldn't just break apart
in our atmosphere.

The amount compression on the air around you might set you on fire.

That would be interesting.

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wlesieutre
Other recent articles I saw about this said twelve, this one says ten. Anyone
know why?

[https://www.npr.org/2018/07/17/629396121/galileo-would-be-
st...](https://www.npr.org/2018/07/17/629396121/galileo-would-be-stunned-
jupiter-now-has-79-moons)

If you look at Wikipedia's list and sort by year of discovery, 10 would
include the ones in 2018 and 2017, while 12 includes two more in 2016. All 12
are discovered by Sheppard et al. Perhaps those two aren't news anymore?

~~~
bmm6o
Right, they are talking about roughly the same discoveries but some include 2
more: [https://astronomy.stackexchange.com/questions/26999/did-
we-d...](https://astronomy.stackexchange.com/questions/26999/did-we-
discover-10-or-12-new-moons-of-jupiter)

------
codeduck
Title should read "At least"

A planet as massive as Jupiter will have captured lots(1) of objects.

1\. Terry Pratchett Troll counting system.

~~~
Jaruzel
I'm gonna paste this in because pterry was a god (small G), and there should
be no point ever where a reference to him shall go unnoticed.

 _‘Everyone knows trolls can’t even count up to four!’

(In fact, trolls traditionally count like this: one, two, three, many, and
people assume this means they can have no grasp of higher numbers. They don’t
realise that many can BE a number. As in: one, two, three, many, many-one,
many-two, many-three, many many, many-many-one, many-many-two, many-many-
three, many many many, many-many-many-one, many-many-many-two, many-many-
three, LOTS.)_

~~~
fabricexpert
Presumably because trolls only have 4 fingers on each hand?

~~~
rainbowmverse
There are finger-based counting systems with more than one number per finger.
Our average of 10 fingers can handle a lot more than 10 numbers.
[https://en.wikipedia.org/wiki/Finger-
counting](https://en.wikipedia.org/wiki/Finger-counting)

~~~
tejtm
And less than a digit per err digit (thumbs included)

If I recall there was a Native American base 8 counting system which utilized
the space between fingers holding sticks or strings as place holders.

Made for a handy octal abacus.

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atulatul
This reminds me of the discussion(s) on BBC QI (Quite Interesting) about 'how
many moons does the earth have'.

A video from QI official channel:
[https://www.youtube.com/watch?reload=9&v=7qZWM8Aatb8](https://www.youtube.com/watch?reload=9&v=7qZWM8Aatb8)

And a compilation of updates over the years. It has Stephen Fry discussing the
topic but in recent seasons Sandy Toksvig discussed the topic as well :
[https://www.youtube.com/watch?v=sGgmmX-
dzgU](https://www.youtube.com/watch?v=sGgmmX-dzgU)

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albemuth
Awesome, new Eclipse IDE versions!

------
walrus01
For reference:
[https://en.wikipedia.org/wiki/Moons_of_Jupiter](https://en.wikipedia.org/wiki/Moons_of_Jupiter)

Its gravitational field is so huge that it is basically the vacuum cleaner of
the solar system. Also a lot of things that are not in orbit of it are
occasionally significantly perturbed in their solar orbits, by Jupiter, such
as a number of long period comets.

~~~
tomp
How exactly does that work though?

An orbit can't _just_ change from a parabola into an ellipse, without the
influence of external forces. Is something slowing down these object so that
Jupiter "catches" them? (Rings?)

~~~
Symmetry
You're right that in a two body system this is totally impossible. But for a
three body system of Jupiter, the Sun, and our new moon then ballistic capture
will _baaaasically_ get you into orbit and then it only takes a small further
interaction with another planet, another moon, or whatever to get into orbit
the rest of the way.

~~~
ars
> the Sun

Jupiter is in a stable orbit about the Sun, I don't think you can use the Sun
as the 3rd body, you need another moon or other object.

~~~
Symmetry
The important thing, as far as I understand it, is the way the future moon
slowly transitions from being primarily under the gravitational influence of
the Sun to primarily under the orbital influence of Jupiter.

We've actually used ballistic transport to transition probes from orbit around
the Earth to basically in orbit around the Moon with just a tiny braking burn
at the far side instead of the serious burn that a Holman Transfer orbit would
require. And the Moon is in a stable orbit around the Earth and doesn't have
any further satellites.

~~~
ars
> slowly transitions

There is no slowly for capturing an incoming object - you get one chance,
unless that object is in a similar orbit to your (and in that case the Sun
doesn't help).

> with just a tiny braking burn

Tiny is relative here. It's still enough of a burn to significantly change the
momentum of the rocket. Moons don't have anything with that kind of energy,
except other moons.

~~~
Symmetry
No, slow is a fair description of this. You need to start out in nearly the
same orbit as the target and while you're both traveling around the Sun or
Earth fairly quickly you approach the target much more slowly than you would
in a Holman Transfer. Then when you're captured you'll be going around the
target a number of times before other gravitational perturbances eject you
again, giving a long time to do a burn to get into a stable orbit. So yes you
have to change the momentum of the rocket but the burn could take place over
the course of a week and won't have to be very large. Your burns to achieve an
intersect giving you a ballistic capture would typically be much larger than
with a Holman transfer but the slowness of the final approach means you're
free to use an ion drive, solar sail, or other high efficiency and low thrust
drive.

------
rezeroed
"All the newfound moons are small, between about 1 and 3 kilometres across.
Seven of them travel in remote orbits more than 20 million kilometres away
from Jupiter"

As a layman with no astronomical credibilty I'd call those satellites, not
moons.

~~~
baseethrowaway
Natural satellite == moon.

"Natural satellite" is a subset of "satellite", thus what you'd call them is
correct by definition. Let's let the astronomers with credibility play and
name the objects.

~~~
jovial_cavalier
So does Saturn have billions of moons?

~~~
Nimelrian
Saturn's rings are a bit more special. They are actually only 10-100m thick.
There's still a difference (however small it may be) between those and
kilometer sized rocks.

------
walrus01
Fun fact, asteroids in the belt can also have natural satellites:

[https://en.m.wikipedia.org/wiki/87_Sylvia](https://en.m.wikipedia.org/wiki/87_Sylvia)

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carrja99
Incredible.

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lamarpye
There goes the neighborhood.

------
Shazbaht
This is a great example of the hubris of scientists that non-scientists can’t
stand.

Most believe Pluto is a planet. Moons are rocks in orbit around planets, etc.

Why the _obsession_ to change these names? Brings painful thoughts of our
future if this is what they care about.

~~~
Symmetry
It's in the nature of science that sometimes things that were previously
believed to be true turn out to be false. The notion that there are nine
planets matched what astronomers knew for many decades but with better
measurements turned out be be an unsupportable idea.

For centuries there were just 6 planets. Then people discovered Uranus and
there were 7. Then people discovered Ceres and there were 8. Then people
discovered lots of other main belt asteroids and there were 7 again. Then
Neptune and 8 again. Then Pluto and 9. And now we've found all these other big
bodies in the outer system and we decided to demote Pluto rather than add a
half dozen new planets, just like we did with Ceres.

I don't think anybody particularly objected to the demotion of Ceres since it
hadn't been around that long when it happened. But I can only imagine the
furor that must have arisen when people tried to claim that Uranus was a
planet despite the number of planets being fixed since the time of Homer.
Well, Ok, there was the bit when they turned Earth into a planet and we all
know about the people like Giordano Bruno literally burned at the stake over
that.

So this demotion of Pluto is just another event in the long, long history of
scientists revising what counts as a planet. I understand that it's
uncomfortable to have to unlearn things you learned as a kid but science is
all about changing our view of the world.

~~~
aeorgnoieang
A great { essay / blog post } about this constant churn:

\- [Ontological remodeling |
Meaningness]([https://meaningness.com/eggplant/remodeling](https://meaningness.com/eggplant/remodeling))

My favorite bit:

> Jupiter doesn’t go around the sun, and therefore is not a planet by the 2006
> definition.

> Don’t believe me? In Newtonian mechanics, two bodies orbit their barycenter,
> or center of mass. If they have equal masses, the barycenter is the midpoint
> between them. If one is heavier than the other, the barycenter is closer to
> it. If one has much greater mass than the other, their common barycenter is
> located within the larger body, and the smaller object goes around that
> point. Only then is the smaller body said to orbit the larger one.
> Otherwise, the two form a binary system.

> Jupiter is ludicrously heavy: it has 2.5 times the mass of everything else
> in the solar system combined, apart from the sun. The sun is much heavier
> still—but the barycenter of their mutual orbit is outside it. Jupiter and
> the sun are a binary system. Their barycenter is, to be fair, quite close to
> the sun, and informally it may be reasonable to say Jupiter goes around it.
> But in terms of the formal definition, it doesn’t, so by the IAU criteria,
> Jupiter is not a planet.

~~~
Symmetry
I'm not sure why you would say that A isn't orbiting B if the barycenter of
the orbit isn't inside B? We normally talk about Jupiter orbiting the Sun or
Charon orbiting Pluto despite this not being true. I can see why someone might
want that to be the definition for aesthetic reasons but I'm not seeing any
evidence that it's the accepted definition. Wikipedia certainly says "near or
within" instead of "within" in the most relevant section and I couldn't find
anything clearer or contradicting that.

~~~
aeorgnoieang
> I'm not sure why you would say that A isn't orbiting B if the barycenter of
> the orbit isn't inside B?

If A and B were roughly the same mass then you'd say they're orbiting each
other. We _don 't_ tho say or write that about Jupiter in my experience.

