Two New Satellites for Jupiter(skyandtelescope.com)
skyandtelescope.com
Two New Satellites for Jupiter
http://www.skyandtelescope.com/astronomy-news/two-new-satellites-for-jupiter/
12 comments
Spotting a 1km moon in orbit around Jupiter is the equivalent of spotting a Rabies virus at the other end of a football field. (1km at 588 million km ~= 185nm at 109m) It's pretty amazing that we can do that.
> Spotting a 1km moon in orbit around Jupiter is the equivalent of spotting a Rabies virus at the other end of a football field. (1km at 588 million km ~= 185nm at 109m)
While a relevant size comparison, this is potentially misleading characterization of our ability to detect these types of satellites. 1 km at 588 million km works out to an angular size of ~6e-6 radians, or 3e-4 arcseconds. The resolution of the telescopes that took the discovery data are typically* 0.3-1 arcseconds (depending on the atmospheric conditions at the site). So the telescope is not even close to being able to directly measure the physical scale of the moon in question. Best case, it's off by ~3 orders of magnitude. (Note of course there are variations in the apparent angular size due to the orbits of Earth and the Moon, but even at the closest the moon is too small to be directly resolved by current ground-based telescopes).
The reason we are able to detect these moons is a combination the albedo of the moon (its reflectiveness) and the still somewhat bright sunlight at the distance of Jupiter. So the main requirement here is that the telescope can see faint objects.
* - Note that near-infrared telescopes on the ground can achieve higher resolution with corrective "adaptive optics", but that is limited to follow-up observations of objects, not initial survey work.
While a relevant size comparison, this is potentially misleading characterization of our ability to detect these types of satellites. 1 km at 588 million km works out to an angular size of ~6e-6 radians, or 3e-4 arcseconds. The resolution of the telescopes that took the discovery data are typically* 0.3-1 arcseconds (depending on the atmospheric conditions at the site). So the telescope is not even close to being able to directly measure the physical scale of the moon in question. Best case, it's off by ~3 orders of magnitude. (Note of course there are variations in the apparent angular size due to the orbits of Earth and the Moon, but even at the closest the moon is too small to be directly resolved by current ground-based telescopes).
The reason we are able to detect these moons is a combination the albedo of the moon (its reflectiveness) and the still somewhat bright sunlight at the distance of Jupiter. So the main requirement here is that the telescope can see faint objects.
* - Note that near-infrared telescopes on the ground can achieve higher resolution with corrective "adaptive optics", but that is limited to follow-up observations of objects, not initial survey work.
"Actually" depending on the wavelengths used, they'revery different things ;)
> The great majority of Jupiter's 69 known moons travel in retrograde orbits, meaning they travel in the direction opposite the planet's spin.
Sketchy! I don't suppose we have a prediction for the next anticipated lunar collision? Surely this system isn't stable.
If two moons in opposite orbits collide, then there would presumably be an even more spectacular event when the majority of the combined mass of those moonlets fall straight down into Jupiter. Imagine a ~1km ball of magma zipping past the Roche limit and punching a hole right through the Jovian atmosphere.
Sketchy! I don't suppose we have a prediction for the next anticipated lunar collision? Surely this system isn't stable.
If two moons in opposite orbits collide, then there would presumably be an even more spectacular event when the majority of the combined mass of those moonlets fall straight down into Jupiter. Imagine a ~1km ball of magma zipping past the Roche limit and punching a hole right through the Jovian atmosphere.
http://home.dtm.ciw.edu/users/sheppard/satellites/jupsatdata...
Having a quick look at the data shows the inner moons travel in prograde, while the outer ones travel in retrograde.
So although there may still be collisions in the future, unfortunately it looks like most of the collisions happened long long ago.
Having a quick look at the data shows the inner moons travel in prograde, while the outer ones travel in retrograde.
So although there may still be collisions in the future, unfortunately it looks like most of the collisions happened long long ago.
Aren't Saturn's rings hypothesised to have come about from moons smashing or disintegrating?
There is another theory suggesting otherwise [1]:
Saturn’s mid-sized moons are like the monsters in late-night horror movies. Smash them into tiny pieces, and they glue themselves back together as new versions of the old moons. This new finding contradicts a theory that Saturn’s rings were caused by moons colliding.
[1]: https://www.newscientist.com/article/2132906-saturns-moons-c...
Saturn’s mid-sized moons are like the monsters in late-night horror movies. Smash them into tiny pieces, and they glue themselves back together as new versions of the old moons. This new finding contradicts a theory that Saturn’s rings were caused by moons colliding.
[1]: https://www.newscientist.com/article/2132906-saturns-moons-c...
Great question. That would be spectacular. Have we ever observed such an event? Seems like, statistically, relatively close solar bodies would rarely collide.
Comet Shoemaker-Levy 9 in 1994. The comet had been captured decades earlier after entering Jupiter's Roche limit and was actually in orbit around Jupiter.
Collisions in the main asteroid belt have been observed, but only after impact. The signature is a puff of dust from the impact that fades after hours or days as the dust spreads. The statistics are imprecise, but collisions between kilometer-scale asteroids might have decades or hundreds of years between them.
Impacts are rare though. There's a reason they call it space. If you were standing on an asteroid in the main belt, the nearest asteroid to you would be so far away that you could not see it.
Collisions in the main asteroid belt have been observed, but only after impact. The signature is a puff of dust from the impact that fades after hours or days as the dust spreads. The statistics are imprecise, but collisions between kilometer-scale asteroids might have decades or hundreds of years between them.
Impacts are rare though. There's a reason they call it space. If you were standing on an asteroid in the main belt, the nearest asteroid to you would be so far away that you could not see it.
Do we know why the lunar orbits are all generally on one plane? Why don't they go over the poles?
The Kozai-Lidov effect (https://en.wikipedia.org/wiki/Kozai_mechanism) causes polar and near-polar orbits to be unstable over long time scales.
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