I won’t go into why I was looking at orbits ( another posting soon…) but along the way, I ran into an annoying picture of the “orbital neighborhood”. That caused me to do a bit more looking that will likely have me not sleeping as well as I’d like for a while. First up, what started this?
OK, I was looking at Milankovitch, and that lead to nutation and obliquity and… the influence of other planets. That lead to looking at the other planets. That lead to:
The animation illustrates Pallas’ near 18:7 resonance pattern with Jupiter. The motion of Pallas is shown in a reference frame that rotates about the sun (i.e., the center dot) with a period equal to Jupiter’s orbital period. Accordingly, Jupiter’s orbit appears almost stationary as the pink ellipse at top left. Mars’ motion is orange, and the Earth-moon system is blue and white. The orbit of Pallas is green when above the ecliptic, and red when below. The near 18:7 resonance pattern with Jupiter only marches clockwise: it never halts or reverses course (i.e., no libration).
For those not familiar with it, Pallas is a fairly large asteroid and one of the first found. It is technically named “2 Pallas” as it was the #2 one found. The wiki:
Has a write up on it that’s not too bad.
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered, by astronomer Heinrich Wilhelm Matthäus Olbers on March 28, 1802. It was at first considered a planet, as were other early asteroids, until the discovery of many more led to their re-classification. Pallas appears to be the largest irregularly shaped body in the Solar System (that is, the largest body not rounded under its own gravity), and a remnant protoplanet.
With a mass estimated to be 7% of the total mass of the asteroid belt, Pallas is one of the largest asteroids. Its diameter is some 530–565 km, comparable to or slightly larger than that of 4 Vesta, but it is 20% less massive, placing it third among the asteroids. The Palladian surface appears to be a silicate material; the surface spectrum and estimated density resemble carbonaceous chondrite meteorites. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the main asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.
So watch that animation for a little bit. Notice that it starts to share the orbit of Mars after a while? One can only hope that it has some kind of ‘orbital resonance’ that prevents it from trying to be in the same place at the same time. If those two whack each other, it’s going to be A Bad Day In The Neighborhood!
Also notice just how wide ranging that orbit is! Way outside the ecliptic AND wandering over a very wide range of orbital diameters. I also note in passing that the Mars band is a lot wider than I’d expected too. That puppy gets around! Mercury has a much wider ranging orbit than I’d imagined too.
OK, so this gets me wondering about just how many OTHER asteroids there are out there whizzing around. That leads to the next picture. That’s also very “Not Good”.
First off, that’s a mighty crowded neighborhood. Second, Mars is a heck of a lot closer to those guys than we are. The odds of it ‘taking a rock’ ought to be fairly high.
Then, third, notice those “Trojans and Greeks”? (and the Hildas)?
This is not at all like that skinny little well defined circle of asteroids shown in the classroom when I was a kid. Far more of them, far wider dispersion. The whole place is crowded with them.
There’s a whole slew of those guys out hanging with Jupiter. (I note in passing that the bumbled definition of a planet they’ve come up with has ‘special allowances’ for the “cleared the neighborhood” rule such that things sharing the orbit but ‘under your gravitational influence’ are not counted as ‘uncleared stuff’. That way Jupiter gets to stay a planet even though it’s orbit is clearly full of uncleared junk while Pluto takes a demotion because… it’s orbit is not cleared of junk…)
OK, that is just one heck of a lot of stuff, and with Pallas getting pally with Mars, I’m wondering what else is a ‘mars crosser’? Turns out there are way to many to list here. I’m just going to put in links to the Mars Crosser and Earth Crosser wiki pages. The notion that we have our own orbit or that it’s clear of ‘junk’ is just wrong. We have an orbital neighborhood full of all sorts of stuff, for all of us “Planets”…
Over 1151 “Potentially Hazardous Objects” are known so far for Earth and we’ve only just started really looking.
Now look back at that Mars orbit. No wonder the poor dear has craters on it.
It looks to me like collisions ought to be pretty common things and like we ought to be seeing a lot more of them than we seem to be having. Yeah, I know, “a couple a year” are big enough to look like small nukes to the Test Ban Treaty satellites so they’ve got to filter out the ‘single light pulse’ from the ‘double light pulse’ of a nuke. Air “burst” mostly over the middle of nowhere with not much around. But still. Some of those things are big!
At any rate, I’m getting that “Duck in a shooting gallery feeling” and not thinking much about CO2 hazards at the moment ;-)
But Wait, There’s More!
This little diagram shows where SOME of the outer objects are located.
Red = The Sun
Aquamarine = Giant Planet
Green = Kuiper belt object
Orange = Scattered disc object or Centaur
Pink = Trojan of Jupiter
Yellow = Trojan of Neptune
Axes list distances in AU, projected onto the ecliptic, with ecliptic longitude zero being to the right, along the “x” axis).
Positions are accurate for January 1st, 2000 (J2000 epoch) with some caveats: For planets, positions should be exact. For minor bodies, positions are extrapolated from other epochs assuming purely Keplerian motion. As all data is from an epoch between 1993 and 2007, this should be a reasonable approximation.
Data from the Minor Planet Center or Murray and Dermott as needed.
Radial “spokes” of higher density in this image, or gaps in particular directions are due to observational bias (i.e. where objects were searched for), rather than any real physical structure. The pronounced gap at the bottom is due to obscuration by the band of the Milky Way
Who needs to invent ‘dark matter’? The whole place is chock full of regular old matter whizzing around!