What we don’t know about them would fill a book and most of the books we’ve already filled are wrong.
These periodic (short and long) or non periodic visitors to the inner Solar System have been observed since the dim mists of history at least, unsurprisingly so since some of them are bright enough to be easily visible even during daytime. They were superstitiously thought to to be that harbingers of great, catastrophic, events.
Since the time of Tycho Brahe we’ve known that comets are exo-atmoshperic objects made more curious by the fact their commas, or tails, always point away from the sun regardless of their actual direction of travel. This observation led to the discovery of Solar Wind.
You see the sun doesn’t just emit radiation (energy, which is matter), it likewise emits particles (which are also matter) and this stream hits the surface of the comet and erodes it, blowing away dust and gasses while ionizing them like a lightbulb and producing the characteristic ‘tail’. Since the ‘wind’ is so much faster than the comet itself the tail points away from the sun even when the comet is moving outward in the Solar System.
The prominence of these tails led scientists to speculate that the composition of a comet was different from an asteroid, those rocky chunks of leftover planet making stuff most of which has been swept out of the Inner System and flung into the Kuiper Belt and Oort Cloud (if not into Interstellar space), or swallowed up by the Sun or Jupiter all because of their massive (heh, only like 99% of the entire mass of the Inner System) gravitational influence, or parked in a more or less safe orbit between Mars and Jupiter.
The scientific myth was that comets were made of more insubstantial things, like ice, and accreted of their own accord far from the warmer, more attractive (because more massive) climes closer to that sustained fusion bomb we call Sol. Indeed some went so far as to assert that the bulk of Earth’s water comes from comet impacts.
Not so much.
As it turns out, comets are not that different from asteroids after all.
Mystery of Earth’s Water Origin Solved
Andrew Fazekas, National Geographic
Published October 30, 2014
To pin down the exact time of the arrival of Earth’s water, the study team turned to analyzing meteorites thought to have formed at different times in the history of the solar system.
First, they looked at carbonaceous chondrite meteorites that have been dated as the oldest ones known. They formed around the same time as the sun, before the first planets.
Next they examined meteorites that are thought to have originated from the large asteroid Vesta, which formed in the same region as Earth, some 14 million years after the solar system’s birth.
“These primitive meteorites resemble the bulk solar system composition,” said Sune Nielsen of the WHOI, a study co-author. “They have quite a lot of water in them, and have been thought of before as candidates for the origin of Earth’s water.”
The team’s measurements show that meteorites from Vesta have the same chemistry as the carbonaceous chondrites and rocks found on Earth. This means that carbonaceous chondrites are the most likely common source of water.
“The study shows that Earth’s water most likely accreted at the same time as the rock,” said Marschall.
But comets have tails!
Comet-Like Asteroid Boasts Dusty Tail
By Jenna Iacurci, Nature World News
Nov 12, 2014 02:37 PM EST
In a case of mistaken identity, a newly active asteroid in our solar system’s famous Main Belt is boasting a dusty tail, thinking it’s more a comet than an asteroid, according to recent research.
Usually it’s easy to tell the difference between a comet and an asteroid. A comet is a body composed of rock and ice that, when it passes close to the Sun, heats up and begins to sublimate, displaying a visible tail or coma. Asteroids, on the other hand, are composed mostly of rock and typically have few comet-like qualities.
But in recent years several asteroids have broken the boundaries of their definition and begun to sport dusty tails. A dozen of such unusual asteroids in the main asteroid belt have been identified thus far, and now a long-known asteroid is joining the club.
Called 62412, it’s the first comet-like object seen in the Hygiea family of asteroids, and only the 13th known active asteroid in the main asteroid belt, located between Mars and Jupiter. Active asteroids, unlike others of their kind, sometimes sport a tail when dust and gas is ejected from their surface, giving them a comet-like appearance.
Not only that, but it should come as no surprise. Quite a large percentage of Meteors are from comet trails rather than Asteroids and you know what? They look exactly the same. To the extent that Asteroids are slightly less volatile you should remember they’ve been sun blasted for about 4.5 Billion years.
Indeed it’s highly likely that most of the far Solar objects originated much closer to the sun than is commonly believed–
(I)t is suggested that this planetary system evolved in the following manner. Planetesimals at the disk’s inner edge occasionally pass through gravitational encounters with the outermost giant planet, which change the planetesimals’ orbits. The planets scatter inwards the majority of the small icy bodies that they encounter, exchanging angular momentum with the scattered objects so that the planets move outwards in response, preserving the angular momentum of the system. These planetesimals then similarly scatter off the next planet they encounter, successively moving the orbits of Uranus, Neptune, and Saturn outwards. Despite the minute movement each exchange of momentum can produce, cumulatively these planetesimal encounters shift (migrate) the orbits of the planets by significant amounts. This process continues until the planetesimals interact with the innermost and most massive giant planet, Jupiter, whose immense gravity sends them into highly elliptical orbits or even ejects them outright from the Solar System. This, in contrast, causes Jupiter to move slightly inward.
The low rate of orbital encounters governs the rate at which planetesimals are lost from the disk, and the corresponding rate of migration. After several hundreds of millions of years of slow, gradual migration, Jupiter and Saturn, the two inmost giant planets, cross their mutual 1:2 mean-motion resonance. This resonance increases their orbital eccentricities, destabilizing the entire planetary system. The arrangement of the giant planets alters quickly and dramatically. Jupiter shifts Saturn out towards its present position, and this relocation causes mutual gravitational encounters between Saturn and the two ice giants, which propel Neptune and Uranus onto much more eccentric orbits. These ice giants then plough into the planetesimal disk, scattering tens of thousands of planetesimals from their formerly stable orbits in the outer Solar System. This disruption almost entirely scatters the primordial disk, removing 99% of its mass, a scenario which explains the modern-day absence of a dense trans-Neptunian population. Some of the planetesimals are thrown into the inner Solar System, producing a sudden influx of impacts on the terrestrial planets: the Late Heavy Bombardment.
Eventually, the giant planets reach their current orbital semi-major axes, and dynamical friction with the remaining planetesimal disc damps their eccentricities and makes the orbits of Uranus and Neptune circular again.
Why is this relevant today?
Well, we just landed a probe on a comet, first time ever.
Philae lander makes historic touchdown on comet
Ian Sample and Stuart Clark, The Guardian
Wednesday 12 November 2014 19.30 EST
The feat marks a profound success for the European Space Agency (ESA), which launched the Rosetta spacecraft more than 10 years ago from its Kourou spaceport in French Guiana. Since blasting off in March 2004, Rosetta and its lander Philae have travelled more than 6bn kilometres to catch up with the comet, which orbits the sun at speeds up to 135,000km/h.
Landing Philae on the comet’s surface was never going to be easy. When ESA managers got their first closeup of the comet in July, its unusual rubber duck shape left some fearing that a safe touchdown was impossible. The shape was not the only problem. The comet’s surface was hostile: hills and spectacular jutting cliffs gave way to cratered plains strewn with boulders. If Philae landed on anything other than even ground it could topple over, leaving it stranded and defunct.
On Tuesday night, hours before Philae had left its mothership, the chances of a safe landing took another dip. Overnight, a thruster on the lander failed to respond to commands sent from Earth. Engineers tried for hours to correct the fault but to no avail. The malfunction threatened to abort the mission, but at 0235 GMT on Wednesday mission controllers decided to go ahead with the landing regardless.
The nitrogen thruster, facing upwards from the top of the lander, was designed to fire for 60 seconds as Philae touched down to prevent it from bouncing off the comet’s surface where the gravitational pull is several hundred thousand times weaker than on Earth.
Can Philae hold on? Fears for comet mission as controllers reveal harpoons that should have tethered lander failed to work – causing it to BOUNCE as it landed
By Jonathan O’Callaghan and Ellie Zolfagharifard and Mark Prigg, Daily Mail
Published: 02:45 EST, 13 November 2014
Philae’s cold thruster is nitrogen-powered and is designed to fire on landing in order to prevent the probe from flying off into space due to the comet’s weak gravity.
In order to prepare cold-gas jets, scientists use one of two pins to puncture a wax seal on the thruster’s gas tank. Experts detect success by the change in pressure in the piping system.
However, this morning mission controllers did not see pressure increases after two attempts with each of the two pins. But according to the industry provider, there may still be a chance that retrying the puncture of the wax seal would succeed, even after four failed attempts.
Now this is an amazing feat of celestial navigation made more so by the irregular shape of the target, its speed and distance, activity of the landing area, and low gravity, but it was not perfect. Since the engineers expected the surface to be icy they employed harpoons as anchors. Well, it’s more rock than ice. Moreover a top thruster was supposed to fire to ensure positive contact during a sub-surface drilling operation has so far been unresponsive and it’s unknown if that part of the mission can be completed.
The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations – then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation – well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
–Sir Arthur Stanley Eddington, The Nature of the Physical World (1927)
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