Human Life and Comets
Lexell found that this space body had passed earth a mere 2.2 million - six times the distance to the moon - away, the closest any comet has ever been to Earth and, a very near miss.
Human Life and Comets
Frenchman Charles Messier was most famous for his catalogue of 110 galaxies, nebulae, clusters and double stars, but at heart was a comet-hunter, who found a total of 13 comets from 1760-1785, most importantly the one spotted in June 1770, a brief spectacle for eighteenth century astronomers to enjoy, that has since become known as Lexell's Comet, after Anders Johan Lexell who first calculated its orbit.
Lexell found that this space body had passed earth a mere 2.2 million - six times the distance to the moon - away, the closest any comet has ever been to Earth and, a very near miss. He discovered that the comet had an orbital period of just under six years, but the comet for unknown reasons never reappeared.
It is now believed that Lexell is a lost comet - Jupiter the reason why -acting as protector to the earth by sweeping up or ejecting many of the long period comets from the solar system, such as the infamous Shoemaker-Levy 9, which spectacularly perished in the Jovian atmosphere.
Today we know of lots more such objects that cross our planet's orbit. and University of New South Wales researcher, Jonathan Horner, along with the UK's Open University Barrie Jones have run new simulations of near-earth approaches, one of which potentially having important consequences.
Employing 21st century computing power, their simulations agreed that Jupiter does play a big part in protecting Earth, and while most asteroids move around the sun on stable orbits, Jupiter's influence should not be under-estimated in sending such bodies inwards within the solar system.
Their simulations showed that asteroid impact numbers on Earth peak whenever a planet 20% of its own mass comes within Jupiter's orbit. The role of the gas giant seems confused though, because even though it offers some protection, it nonetheless definitely sends asteroids and comets our way, so it may be such a friend after all.
In the grand scheme of things, the question remains whether comet impacts are positive or negative evolutionary influence over billions of years, and if indeed the search for potentially habitable exoplanets, should avoid systems containing such gas giants. That impacts have caused mass extinctions is not in doubt, nor that this almost certainly helped human evolution, but the usefulness of comet impacts may depend on frequency.
Earth's geological record suggests it taking around 10 million years for the planet to recover from big hits, so one every 1000 million years or so allows plenty of time between for life to flourish. Impacts also, it appears, have additional benefits in that they often bring water to dry worlds.
Earth is believed to have got its water from objects crashing into it, which in the beginning were countless, and evidence for this lies in the fact that isotopes of Earth water hydrogen are not a match to cometary isotopes.
Impacts undoubtedly brought water to Earth and paved the way for humanity to evolve, but we must never, ever stop watching the skies for those near Earth objects which get by our not-so-friendly Jovian guardian, because one of them just might have our number on it.