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Since space scientists spotted for the first time the strange cigar-shaped object known as Oumuamua in the sky, they debated what it is exactly. Suggestions included an asteroid, a comet and even an extraterrestrial spaceship. Now, a study, published in Nature, suggests that it could be a comet – but an unusual one.
The fascinating discoveries add to recent discoveries that suggest that it may be time to think beyond the division between asteroid and comet. The force of gravity is what keeps our feet on the ground, the moon orbiting the Earth and the planets orbiting the sun. It is also the main agent that governs the trajectory of comets as they traverse the solar system. It was his laws of gravity that allowed Newton's colleague, Edmond Halley, to predict the return of the comet that is now known as Halley's Comet. The same laws allowed the Rosetta probe to catch up and fly next to the comet 67P / Churyumov-Gerasimenko
There are non-gravitational effects that can influence the orbit of a comet, but they are so small compared to the gravity that they are usually negligible. But in the case of the object I / Oumuamua, its orbit is so unusual that non-gravitational effects could play a role in its geometry.
1I / & # 39; Oumuamua was first sighted in October 2017. Calculations based on the laws of gravity showed that its trajectory was neither the ellipse of a comet nor the circle of a asteroid. Instead, "Oumuamua seemed to have a hyperbolic orbit (see video), meaning that it was not bound by gravitation to the sun. This means that he is almost certainly an intruder from beyond the solar system. From where the "1I" part of his name: the first interstellar body.
Oumuamua rose to 0.25 astronomical unit (AU) of the sun (1 AU is the distance between the sun and the Earth). As a well-bred comet, she should have developed a coma and tail in the form of ice and dust vaporized on the surface by a process called sublimation. But, despite an organized and concerted observation campaign by the international astronomical community, no trace of coma or tail has been observed.
Indeed, measurements of its composition show that it has more in common with asteroids than with comets: its surface indicated that there was little or no ice. It also appeared to have organic materials that had been altered by cosmic radiation exposure – like many asteroids.
Beside these observations was the very unusual form of Oumuamua: although the object can not be seen directly how the amount of reflected light varies as an object rotates) suggested that he was long and thin – cigar-shaped rather than the more usual semi-rounded appearance of asteroids and comets. This led the most fanciful to posit that "Oumuamua could be a visitor manufactured, rather than natural, to the internal solar system, perhaps an alien spacecraft.
Last proof
So: comet, asteroid or spaceship? We can probably avoid the interstellar spacecraft – surely he would have at least paused to study the mix of signals emanating from our planet. So the comet or the asteroid are the remaining options. The same group that published the Oumuamua discovery report continued to observe the object, acquiring instrumentation data on virtually every major ground-based telescope in the world, as well as the space telescope Hubble
What They Have discovered was that there was a deviation from the path that Oumuamua should be next if it was influenced by gravitational effects alone. It seemed that 'Oumuamua was slightly away from the sun. The team studied all possible non-gravitational effects: "Was Oumuamua influenced by solar radiation? Was the effect of thermal emission important when the object was rotated? Had he had a collision that changed the trajectory?
Other options included an unusual mass distribution or perhaps a center of gravity displaced from the center of his body. It could also be strongly magnetized, so that it interacts more strongly with the solar wind. On the basis of their extremely accurate and complete series of observations, the team assumed that none of these effects was sufficient to explain the orbit of Oumuamua. Instead, they concluded that a significant release of sublimation gas was the main feasible mechanism – and this would suggest that it is a comet.
Deep Implications
This is an important finding to reach, not just for & # 39; Oumuamua, but for cometary and asteroidal science more generally. There has been only a handful of previous observations of non-gravitational effects on cometary orbits, mainly because the measurements needed to perform the calculations must be performed over a very large distance . Assuming that 'Oumuamua degassed in the same way as the other comets, the body was losing water and dust at a rate of about 2.4 kg per second
against 20 kg per second for 67P / Churyumov-Gerasimenko. It would seem that although Oumuamua seems to have a density similar to comets in the solar system, it must be much stronger, given the lack of released dust. Or perhaps the dust was more firmly bound to the surface by the organic compounds present.
So can we really say, "Oumuamua is a comet, although possessing asteroidal properties? Or could it be an asteroid with cometary properties? Does it matter? I'm not sure it does. The division between asteroids and comets in the solar system was obvious: the asteroids had circular orbits, were made of rock and metal and lived in the asteroid belt between Mars and Jupiter. Comets had inclined elliptical orbits, were made of rocks and ice and came from farther away.
But in recent years, we have discovered asteroid families scattered throughout the solar system, as well as Kuiper belt objects. We also found comets with short and long orbits. And we saw unusual features on objects that we thought we knew well. The asteroid Ceres, for example, has salts left as ice by sublimation – a process often seen in comets, but not in asteroids.
Since 'Oumuamua, with small bodies in the solar system and instestellar guests, appear to present a spectrum of compositional and dynamic properties, it may be time to think beyond the asteroid vs. comet division, and to look for a new way of classifying these objects. Maybe we need a lot more categories. Or maybe just one – the Small Body class. It is clear that Oumuamua shows us that we have not yet discovered the variety of objects found in our own neighborhood.
This article originally appeared on The Conversation. Read the original article
<p class = "web-atom canvas-text Mb (1.0em) Mb (0) – sm Mt (0.8em) – sm "Monica Grady is a professor of planetary and space sciences at the Open University and a researcher at the Natural History Museum in London, receiving funding from the STFC, the British Space Agency and the European Union. . Horizon 2020 Program. She is the CEO of MonicaGrady MeteoriteLady Ltd, a company that produces educational resources and awareness events for schools and the public. "data-reactid =" 87 "> Monica Grady is a professor of planetary and space sciences at the Open University and a researcher at the Natural History Museum in London, receiving funding from the STFC, the University of California 39 British Space Agency and the EU's Horizon 2020 program.It is Chief Executive Officer of MonicaGrady MeteoriteLady Ltd, a company that produces educational resources and awareness events for schools and the public.
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