New observations of interstellar Oumuamua give it comet-like properties

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The uncertainty of science: New observations of interstellar object Oumuamua suggest that it is a comet, not an asteroid.

[B]y combining data from the NASA/ESA Hubble Space Telescope, the Canada-France-Hawaii Telescope, ESO’s Very Large Telescope and the Gemini South Telescope, an international team of astronomers has found that the object is moving faster than predicted. The measured gain in speed is tiny and `Oumuamua is still slowing down because of the pull of the Sun — just not as fast as predicted by celestial mechanics.

The team, led by Marco Micheli (European Space Agency) explored several scenarios to explain the faster-than-predicted speed of this peculiar interstellar visitor. The most likely explanation is that `Oumuamua is venting material from its surface due to solar heating — a behaviour known as outgassing. The thrust from this ejected material is thought to provide the small but steady push that is sending `Oumuamua hurtling out of the Solar System faster than expected — as of 1 June, it is travelling with about 114 000 kilometres per hour.

Such outgassing is a typical behaviour for comets and contradicts the previous classification of `Oumuamua as an interstellar asteroid. “We think this is a tiny, weird comet,” comments Marco Micheli. “We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets.”

If I was to speculate wildly, I could also wonder if maybe the aliens on board have decided they needed to get the heck out of here as fast as possible, and have fired their thrusters to make that happen.


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  • Edward

    You wrote: “If I was to speculate wildly, I could also wonder if maybe the aliens on board have decided they needed to get the heck out of here as fast as possible, and have fired their thrusters to make that happen.

    That would be wild indeed, since the best time to apply thrusters is at perihelion in order to take advantage of the Oberth effect.

  • Localfluff

    Could you give an intuitive explanation of the Oberth effect? It is plain Newtonian mechanics, easily evident in simple formulas. But that’s not how it apparently works in everyday life. That it doesn’t work for cars (that you accelerate faster and faster for the same amount of fuel the faster you already go) I understand is because of mounting air resistance.

    Is it really more efficient to push the swing of a child when it is vertical and has the highest speed, than when it stands still while turning around?

  • wayne

    (I’m just an amateur.)

    “When travelling at a low speed, an increase in velocity adds a small amount of kinetic energy. Yet if you’re travelling at a high speed, the same amount of velocity increase will add many more times the energy.”

  • Localfluff

    Yes, that is mathematically indisputable. But why isn’t apparent by intuition to push the child’s swing when it moves at its fastest, rather than when it stands still, as everyone I’ve seen doing it, including myself. Could the lazy baby sitter save a few calories here?

    Even Aristoteles thought that heavier objects fall faster than light ones. And that stuck for a couple of thousand years. It is funny how counter intuitive it is how things move, while we are intuitive masters at throwing a ball. Even dogs don’t get it but run towards where the ball is at the moment.

  • Edward

    Localfluff Asked: “Could you give an intuitive explanation of the Oberth effect?

    Yes. This is an impressive part of the counterintuitive nature of orbital mechanics.

    Imagine a highly elliptical orbit (mathematically, this is a conic section), such as a comet. It travels fastest when it is nearest the sun, at perihelion (generic term is “periapsis”), and slowest when it is farthest from the sun, at aphelion (apoapsis). In order to change that orbit from elliptical to parabolic or hyperbolic* a delta v is necessary. If that delta v is applied at aphelion, away from the sun, then the delta v will first have to be enough to create a circular orbit around the sun, and that is more delta v, then create an elliptical orbit in which apohelion becomes perihelion, then create a parabolic or hyperbolic trajectory. The slow speed at apohelion means that the delta v required is not great, but it is still more than needed at perihelion.

    However, since the velocity at perihelion is already so great, the orbit is already far beyond being circular, it takes much less delta v to open up the orbit (conic section) to a parabola or hyperbola.

    In orbital mechanics, there are two kinds of energy that are tracked most: kinetic energy, the energy of motion, and potential energy, the energy of height above a gravity source. Combining both gives a total energy that remains the same throughout the orbit. Kinetic energy is greatest and potential energy is least at periapsis, and the other way around at apoapsis.

    Confusion about the energy requirements may abound, because the propellant (chemical energy) expended by the spacecraft is the same for the same delta v at either apsis. However, the wikipedia article I linked, above, helps to explain this:

    At very high speeds the mechanical power imparted to the rocket can exceed the total power liberated in the combustion of the propellant; this may also seem to violate conservation of energy. But the propellants in a fast moving rocket carry energy not only chemically but also in their own kinetic energy, which at speeds above a few km/s exceed the chemical component. When these propellants are burned, some of this kinetic energy is transferred to the rocket along with the chemical energy released by burning.

    To help explain the seeming discrepancy of the energy difference, recall that the exhaust velocity of the rocket engine is the same relative to the spacecraft at either apsis. However, the velocity of the exhaust is different relative to the center of attraction, such as the Sun. From the Sun’s point of view (or the universe’s), more energy is added to the spacecraft when its engines fire at perihelion than when they do at aphelion. The energy of the universe is conserved despite the intuited discrepancy.

    I hope that helped.

    * Both are escape trajectories and will never return to the sun, but parabolic is only just barely enough, theoretically coming to a stop at infinity, while hyperbolic is faster than the minimum needed, never stopping, even at infinity.

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