I see a lot of cool space stuff in the tweet and in these comments. Can someone explain this to someone without an astrophysics background? What does the eccentricity of 3 mean, why is that important, and why is this discovery important? Thanks!
If it was flying by slowly, it would get caught by the sun’s gravity and end up in a circular orbit around the sun. If the orbit was perfectly circular, it would have an eccentricity of 0.
If it was flying by a little bit faster, it might barely get caught by the sun and end up with an orbit like Halley’s comet where it comes close to the sun once every hundred years and then flies off to the far reaches of the solar system before coming back again. It would have an eccentricity of 0.9-ish.
If it was flying a little bit faster, the sun’s gravity would try to catch it, but it would fail! The object would “swoop” around the sun then go flying back the direction it came, never to return again. It would have an eccentricity of about 1 or a little more (Less than 1 means it’s in orbit, 1 or more means it’s not coming back).
If it was flying REALLY REALLY fast, the sun’s gravity would try to pull it in, but this time the object has other plans. It barely even changes course and rockets through the solar system in an almost perfectly straight line. Its eccentricity would be something like 89 (there’s no upper limit, although at a certain point the object would have to travel at close to lightspeed to acheive certain really high eccentricities if flying close to the inner solar system).
This object (the real-life one we’re talking about) is going faster than the “swoop” object but slower than the “other plans” object. Its path is being bent somewhat by the sun’s gravity, but it is going to leave an never come back. So its eccentricity is 3.
One last thing: eccentricity is PURELY a math thing that describes circles, ellipses and curves. It’s just that when you’re talking about orbital mechanics, it gets interwoven with other aspects of an object’s orbit, like its velocity and its altitude at the closest point in the orbit.
"If it was flying by slowly, it would get caught by the sun’s gravity and end up in a circular orbit around the sun. If the orbit was perfectly circular, it would have an eccentricity of 0."
An interstellar object cannot be flying by slowly enough to enter a circular orbit. If it were, it would already have been in a circular orbit. Trajectories can be extrapolated both into the future and into the past. It has enough momentum to escape the Solar System if and only if it came from outside the Solar System.
This assumes the only interaction is the gravitational pull between the Sun and the object. A close approach to Jupiter, for example, might slow an incoming object into an elliptical orbit, or speed a Solar System object into a hyperbolic escape trajectory.
Or, apparently, if it crosses an event horizon, although I've never properly understood how that works. (Explanations I've found typically just say: "All paths from a black hole are toward the black hole."
Captured into a stable orbit, sure, but it will never have an eccentricity of 0 (circular). The limit can approach zero, but will never be equal to it. Even the planets don’t have a circular orbit FWIW.
An interstellar object cannot be captured into a stable orbit unless some other force (such as Jupiter's gravity) acts on it.
Orbits are time-symmetric, so an interstellar object being captured into a stable elliptical orbit would be equivalent to an object in a stable elliptical orbit escaping the Solar System (again, ignoring other influences).
Is that true of comets as well? It seems conceivable to me that the sun could spin it around such that the comet's off-gassing could reduce its speed and allow it to be captured. Obviously unlikely, and highly dependent on conditions (in particular incoming velocity) but possible.
If you're ignoring all other influences then there is no such thing as interstellar objects. By definition, every object in a two body system are gravationally bound to each other.
This is so wrong. Objects can have enough energy that will become arbitrary far apart as time passes. In no sense are the objects bound together in that case.
Eccentricity describes the shape of an orbit. 0 is perfectly circular. Between 0 and 1 is elliptical. 1 is parabolic. Above 1 is hyperbolic. So an eccentricity of 3.0 is hyperbolic, which means the object is of interstellar origin. (Picture the shape of a hyperbola in your mind. An object on that trajectory can't come back to the same point.)
With the uncertainties that come from fitting early observations, these numbers can change as more observations come in. At 1.07, it would still be possible that the final orbit turns out to not be hyperbolic. At 3.0, that's much less likely.
What makes this discovery exciting is that it would be the second object of interstellar origin that we've discovered in our own solar system (and both discoveries are fairly recent). That's a new class of object to be studied and presents an opportunity to learn something new. The fact that these two discoveries are temporally close suggests that we'll discover many more as our technology and technique improve.
Eccentricity is a measure of how elongated/oval shaped your orbit is, eccentricity of 0 means a circular orbit, 0.5 is "very elongated orbit" (that is coming relatively close to the sun, then going very far away, think deep space comets) 1 means a parabola escape trajectory, and anything higher than 1 means a very fast escape trajectory.
The fact its eccentricity is 3 means its going through the solar system extremely fast (relative to the orbits of other things at comparable distance), and is going to exit again after slingshotting around the sun, that means its origin is extremely likely to be extra-solar because we know of no real mechanism for a body to generate such extreme speed while originating from within the solar system.
In this image, red has an eccentricity of 0.7, green 1.0 and blue 1.7, the sun would be the focus. The the green and blue "orbits" never return, just like a parabola/hyperbola never curve into an ellipse, think of a graph like y=x^2.
Eccentricity is a way of characterizing the curve the object makes around the sun. <1 is an ellipse, and so the object would be in orbit around the sun, and hence probably not interstellar. >1 is hyperbolic, and thus not gravitationally bound to the sun, and so probably interstellar.
The closer the eccentricity is to 1 without being equal or less, the more "curved" the trajectory is and the nearer its closes approach to the sun will be.
> The closer the eccentricity is to 1 without being equal or less, the more "curved" the trajectory is and the nearer its closes approach to the sun will be.
Is this true? I thought it depends on the object’s speed. You can have an object with e=3 have a closer approach to the sun than an object with e=2 if the first object is traveling sufficiently faster.
In other words, at a given perihelion, you can change an object’s e by accelerating or decelerating. Not a orbital mechanics major, just played too much KSP.
