That's a trick of perspective. The estimated orbital elements [1] show an inclination of roughly 45 degrees between the object and the ecliptic. If you click the link in the tweet, you can rotate the model and see for yourself.
The orbital plane of the solar system is not aligned in any special orientation relative to nearby stars or the Milky Way, so we would expect interstellar objects to arrive from all directions with equal probability.
Thanks, I was wondering if there was a perspective thing going on.
But also thinking that the larger the angle to the ecliptic, the less likely it was something that just happened to be in a very large orbit around our solar system.
From the discussion above though it sounds like the eccentricity tells us that anyway.
Interesting. This came in at ~30 km/sec. And Oumuamua came in at ~26 km/sec.
It seems that "the Sun moves through the Milky Way at about 20 km/s faster than the local average", or "local standard of rest (LSR)".[0] With the components being ...
11.1 km/s toward the galactic center
12.24 km/s extra in the direction of galactic rotation
7.25 km/s toward the north galactic pole
Some ~nearby stars are coming through the galactic plane at 150 km/sec or so relative to LSR.
These objects probably came from stars with relatively low velocities relative to LSR. There might be some moving a lot faster, but the detection window would be much smaller.
Ironically, Arthur C. Clarke never intended for Rendezvous With Rama to be the start of a trilogy when he wrote that, even though it's an obvious trilogy hook in hindsight. It never occurred to him until readers kept asking when the other two books were coming out.
In the book by Arthur C Clarke Rendezvous with Rama the eponymous Ramans designed their interstellar craft with triple redundancy. Everything is built in threes - 3 airlocks, for example.
The line I quoted is the very last line in the book, and is equal parts fascinating and chilling, a very apt end for a great book.
I highly recommend you read it. If you are in the USA and have a library card, may I suggest the Libby app for Android and iPhone? Or of course buying it second hand from online bookstores is also an option: looks like a new paperback is about $4.55 at smile.amazon.com, used is about 1 to 3 dollars, Kindle version is $7.59. I don't recommend the other books in the series, they don't live up to the original, in my opinion.
A similar book is Eon by Greg Bear, although less pure sci-fi exploration and more sci-fi adventure.
Sorry if anyone felt offended, not being a SF expert myself I actually did Google "The Ramans do everything in threes" and wanted to save others the trouble of doing so...
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.
I don't think it necessarily has to be surprising to be interesting. If it came from another solar system, it's traveled more than 4 light years (obviously much slower than light speed) to get here. That's a long time, and our solar system is a very small target, so the odds are (literally) astronomical.
There's a lot we don't know about the space between solar systems or the space outside of our own heliosphere. This is evidenced by our lack of understanding of when (or if) Voyager actually left our solar system. We just don't know enough to say. Having the opportunity to see something that came from outside our solar system is a good thing for science.
I was under the impression we did know that Voyager left our solar system -- could you clarify what you mean by this to someone not familiar with these concepts?
For several years there were a series of "voyager left the solar system today" headlines, each using a different definition of the solar system boundary because very little is known about where it is.
It's not particularly surprising, but this is (probably) only the second such object we've actually seen, which makes it very interesting.
The first such object, ʻOumuamua, was both interesting (because it was the first one we saw) and surprising (because of its apparent odd shape as indicated by its observed light curve).
A lot of rocks get ejected from solar systems. Current dynamical models suggest there are as many as 3 of these sorts of objects passing through on closest approach every day. We just aren’t very well equipped to detect them.
I'm excited that as our technology improves we will likely find objects like this increasingly frequently, and in the near future may have the ability to rapidly send a mission to intercept one.
It’s also because I think we are more actively looking for them now since we discovered one kinda by accident.
Overall I think we’ll discover that they are more common than we think which makes me wonder if we could use them to piggy back probes on them to the outer solar system and beyond since they move quite darn fast.
Sadly space is super complicated and you can't piggy back on these. To land on one you need to precisely match their speed so if you were to do that, you could already go wherever it was going in the first place.
You don’t need to match their speed necessarily if they are large enough we might be able to bleed off enough kinetic energy to safely collide with them via mechanical means.
If you want to collide with it softly then you pretty much have to go at about the same speed: there are no soft collisions with relative velocities over 500 m/s, and 500 m/s is small when talking about escape velocities.
In short: to have any sort of survivable encounter with the object the relative velocities need to be so small you have essentially "matched their speed". Those last 0.5 km/s you might gain aren't important compared with the 29.5 km/s you need to put in.
> Matching DeltaV for a short duration is easier than for a long duration, no?
you don't match deltav, you'd match trajectory. you may be able to do that at the cost of more or less deltav, depending on how clever and patient you are. once you've matched trajectory, you'll (basically) stay matched, as you're in space and there's nothing to disturb you.
You match velocity, not deltaV. Since natural objects accelerate only under environmental forces (mainly gravity for something like this), once you match velocity and are subject to essentially the same gravitational fields, you will stay matched (roughly).
I shouldn’t have asked the question while commuting, clearly didn’t explain myself well.
Elsewhere I saw what I thought was the speed of the comet, which was around 69k mph. Didn’t Helios 1 and 2 do something like ~150k mph[0]?
Assuming those numbers are correct (please say so if not), then what would stop an intercept from being technically possible (even if very very very hard)?
The Helios probes reached those speeds by starting out much slower than that and then falling toward the sun, speeding up in the process.
To actually hitch a ride with another object you have to match their velocity _and_ their location at the same time.
If the object you are trying to catch started further from the sun than you, and was already moving faster than you, then you can't match its speed and location by falling toward the sun: when you arrive at the same location, it will have fallen further than you and hence picked up more kinetic energy per unit mass than you did, and it started off moving faster than you to start with, so it's still moving faster.
Sorry, yeah, definitely was very poorly worded. I meant to say:
From what I know from reading on Helios-A and Helios-B, we already have a probe which can go over twice the speed of what I thought I saw the comet is traveling at. So couldn’t the probe match its velocity (dunno why I said Delta V before) to the comet, if even just for long enough to land without total destruction?
We probably could if it's travelling at low enough velocity. But if you match its orbit you're just going the same place at it is anyway, at which point "piggybacking" is just code for "getting some company".
And if you don't match it well enough that you end up roughly the same place you're going to have a fast, and therefore violent, encounter.
>you could already go wherever it was going in the first place
Yeah, but we could go to other stars already, in like a zillion years. Voyager is on an interstellar path, although it isn't pointed at a nearby star. If the object has made the journey it might take the same delta-V for us as an interstellar trip, but it would happen a whole lot faster.
I think we might be able to design something that would be able to absorb most of the energy as well as possibly decelerate the probe impact like an explosive charge that won’t work that well as an impulse engine in vacuum but would work against an object.
Some of these objects are also not very dense so some ablative enclosure might also work.
This is not a ridiculous approach. Forget about crashing into it. Instead, imagine a very large, very stretchy net. Large meaning many, many kilometers long.
On impact, only the nearly massless front bit of the net needs to accelerate instantly. The acceleration gradually spreads back to the probe, which is then accelerated gradually to match the object.
After it matches, there is still a great deal of energy in the stretched-out net, which in principle could accelerate your probe to be as much faster than the object as it started out slower than.
Another way to think of this is with a non-stretchy tether perpendicular to the object's path. The object hits one end, and the probe swings around behind it, gaining 2x before it gets to the other side. It could let go at various other points to head off in some other direction at less than 2x.
This is not too different from what we do when we fly probes past planets to give them a speed boost. In that case, the tether is gravity.
So if my back of the envelope math is correct (and I've probably screwed something up as it's past midnight here), to accelerate a 100 kg probe to the speed of the object (30 km/s) at a maximum of 1000 m/s² using a steel spring would require a spring of modulus about 0.11 kg/s². (For reference, a slinky is less "stretchy" at 0.8 kg/s².)
It would take about 47 seconds, and the spring would elongate by a maximum of 530 km. To withstand this tension (100 kN), the steel wire (ultimate tensile strength ~400 MPa) would need to have a cross-sectional area of 250 mm², or diameter of 16 mm.
Given this wire diameter, and a Young's modulus for steel of 200 GPa, the spring itself would require a diameter of ~42 cm and ~200,000 turns, if we allow it to stretch completely during acceleration. Fully relaxed, the spring would be 3.2 km in length.
This works out to at least 130 m³ of steel, or 1000 metric tons, which is somewhere around USD$500k. Eminently affordable.
Of course, that's a drop in the bucket compared to the cost of launching a 3.2 km long, 1000 metric ton slinky into outer space.
Steel is not usually chosen when elasticity or tensile strength per unit mass are goals.
I am actually more worried about the propagation of tension in the tether being restricted to the speed of sound in the material. It seems like the end would just snap off, for any realizable material.
The speed of sound in graphene is 22 km/s. If your tether were made of monocrystalline carbon fiber, it looks like you would have to boost the whole apparatus to 8+ km/s first to catch a 30km/s object, and you could then leave the solar system at something less than 52 km/s.
For comparison, the New Horizon probe to Pluto and beyond left at 23 km/s.
With a thousand km of graphene-fiber tether standing perpendicular to the object's path, it would take 45s to accelerate the probe at a peak of 484 km/s/s. For a 10kg probe, that would put 4.8e9 N of tension on the tether.
Graphene has a tensile strength of 130000 MPa, or 1.3e11 N/m^2, so the cable would need to be some 20cm thick.
Boosting to a higher speed would reduce the cable thickness needed proportionally. At 19 km/s, quite doable, it's 10 cm.
However, boosting 1000 km of 10cm cable to 19 km/s would take quite a fair bit more fuel than just boosting the probe itself to the target speed of 41 km/s.
What about tethers instead ? You could have a two part probe connected by tether rotate about common center quickly enough to have a part encounter the body at pretty low to zero.relative speed at the time of encounter.
What if it “landed” (more like impacted) at a 90° angle to the comet’s direction of travel? Discounting the extreme difficulty of timing that right (like trying to hit a bullet with a smaller bullet, whilst wearing a blindfold, riding a horse — haha), wouldn’t it dramatically decrease the impact energy?
Think about a car travelling 60mph, and you want to climb aboard. You can either jump on the hood (and likely get made into paste) or jump onto the side and immediately have a 60mph 'jerk' forward.
In either case, you are going from 0 to 60 more or less instantly and the results on your fleshy meat body will be the same.
You could try to run real fast in front of the car, away from it, but you are limited by your meat body's technology to ~10mph. You still hit the car at a difference of 50mph. Meat paste once again.
To avoid a painful collision, you'd need to get up to speeds of about 57mph, which is pretty hard for your body to do. And if you could get up to 57mph, why do you need tha car? There's no friction in space, so you'd just keep going until something bumped you.
If you lasso the car with a deceleration rig or an elastic cord you can accelerate safely tho because it adds delay to how fast the energy is transferred to you.
Yep, the trick is that the lasso needs to be able to stretch enough to accelerate more slowly. You'd also need whatever harpoon you fired into the comet to be able to survive thousands of times its own mass in acceleration.
What are the theoretical limits to litobreaking? I imagine the best you can do is shooting a cable at it and constructing something like an space elevator. Small accelerations will require a larger cable, so there is a limit somewhere.
The practical limits are the structural integrity of the craft or its payload package(s). In most cases, that's somewhere on the order of 10-100,000 Gs acceleration.
(If my maths are right, and the rail of the US Navy's 2km/s railgun is 10m, the projectile has an initial accelleration of about 20,000 gs. Though its internal complexity is fairly low.)
Keep in mind that if you manage to harpoon your extrasolar whale, you haven't landed, you've only attached yourself to it. If you thought ahead and packed a bungee cord, rather than a completely inelastic cable, you could take up the initial acceleration, but would then find yourself dealing with the elastic rebound. You'd eventually contact at twice the original negative delta, assuming elastic limits on the cord weren't exceeded.
(Keep in mind that you were initially travelling faster, slower, or with some relative motion to the interstellar whale, and hence are implicitly counting on your harpoon cord to take up the difference. Unlike Ahab, you don't have the medium of water to supply friction or shock absorption.)
You could carry airbags to cushion the impacts. This was actually done for the Mars Pathfinder mission. The critical differences between Pathfinder and Extrasolar Ahab is that Pathfinder had an aeroshell and parachutes, all of which reduced the terminal impact eleasticity accelleration to well below Mars escape velocity, as well as a substantial surface gravity to deal with, while Extrasolar Ahab has the cold hard vacuum of space and a microgravity measured in single-digit metres/s^2. Rather than bounce and come to a rest, you'll bank off. Instead of Ahab, you're now "Fast Eddie" Felton, and your balls are no longer on the felt.
The problem in both these cases is elasticity, so what you're looking for is something that's deformable rather than elastic, probably at both the 'poon cord and crash buffer side. The longer you can stretch out (so to speak) the accelleration and impact, the softer your ultimate kiss.
If you hook a bungee cord onto the object can’t you just use the stored elastic energy to propel yourself forward? The whole point of the piggy back is to take some of its kinetic energy as your own velocity it doesn’t matter if you actually land on it or not.
I was imagining eventually landing on the object. Interstellar space is probably not very interesting on its own.
But how much delta-v one can extract from a bungee jump is just as interesting a problem as how do you break from a huge delta-v and no atmosphere. Maybe one can even direct the jump into a useful direction: interstellar body based propulsion.
No need to invoke aliens. We're seeing these now because our ability to find smaller objects in the solar system has improved rapidly over the last few decades, as is evident from this animation of asteroid discoveries: https://www.youtube.com/watch?v=BKKg4lZ_o-Y
We already have three functioning interstellar probes - there might not be much benefit in building one specifically to accompany an interstellar object, unless the idea is to sample it.
frankly, having read the three body trilogy, these kinds of reports scare the pants off of me. i always put a small prior that astronomers would look at this and say "uh, oddly, it seems to be headed precisely for a direct collision with Earth."
Just as a reminder,
So at e = 3, this object is in a sense bouncing off the sun at an angle of 180 * 2 * invsec(3) / pi or ~141 degrees.https://en.wikipedia.org/wiki/Eccentricity_(mathematics)
An animation of the orbit as it is known right now: https://twitter.com/tony873004/status/1171917219485192192
This animation seems to imply it is traveling orthogonal to the ecliptic. Is that normal (maybe expected is a better word) for this kind of thing?
That's a trick of perspective. The estimated orbital elements [1] show an inclination of roughly 45 degrees between the object and the ecliptic. If you click the link in the tweet, you can rotate the model and see for yourself.
The orbital plane of the solar system is not aligned in any special orientation relative to nearby stars or the Milky Way, so we would expect interstellar objects to arrive from all directions with equal probability.
[1]: https://twitter.com/AscendingNode/status/1171845027099795456
Thanks, I was wondering if there was a perspective thing going on.
But also thinking that the larger the angle to the ecliptic, the less likely it was something that just happened to be in a very large orbit around our solar system.
From the discussion above though it sounds like the eccentricity tells us that anyway.
Interesting. This came in at ~30 km/sec. And Oumuamua came in at ~26 km/sec.
It seems that "the Sun moves through the Milky Way at about 20 km/s faster than the local average", or "local standard of rest (LSR)".[0] With the components being ...
Some ~nearby stars are coming through the galactic plane at 150 km/sec or so relative to LSR.These objects probably came from stars with relatively low velocities relative to LSR. There might be some moving a lot faster, but the detection window would be much smaller.
0) https://en.wikipedia.org/wiki/Stellar_kinematics
...wakened from a restless sleep with the message from his subconscious still echoing in his brain: The Ramans do everything in threes...
>The Ramans do everything in threes...
Ironically, Arthur C. Clarke never intended for Rendezvous With Rama to be the start of a trilogy when he wrote that, even though it's an obvious trilogy hook in hindsight. It never occurred to him until readers kept asking when the other two books were coming out.
This always comes to mind when an event like this is announced. Paging Captain Norton.
who are the Ramans? are you suggesting these are callibration shots?
In the book by Arthur C Clarke Rendezvous with Rama the eponymous Ramans designed their interstellar craft with triple redundancy. Everything is built in threes - 3 airlocks, for example.
The line I quoted is the very last line in the book, and is equal parts fascinating and chilling, a very apt end for a great book.
I highly recommend you read it. If you are in the USA and have a library card, may I suggest the Libby app for Android and iPhone? Or of course buying it second hand from online bookstores is also an option: looks like a new paperback is about $4.55 at smile.amazon.com, used is about 1 to 3 dollars, Kindle version is $7.59. I don't recommend the other books in the series, they don't live up to the original, in my opinion.
A similar book is Eon by Greg Bear, although less pure sci-fi exploration and more sci-fi adventure.
LMGTFY: https://en.wikipedia.org/wiki/Rendezvous_with_Rama :D
> LMGTFY
Even with a smiley face this is still a tiresome and rude response.
Sorry if anyone felt offended, not being a SF expert myself I actually did Google "The Ramans do everything in threes" and wanted to save others the trouble of doing so...
It's not the link itself, it's the LMGTFY in front. It's a form of snark.
Opened the thread looking for this comment :)
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!
Think of it this way:
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.
It it not at all possible for an interstellar object to be captured into a stable solar orbit?
Not without interacting with a 3rd body, such as Jupiter.
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."
Most things flying towards a black hole don't cross the event horizon thanks to angular momentum.
Stars tend to get torn apart, go into the accretion disk, and then eventually fall in. But we don't have a big black hole in the solar system.
For present purposes the Newtonian approximation suffices, and in Newtonian gravity there are no black holes and no event horizons.
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.
Thank you, great explanation.
Do we have any sense of how large this object is? Or is that a silly question that only non-astronomers like me ask?
According to this tweet[0] the object's coma[1], the cloud of ice and dust surrounding the object, makes it too difficult to measure.
[0]https://twitter.com/AscendingNode/status/1171913641647493120 [1]https://en.wikipedia.org/wiki/Coma_(cometary)
Thank you. I learnt something very cool today!
Thank you, that was a great explanation with lovely examples.
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.
These things probably are not rare. They're just new to us because we can detect them now.
Or we're about to get hit with a ton of space rocks. 2020 is really the end of the world!
/s
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.
https://upload.wikimedia.org/wikipedia/commons/b/b7/Kepler_o...
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.
... for a given semimajor axis, is what the OP forgot to add.
The absolute nearness of approach depends on the energy of the orbit as well as it's eccentricity.
Talking about the "speed" is potentially unclear as an object's speed can vary greatly within its orbit.
A helpful comment from the twitter thread:
"Not aliens. And surely they're common. But we just haven't been able to detect these things until recently. It's new to us, not new to the universe."
[1] https://twitter.com/twit_spires/status/1171939194282762240
Short Wiki article on the object: https://en.wikipedia.org/wiki/C/2019_Q4_(Borisov)
See also the initial professional report at http://www.astronomerstelegram.org/?read=13100
Something I don't understand - why is it surprising that there could be rocks floating around that come from other solar systems?
I don't think it necessarily has to be surprising to be interesting. If it came from another solar system, it's traveled more than 4 light years (obviously much slower than light speed) to get here. That's a long time, and our solar system is a very small target, so the odds are (literally) astronomical.
There's a lot we don't know about the space between solar systems or the space outside of our own heliosphere. This is evidenced by our lack of understanding of when (or if) Voyager actually left our solar system. We just don't know enough to say. Having the opportunity to see something that came from outside our solar system is a good thing for science.
I was under the impression we did know that Voyager left our solar system -- could you clarify what you mean by this to someone not familiar with these concepts?
https://www.nytimes.com/2013/09/13/science/in-a-breathtaking...
For several years there were a series of "voyager left the solar system today" headlines, each using a different definition of the solar system boundary because very little is known about where it is.
oh ok, thanks
It's not particularly surprising, but this is (probably) only the second such object we've actually seen, which makes it very interesting.
The first such object, ʻOumuamua, was both interesting (because it was the first one we saw) and surprising (because of its apparent odd shape as indicated by its observed light curve).
A lot of rocks get ejected from solar systems. Current dynamical models suggest there are as many as 3 of these sorts of objects passing through on closest approach every day. We just aren’t very well equipped to detect them.
You can assume but you don't know until you actually observe them. It's no longer an assumption.
Update: eccentricity closer to 3.0 (still hyperbolic). https://twitter.com/AscendingNode/status/1171845027099795456
Or the object is speeding up...
The orbit didn't physically change. The author fixed his code and reran the analysis.
I'm excited that as our technology improves we will likely find objects like this increasingly frequently, and in the near future may have the ability to rapidly send a mission to intercept one.
It’s also because I think we are more actively looking for them now since we discovered one kinda by accident.
Overall I think we’ll discover that they are more common than we think which makes me wonder if we could use them to piggy back probes on them to the outer solar system and beyond since they move quite darn fast.
Sadly space is super complicated and you can't piggy back on these. To land on one you need to precisely match their speed so if you were to do that, you could already go wherever it was going in the first place.
At least it would be a source of materials on the way there.
You don’t need to match their speed necessarily if they are large enough we might be able to bleed off enough kinetic energy to safely collide with them via mechanical means.
If you want to collide with it softly then you pretty much have to go at about the same speed: there are no soft collisions with relative velocities over 500 m/s, and 500 m/s is small when talking about escape velocities.
In short: to have any sort of survivable encounter with the object the relative velocities need to be so small you have essentially "matched their speed". Those last 0.5 km/s you might gain aren't important compared with the 29.5 km/s you need to put in.
Matching DeltaV for a short duration is easier than for a long duration, no? I’m not an astrophysicist, but that seems to make sense to me.
> Matching DeltaV for a short duration is easier than for a long duration, no?
you don't match deltav, you'd match trajectory. you may be able to do that at the cost of more or less deltav, depending on how clever and patient you are. once you've matched trajectory, you'll (basically) stay matched, as you're in space and there's nothing to disturb you.
You match velocity, not deltaV. Since natural objects accelerate only under environmental forces (mainly gravity for something like this), once you match velocity and are subject to essentially the same gravitational fields, you will stay matched (roughly).
I shouldn’t have asked the question while commuting, clearly didn’t explain myself well.
Elsewhere I saw what I thought was the speed of the comet, which was around 69k mph. Didn’t Helios 1 and 2 do something like ~150k mph[0]?
Assuming those numbers are correct (please say so if not), then what would stop an intercept from being technically possible (even if very very very hard)?
[0] https://en.m.wikipedia.org/wiki/Helios_(spacecraft)
The Helios probes reached those speeds by starting out much slower than that and then falling toward the sun, speeding up in the process.
To actually hitch a ride with another object you have to match their velocity _and_ their location at the same time.
If the object you are trying to catch started further from the sun than you, and was already moving faster than you, then you can't match its speed and location by falling toward the sun: when you arrive at the same location, it will have fallen further than you and hence picked up more kinetic energy per unit mass than you did, and it started off moving faster than you to start with, so it's still moving faster.
No, that doesn't make much sense does it? Delta V is a change in velocity, but comets don't change velocity very much.
That remark makes more sense in the context of missile defence than when talking about space travel.
Sorry, yeah, definitely was very poorly worded. I meant to say:
From what I know from reading on Helios-A and Helios-B, we already have a probe which can go over twice the speed of what I thought I saw the comet is traveling at. So couldn’t the probe match its velocity (dunno why I said Delta V before) to the comet, if even just for long enough to land without total destruction?
We probably could if it's travelling at low enough velocity. But if you match its orbit you're just going the same place at it is anyway, at which point "piggybacking" is just code for "getting some company".
And if you don't match it well enough that you end up roughly the same place you're going to have a fast, and therefore violent, encounter.
s/DeltaV/velocity and trajectory/
Dunno why I said DeltaV, but clearly it was inappropriate.
>you could already go wherever it was going in the first place
Yeah, but we could go to other stars already, in like a zillion years. Voyager is on an interstellar path, although it isn't pointed at a nearby star. If the object has made the journey it might take the same delta-V for us as an interstellar trip, but it would happen a whole lot faster.
A probe would have to crash pretty hard into one to easily absorb that much kinetic energy, I imagine, otherwise it's the same old problem of delta-V.
I think we might be able to design something that would be able to absorb most of the energy as well as possibly decelerate the probe impact like an explosive charge that won’t work that well as an impulse engine in vacuum but would work against an object.
Some of these objects are also not very dense so some ablative enclosure might also work.
This is not a ridiculous approach. Forget about crashing into it. Instead, imagine a very large, very stretchy net. Large meaning many, many kilometers long.
On impact, only the nearly massless front bit of the net needs to accelerate instantly. The acceleration gradually spreads back to the probe, which is then accelerated gradually to match the object.
After it matches, there is still a great deal of energy in the stretched-out net, which in principle could accelerate your probe to be as much faster than the object as it started out slower than.
Another way to think of this is with a non-stretchy tether perpendicular to the object's path. The object hits one end, and the probe swings around behind it, gaining 2x before it gets to the other side. It could let go at various other points to head off in some other direction at less than 2x.
This is not too different from what we do when we fly probes past planets to give them a speed boost. In that case, the tether is gravity.
So if my back of the envelope math is correct (and I've probably screwed something up as it's past midnight here), to accelerate a 100 kg probe to the speed of the object (30 km/s) at a maximum of 1000 m/s² using a steel spring would require a spring of modulus about 0.11 kg/s². (For reference, a slinky is less "stretchy" at 0.8 kg/s².)
It would take about 47 seconds, and the spring would elongate by a maximum of 530 km. To withstand this tension (100 kN), the steel wire (ultimate tensile strength ~400 MPa) would need to have a cross-sectional area of 250 mm², or diameter of 16 mm.
Given this wire diameter, and a Young's modulus for steel of 200 GPa, the spring itself would require a diameter of ~42 cm and ~200,000 turns, if we allow it to stretch completely during acceleration. Fully relaxed, the spring would be 3.2 km in length.
This works out to at least 130 m³ of steel, or 1000 metric tons, which is somewhere around USD$500k. Eminently affordable.
Of course, that's a drop in the bucket compared to the cost of launching a 3.2 km long, 1000 metric ton slinky into outer space.
Steel is not usually chosen when elasticity or tensile strength per unit mass are goals.
I am actually more worried about the propagation of tension in the tether being restricted to the speed of sound in the material. It seems like the end would just snap off, for any realizable material.
Are there materials available with three orders of magnitude better properties?
The speed of sound in graphene is 22 km/s. If your tether were made of monocrystalline carbon fiber, it looks like you would have to boost the whole apparatus to 8+ km/s first to catch a 30km/s object, and you could then leave the solar system at something less than 52 km/s.
For comparison, the New Horizon probe to Pluto and beyond left at 23 km/s.
With a thousand km of graphene-fiber tether standing perpendicular to the object's path, it would take 45s to accelerate the probe at a peak of 484 km/s/s. For a 10kg probe, that would put 4.8e9 N of tension on the tether.
Graphene has a tensile strength of 130000 MPa, or 1.3e11 N/m^2, so the cable would need to be some 20cm thick.
Boosting to a higher speed would reduce the cable thickness needed proportionally. At 19 km/s, quite doable, it's 10 cm. However, boosting 1000 km of 10cm cable to 19 km/s would take quite a fair bit more fuel than just boosting the probe itself to the target speed of 41 km/s.
What about tethers instead ? You could have a two part probe connected by tether rotate about common center quickly enough to have a part encounter the body at pretty low to zero.relative speed at the time of encounter.
30 kilometers per second is 67,108 miles per hour. That's a lot of probe impact energy to absorb.
Something like a Newton's cradle with different radii? One to deform on impact, another to take a sizable portion of the force?
What if it “landed” (more like impacted) at a 90° angle to the comet’s direction of travel? Discounting the extreme difficulty of timing that right (like trying to hit a bullet with a smaller bullet, whilst wearing a blindfold, riding a horse — haha), wouldn’t it dramatically decrease the impact energy?
Think about a car travelling 60mph, and you want to climb aboard. You can either jump on the hood (and likely get made into paste) or jump onto the side and immediately have a 60mph 'jerk' forward.
In either case, you are going from 0 to 60 more or less instantly and the results on your fleshy meat body will be the same.
You could try to run real fast in front of the car, away from it, but you are limited by your meat body's technology to ~10mph. You still hit the car at a difference of 50mph. Meat paste once again.
To avoid a painful collision, you'd need to get up to speeds of about 57mph, which is pretty hard for your body to do. And if you could get up to 57mph, why do you need tha car? There's no friction in space, so you'd just keep going until something bumped you.
If you lasso the car with a deceleration rig or an elastic cord you can accelerate safely tho because it adds delay to how fast the energy is transferred to you.
Yep, the trick is that the lasso needs to be able to stretch enough to accelerate more slowly. You'd also need whatever harpoon you fired into the comet to be able to survive thousands of times its own mass in acceleration.
What are the theoretical limits to litobreaking? I imagine the best you can do is shooting a cable at it and constructing something like an space elevator. Small accelerations will require a larger cable, so there is a limit somewhere.
The practical limits are the structural integrity of the craft or its payload package(s). In most cases, that's somewhere on the order of 10-100,000 Gs acceleration.
(If my maths are right, and the rail of the US Navy's 2km/s railgun is 10m, the projectile has an initial accelleration of about 20,000 gs. Though its internal complexity is fairly low.)
Keep in mind that if you manage to harpoon your extrasolar whale, you haven't landed, you've only attached yourself to it. If you thought ahead and packed a bungee cord, rather than a completely inelastic cable, you could take up the initial acceleration, but would then find yourself dealing with the elastic rebound. You'd eventually contact at twice the original negative delta, assuming elastic limits on the cord weren't exceeded.
(Keep in mind that you were initially travelling faster, slower, or with some relative motion to the interstellar whale, and hence are implicitly counting on your harpoon cord to take up the difference. Unlike Ahab, you don't have the medium of water to supply friction or shock absorption.)
You could carry airbags to cushion the impacts. This was actually done for the Mars Pathfinder mission. The critical differences between Pathfinder and Extrasolar Ahab is that Pathfinder had an aeroshell and parachutes, all of which reduced the terminal impact eleasticity accelleration to well below Mars escape velocity, as well as a substantial surface gravity to deal with, while Extrasolar Ahab has the cold hard vacuum of space and a microgravity measured in single-digit metres/s^2. Rather than bounce and come to a rest, you'll bank off. Instead of Ahab, you're now "Fast Eddie" Felton, and your balls are no longer on the felt.
The problem in both these cases is elasticity, so what you're looking for is something that's deformable rather than elastic, probably at both the 'poon cord and crash buffer side. The longer you can stretch out (so to speak) the accelleration and impact, the softer your ultimate kiss.
https://invidio.us/watch?v=E65F86kMu48
At which you've probably got one more question:
https://invidio.us/watch?v=uJixQ16L5zc
To which I can only reply: as you wish.
If you hook a bungee cord onto the object can’t you just use the stored elastic energy to propel yourself forward? The whole point of the piggy back is to take some of its kinetic energy as your own velocity it doesn’t matter if you actually land on it or not.
How do you plan on stopping?
The question specifically addressed lithobraking. Which means a terminal state on the surface with matched velocity.
You don’t stop it you can mechanically extract energy form it you just slingshot swag out of the solar system.
I was imagining eventually landing on the object. Interstellar space is probably not very interesting on its own.
But how much delta-v one can extract from a bungee jump is just as interesting a problem as how do you break from a huge delta-v and no atmosphere. Maybe one can even direct the jump into a useful direction: interstellar body based propulsion.
Or they are more actively looking at us since they first picked up our radio signals several decades ago :)
No need to invoke aliens. We're seeing these now because our ability to find smaller objects in the solar system has improved rapidly over the last few decades, as is evident from this animation of asteroid discoveries: https://www.youtube.com/watch?v=BKKg4lZ_o-Y
SEND MORE I LOVE LUCY.
Why does Ross, the largest friend, not simply eat the other friends?
Have they already listened to Chuck Berry's entire catalog?
At these speeds, millennia.
We already have three functioning interstellar probes - there might not be much benefit in building one specifically to accompany an interstellar object, unless the idea is to sample it.
An interesting podcast interviewing Avi Loeb at the time on his speculative theory from the previous visitor:
https://after-on.com/episodes-31-60/040
See also the Minor Planet Electronic Circular at https://minorplanetcenter.net/mpec/K19/K19RA6.html
It's a comet. https://twitter.com/AscendingNode/status/1171894551780290560
Is it even feasible to think of scraping something off this object for study? I mean in far future it will happen, but does current tech allow it?
So interesting. There is so much that we don't know about the solar system.
Does anyone have more helpful links on this topic?
Can we not miss this one this time? I’m still bummed over not catching sight of the first interstellar solar sail.
Apparently it will come within 1.7 AU of Mars and 1.9 AU of Earth.
I'm not sure if they've figured out how big it is, or what other analysis they can do of it.
Eye-balling the animation someone else link to, it looks like it will be in our solar system area for about half a year.
I have no idea if we have telescopes good enough for this. Does anyone else know?
ESA is planning to launch the Comet Interceptor mission in 2028: https://en.wikipedia.org/wiki/Comet_Interceptor
Hope a rewarding target comes up.
Sadly, there has to be lots more that are undetected
Maybe its just waiting for us to destroy ourselves and then come and make a new home out of our radiated shell of a planet like a hermit crab.
All I need to know is are we Gona die ?
frankly, having read the three body trilogy, these kinds of reports scare the pants off of me. i always put a small prior that astronomers would look at this and say "uh, oddly, it seems to be headed precisely for a direct collision with Earth."
That book has junk science. I wouldn’t be so concerned.
The word you're looking for is "fiction"
And one shouldn't base their worldview on fiction.
HN has no sense of humor.