Why Didn't 'Oumuamua Hit The Sun?

If Oumuamua has come from so far away, and in it’s final approach was primarily attracted by the Sun, why didn’t it hit the Sun? Were the gravitational forces of other planets sufficient to make it miss?
This artist’s impression shows the first interstellar asteroid: `Oumuamua. This unique object was discovered on 19 October 2017 by the Pan-STARRS 1 telescope in Hawai`i. Subsequent observations from ESO’s Very Large Telescope in Chile and other observatories around the world show that it was traveling through space for millions of years before its chance encounter with our star system. Image credit: European Southern Observatory/M. Kornmesser

This artist’s impression shows the first interstellar asteroid: `Oumuamua. This unique object was discovered on 19 October 2017 by the Pan-STARRS 1 telescope in Hawai`i. Subsequent observations from ESO’s Very Large Telescope in Chile and other observatories around the world show that it was traveling through space for millions of years before its chance encounter with our star system. Image credit: European Southern Observatory/M. Kornmesser

Originally posted on Forbes!

The answer to this question lies in how gravity acts over large distances, with a bit of interstellar aiming thrown in for flavor.

On the surface of planet Earth, the force of gravity is pretty much a constant through our entire lives. We recognize it as the influence which grounds us to the surface of our planet - but it remains a constant feature on our planet. That’s because we are all living (more or less) at the same distance from the center of the Earth. If you change the distance between us and the center of the Earth, the force of gravity will change.

It actually changes reasonably quickly - the equations go as one over the square of the distance - so if you double the distance between you and a massive object, you’ll cut the gravitational force in fourths. If you keep going, and double the distance again, your already quartered gravitational force is cut into fourths once more, to one sixteenth its original strength. In the distances considered in a solar system, the gravitational influence of the Earth is fairly rapidly diminished down to a tiny disturbance to the surrounding space.

Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time. Image credit: NASA

Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time. Image credit: NASA

On the scale of the solar system, the entire mass of the Earth is peanuts compared to the mass contained in the Sun. This is probably not surprising - we’re relatively familiar with the Earth being one of our solar system’s smaller planets. Of the planets, Jupiter is where the bulk of the mass in the solar system lies - Jupiter is more than three hundred times the mass of the Earth, which puts it at more than twice the mass of all the other major planets in the Solar system. But the Sun is a thousand times more massive than Jupiter, and so while we need to account for Jupiter when calculating out where our outer-solar-system-exploring spacecraft will go, for an interstellar visitor, it’s the Sun that’s going to be the most influential, not the planets.

However, if we want to compare the Sun's gravitational distortions with the distances involved in the spaces between the stars in the Galaxy, we find that the Sun’s gravity also dwindles very quickly out to insignificance. For the majority of ‘Oumuamua’s journey through the vast spaces between the stars, our Sun’s gravitational pull would have had no effect whatsoever on the direction that space rock was traveling.

This animation shows the path of A/2017 U1, which is an asteroid -- or perhaps a comet -- as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system. Image credit: NASA/JPL-Caltech

This animation shows the path of A/2017 U1, which is an asteroid -- or perhaps a comet -- as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system. Image credit: NASA/JPL-Caltech

If ‘Oumuamua had been traveling directly at the Sun, the force of the Sun’s gravity would have served just to speed it up, without needing to reorient the direction of its travels in any way. However, as we mentioned in another article, the likelihood of hitting the Sun directly is astonishingly low, and so it’s much more likely that this object would travel through our solar system without crashing into anything.

Why didn’t the force of the Sun’s gravity redirect the object into itself? Primarily because ‘Oumuamua was traveling fast enough. Our interstellar visitor only spent a short period of time close to the Sun, where the force of gravity was particularly strong. During the majority of its journey inwards towards our Sun, its path was only slightly adjusted by the Sun. During its close approach to the Sun, the force of gravity was considerably stronger, but ‘Oumuamua was only in this region of strong gravitational disturbance for a short period of time.

While the force of the Sun's gravity did deflect the path of ‘Oumuamua significantly, it could only do so in a brief window of time before our interstellar visitor was swinging its way back out of the solar system. If it had been moving slower with respect to the Sun, there would have been more time, and it could have been more effectively pulled into the Sun. On the other hand, the speed with which it came into our solar system was typical for an object outside of our solar system, so it coming in slower would be unusual, considering where it was coming from!

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What Would Have Happened To That Interstellar Object If It Had Hit The Sun?

What would have happened if A/2017 U1 had hit or grazed the sun? Would we have noticed?
This animation shows the path of A/2017 U1, which is an asteroid — or perhaps a comet — as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system. Image credit: NASA/JPL-Caltech

This animation shows the path of A/2017 U1, which is an asteroid — or perhaps a comet — as it passed through our inner solar system in September and October 2017. From analysis of its motion, scientists calculate that it probably originated from outside of our solar system. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

The snappily-named object A/2017 U1 may be more familiar to you as the interstellar visitor that zipped through our solar system at nearly 16 miles per second, discovered in mid-October. It has now been given a less alphanumeric name by the Minor Planet Center: ‘Oumuamua. That Hawai'ian name “reflects the way this object is like a scout or messenger sent from the distant past to reach out to us (ʻou means reach out for, and mua, with the second mua placing emphasis, means first, in advance of)

At 400 meters (about a quarter mile) across, ‘Oumuamua is a relatively small visitor to our solar system. Though it passed through the innermost regions of the Solar system, closer to the Sun than Mercury, that’s not nearly close enough to be considered a sun-grazing comet, and well too far away to hit the Sun directly.

At 400 meters across A/2017 U1 is considerably larger than the vast majority of the comets spotted by the SOHO satellite, one of our Sun-monitoring satellites. SOHO’s main goal is to watch out for solar flares and other events on the surface of the Sun which could pose a hazard to the Earth, but its continual monitoring of the sun has also discovered a huge number of comets - in 2015, NASA celebrated SOHO’s 3,000th comet discovery.  These comets are usually only a few tens of meters across, ten times smaller than our interstellar visitor. SOHO has also spotted objects which blur the boundaries between comets and asteroids, probably a fairer comparison to our interstellar wanderer. One such discovery, comet 322P, is estimated to be around 100m in diameter, not so far off of the estimated size of 'Oumuamua.

If the object had hit the Sun directly, it would have been astoundingly bad luck for our interstellar wanderer. Imagine travelling for billions of years, only to run smack into a star - that’s like skiing into the only tree on the entire mountain. If that had happened, though, that’s a straightforward end to this interstellar object. Plunging into the incredible heat of our Sun would have destroyed that object, however rocky it was.

A sun grazing comet as witnessed by the ESA/NASA Solar and Heliospheric Observatory, or SOHO, as it dived toward the sun on July 5 and July 6, 2011. SOHO is the overwhelming leader in spotting sungrazers, with almost 3000 spotted to date. SOHO can see the faint light of a comet, because the much brighter light of the sun is blocked by what's known as a coronograph. Image credit: ESA&NASA/SOHO

A sun grazing comet as witnessed by the ESA/NASA Solar and Heliospheric Observatory, or SOHO, as it dived toward the sun on July 5 and July 6, 2011. SOHO is the overwhelming leader in spotting sungrazers, with almost 3000 spotted to date. SOHO can see the faint light of a comet, because the much brighter light of the sun is blocked by what's known as a coronograph. Image credit: ESA&NASA/SOHO

Grazing the Sun involves swinging past the Sun at such a close distance that your object is traveling within a contour that’s less twice the size of the Sun. Generally, from observations by satellites like SOHO, it seems that only comets which are more than a few kilometers across will survive the intense environment that close to the Sun - comets smaller than that will evaporate entirely away, reaching the same fate as their plunge-diving cousins. Asteroids and other rocky objects are a little more durable than the ice of a comet, but the harshness of the space immediately surrounding the Sun will abrade away the surface of even very durable materials.

Would we have been able to spot this abrasion of a small rock? The more comet-like our visiting object were, the easier it would be, since SOHO easily spots comets a tenth the size of our visitor. Rocky objects are harder to spot because they tend not to form large tails, but they will still reflect light into any waiting cameras, and as the detection of 322P proves, intermediate objects are still readily detectable at the size of 'Oumuamua. If the object were 100% rock, it reflects so little light that it would be much more difficult to observe with SOHO unless the object were another factor of ten or so larger - kilometers instead of hundreds of meters across. However, since it seems that 'Oumuamua was one of these mysterious, rocky/icy objects like the objects in our own Kuiper belt, it might have been more analogous to the hybrid comets we've spotted so far. In that case, as long as it had gone within SOHO’s field of view, we might have had a good chance of seeing the reflected sunlight from its surface. SOHO can spot objects a little beyond the surface of the Sun out to 30 times the radius of the Sun (the very surface of the Sun is too bright, and so it’s blocked from view). It might have been harder, given the brief flash of observation time we would have had before it annihilated, to determine exactly where it had come from, and we certainly wouldn’t have had time to get more information on our first interstellar visitor, like its color (red)!

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Can You Make An Object Pretend It Has Less Mass Than It Does?

Speed can make objects act as if they have more mass, but can you ever make an object appear to have less mass?
A NASA DC-9 reduced-gravity aircraft is featured in this image during a parabolic flight photographed from a T-38 aircraft. Image credit: NASA

A NASA DC-9 reduced-gravity aircraft is featured in this image during a parabolic flight photographed from a T-38 aircraft. Image credit: NASA

Originally posted on Forbes!

It’s true that speeding an object up to considerable fractions of the speed of light will make things behave like they are more massive than they do when they’re moving at slower, more human, speeds. But going the other direction is not as easy. Objects which are not moving at relativistic speeds are measured to have what’s formally known as their rest mass, the mass that you get when you sum up the masses of all the individual particles that make them up. For a human, this rest mass usually puts people at a mass of tens of kilograms (between 50 and 80 kilograms is 110-175 pounds).

It’s relatively easy to make something weigh less, as weight is the interaction between a mass and the gravitational field of the Earth. Want to make something weigh less? Try putting it in a gravitational field that isn’t as strong. The Moon is a good place for this kind of experiment; with a gravitational field only a sixth as strong as the Earth, a 60 kilogram person would feel about as heavy as a 10 kilogram baby (a milestone the median girl hits at 13 months old, and the median boy hits at 11 months). 

Scientist-Astronaut Harrison H. Schmitt is photographed standing next to a huge, split boulder at Station 6 (base of North Massif) during the third Apollo 17 extravehicular activity (EVA-3) at the Taurus-Littrow landing site on the Moon. Image credit: NASA

Scientist-Astronaut Harrison H. Schmitt is photographed standing next to a huge, split boulder at Station 6 (base of North Massif) during the third Apollo 17 extravehicular activity (EVA-3) at the Taurus-Littrow landing site on the Moon. Image credit: NASA

There’s also weightlessness, which we can experience on Earth in amusement park rides which drop you from a height, specialized planes like the Vomit Comet, or if you’ve gone skydiving. All of these situations have something in common; you’re falling. Technically speaking, the sensation of weight is provided by a contact between your body and something else - for us humans, this is typically the floor, a bed, or a chair. If you’re falling freely, that contact is missing, and you can feel like you weigh nothing. The astronauts in the International Space Station get this sensation for longer periods of time, but the ISS can be thought of as very carefully perpetually falling. (The sideways speed of the ISS allows it to fall around the planet instead of down towards the surface.)

However, in all these weight-altering situations, the mass of our people or objects would still fundamentally remain the same as it was when they were sitting, on their own, on the surface of our planet. Measurements would bear that out no matter how paltry the gravitational field of the world you’re standing on, or how much you happened to be falling at the time. These are all completely expected behaviors of a massive object, given a particular motion within a particular gravitational field.

Can you make something behave like it has less mass? Not unless you remove some material from the object. There are lots of ways to remove mass from an object, but most of them involve physically removing some of the object. You can remove mass either mechanically, by making the object physically smaller, or made of different materials, or by converting some of the mass into energy. Converting mass into energy is not an easy thing to do (which I find very fortunate, as I personally like being stable as an entity of matter and not an unstable energy bomb) but it can happen in the Universe. The most dramatic recent example is that of the binary, merging black holes detected by LIGO - the final black hole is considerably less massive than the sum of the two black holes - the remaining mass was lost, converted into the energy required to produce the gravitational waves in the first place.

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How Do We Track Photons Through Space?

When a particle moves through spacetime, how do we know it is the same particle and not some excitation that is passed from place to place?
Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop Nebula, taken by NASA’s Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred 5,000-8,000 years ago. Image credit: NASA/JPL-Caltech

Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop Nebula, taken by NASA’s Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred 5,000-8,000 years ago. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

We don’t! This is a really interesting feature of our universe, and it comes from the observation that all subatomic particles are described by a few key properties, but are otherwise completely and utterly identical. Electrons appear to be identical to all other electrons. All photons (if they carry the same energy within them) are identical to all other photons of that energy. Protons are identical to other protons, and neutrons are identical to other neutrons.  All of these particles are distinguished from each other by their mass, electric charge, and a property called their spin. However, each and every single electron in the Universe has the exact same mass, electric charge, and spin. There are no other measurements we can do to distinguish a given electron from another.

We could think of it along these lines: let’s say I give you a ping pong ball, and tell you that this one is special because it’s yours. But then we throw that ping pong ball into a bag full of other balls which look just like yours and mix them up. It’d be quite difficult to tell if the one I pull out of the bag next is the one I initially gave you or another one. If that newly drawn ball is identical in all measurable ways to the original one I told you was yours, there’s really no way to tell if it’s the one I originally handed to you or not.

Photons have one extra parameter that can distinguish them from each other and it’s the amount of energy they’re carrying. This energy corresponds to the color of the light - the more energy, the further to the blue the light appears, and the less energy, the harder to the red it falls. I can distinguish a blue photon from a red one as it hits my camera because of this difference in energy, but the mass, electric charge, and spin of those two photons are the same.

If the photons have the same energy when they arrive, then I’ve run out of ways to distinguish them.

Flaring, active regions of our sun are highlighted in this new image combining observations from several telescopes. High-energy X-rays from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue; low-energy X-rays from Japan's Hinode spacecraft are green; and extreme ultraviolet light from NASA's Solar Dynamics Observatory (SDO) is yellow and red. Image credit: NASA/JPL-Caltech/GSFC/JAXA

Flaring, active regions of our sun are highlighted in this new image combining observations from several telescopes. High-energy X-rays from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue; low-energy X-rays from Japan's Hinode spacecraft are green; and extreme ultraviolet light from NASA's Solar Dynamics Observatory (SDO) is yellow and red. Image credit: NASA/JPL-Caltech/GSFC/JAXA

So if I dump a bunch of photons into my metaphorical bag, and they all come out again, there’s no way for me to tell if my favorite photon came out first or last. The closest astrophysical approximation to this simple setup is light which strikes the surface of the Sun and is then absorbed. That photon is now mixing with a huge number of other photons created within the depths of the Sun, and I have no way of flagging that particular photon to distinguish it from the flood of other, identical photons which are streaming outwards away from the Sun.

The energy that a photon carries isn’t a fundamental property of the photon in the way that its electric charge (which is neutral) and its spin are fundamental properties. Fundamental properties cannot be changed, no matter what happens to these photons in the course of bouncing around the Universe. So the energy of a photon, not being a fundamental property, canbe changed. And this energy often is changed, making our attempt to keep track of individual photons even more difficult. The photons that stream from the Sun and onto the surface of the Earth deposit some of their energy into the matter of the Earth, heating up the ground. That heating process depletes the energy remaining in the photon, and so the photon which reflects away has changed the amount of energy that it carries with it. So, if I see photons streaming into a region of space where they must interact with other objects, the identities of individual photons are even more scrambled than they would have been while they were streaming freely through space.

Your idea of energy excitations passing from place to place is precisely the right one for fundamental particles - nothing we can measure will tell me which electron is my favorite.

 

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Can A Star Ever Turn Its Spin Backwards?

Can a star reverse its rotational direction during some time in their life, and if so, how would it affect any planets around it?
This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

Originally posted on Forbes! 

The stars in the night sky all have their preferred direction of rotation, which depends directly on the exact way that the cloud of gas and dust that the star formed out of collapsed. If there was slightly more random motion in a clockwise or a counterclockwise direction, as the cloud of gas collapsed, the star would have magnified that hint of rotation, spinning up the same way that a figure-skater does, pulling in their arms and legs.

Once spun up this way, another piece of fundamental physics comes into play - inertia. Inertia tells us that objects in motion tend to stay in motion unless there’s something else that’s causing that object to slow down. On Earth, that something else can come in many forms - we have the mass of the Earth, whose gravity will pull objects down towards the surface, an atmosphere to move through, which will slow objects moving through it, or quite simply mountains and buildings which objects can bounce off of and away from.

If you’re in space, these sorts of Earthly obstacles which can serve to stop a moving object aren’t around. There are many fewer things which could serve to stop an object’s motion - without a thick atmosphere to move through, the planets and our spacecraft continue at their current speeds without any impediments. The most common method of slowing down (or speeding up) an object in space is by traveling near another large object (ones that are of a similar mass to yourself are the most effective) and letting the force of gravity alter your path.

These requirements for an external force hold both for motion as we normally think about it (a forward or sideways motion) and for spin. So if we think about spinning a bicycle tire, that wheel will continue to spin until the forces of friction in the axle (primarily) will slow it down, or until you clamp down on the brakes. An object which is spinning in space has no friction-containing axle around which to spin, and so if it’s isolated, without any external objects which can act as a braking force, that object should continue to spin as it is, ad infinitum. This is basically the situation that stars find themselves in. Stars do not reverse their spins as a standard part of their lifetimes.

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

A reversal in the rotation of a star is extremely difficult to accomplish without something external to the star punching it backwards in the other direction, to slow down each rotating particle that makes up the star. If you could, using magic, reverse the rotation of our own star, without changing anything else, the planets surrounding our Sun wouldn’t be influenced at all - the orbits of the planets are determined by the gravitational pull of the Sun, which hasn’t changed if we haven’t changed its mass, plus the planets’ own velocities.

What sort of objects could act as a brake in space? The is easiest is always a good old large-scale collision. This is how we think the Earth formed the Moon, how Uranus got tipped onto its side, and Venus got tipped completely upside down. So if we wanted to reverse the spin of the Sun, we’d have to hit it with something pretty catastrophic in order to both stop the rotation of the gas & plasma which currently makes up the Sun, and reverse the direction of the spin. Any impact on that scale would definitely impact the planets. If it was another star that hit our own, we’d have gravitational and temperature-related chaos even before the impact, setting aside whatever cataclysm of energy would be unleashed during the collision.

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