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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How Do We Map The Earth’s Gravity?

Can Earth’s center of gravity be located? And if so, to what precision?
Satellite measurements offer scientists a new view of our planet. Warm colors (red, orange, yellow) represent areas with strong gravity. Cool colors (green, blue) represent areas with weak gravity. Image credit: NASA's Goddard Space Flight Center

Satellite measurements offer scientists a new view of our planet. Warm colors (red, orange, yellow) represent areas with strong gravity. Cool colors (green, blue) represent areas with weak gravity. Image credit: NASA's Goddard Space Flight Center

Originally posted on Forbes!

Earth’s center of gravity can be located! We talked a few months ago about measuring the force of gravity surrounding the Moon, and that the way we do this measurement is by having twin satellites, and calculating the difference in gravitational pull on each satellite. As the satellites go over high density regions, the one of them will feel an increased pull before the other, and the distance between the two satellites will change. These tiny changes in the distance between the two satellites allow us to map out the density of the ground below, but it's fundamentally a measure of the strength of the gravitational pull of the ground below the satellites.

We can do the exact same thing for pairs of satellites around the Earth, and we have! The Gravity Recovery and Climate Experiment (GRACE) is a NASA mission to do precisely this. It was a pair of satellites, launched in 2002, which bounced microwaves back and forth between them, very precisely measuring the distance between them, to a sensitivity of about a micron (many times smaller than the width of a human hair.) By additionally communicating with GPS satellites, the GRACE satellites were able to precisely communicate both their absolute positions in orbit around the Earth (to a precision of about a centimeter), and their motions relative to each other. Any deviations in their relative distances should be due to something down below, on Earth.

Artist's rendering of the twin satellites that will compose NASA's Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission. Image credit: NASA/JPL-Caltech

Artist's rendering of the twin satellites that will compose NASA's Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission. Image credit: NASA/JPL-Caltech

ESA has also had one of these earth-measuring satellites, called GOCE (Gravity field and steady-state Ocean Circulation Explorer) which operated between 2009 and 2013. Instead of having two independent satellites, it had two sets of accelerometers at opposite ends of one, long, tubelike satellite, which each measured gravity at their end of the satellite.

Both experiments were able to generate maps of the Earth’s gravitational field strength from their locations in orbit. In practice, these are often reported back as a “geoid”, which is a way of deforming the Earth’s sphere so that any point on its surface would have an equal gravitational strength. Anything built up into a lump outwards indicates that there’s extra mass there, and anything sunken inwards indicates that there’s less mass. GOCE managed to map the gravitational strength of the Earth beneath it to a precision of 10^–5 m/s2.  While we commonly quote the gravitational force as 9.81 m/s^2, this satellite was measuring it out to 0.00001.

ESA's GOCE mission has delivered the most accurate model of the 'geoid' ever produced, which will be used to further our understanding of how Earth works. The colours in the image represent deviations in height (–100 m to +100 m) from an ideal geoid. The blue shades represent low values and the reds/yellows represent high values. Image credit: ESA/HPF/DLR

ESA's GOCE mission has delivered the most accurate model of the 'geoid' ever produced, which will be used to further our understanding of how Earth works. The colours in the image represent deviations in height (–100 m to +100 m) from an ideal geoid. The blue shades represent low values and the reds/yellows represent high values. Image credit: ESA/HPF/DLR

You’ll notice that both of these experiments have another facet to their names - GOCE also says it’s monitoring the ocean circulations, and GRACE is also a climate experiment. That’s because these very precise gravitational measurements can also track the motion of water around our planet. Not just the locations of surface water, or the amount of water in the oceans versus at the poles, but underground water, in reservoirs. Water is a relatively dense material, and so its presence or absence in a certain location will alter the average density of the planet underneath either of these observatories.

GRACE has a follow-up mission, intended for launch this year - GRACE-FO. The FO stands for Follow-On, and is intended to increase the accuracy of the GRACE experiment dramatically, by using laser beams to check the distances between the satellites, instead of microwaves. GRACE-FO will also help us continually monitor our fragile world’s water supplies as the original GRACE satellites age. Not every satellite lasts 15 years, after all.

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Would You Float At The Core Of The Earth?

Thought experiment...if you built a bedroom sized room at the center of the Earth, and you are in that room, which way is down? Please explain to me why if you are surrounded by the same amount of mass in every direction, how does that NOT EQUAL NET ZERO? In other words, would that not be the exact same thing as weightlessness? So what would an illustration of the curvature of Space look like at the center of a massive body? Wouldn’t there be a vortex of some sort? It’s kind of important to me to understand where I’m going wrong?
A 'Blue Marble' image of the Earth taken from the VIIRS instrument aboard NASA's most recently launched Earth-observing satellite - Suomi NPP. This composite image uses a number of swaths of the Earth's surface taken on January 4, 2012. The NPP satellite was renamed 'Suomi NPP' on January 24, 2012 to honor the late Verner E. Suomi of the University of Wisconsin. Image Credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring

A 'Blue Marble' image of the Earth taken from the VIIRS instrument aboard NASA's most recently launched Earth-observing satellite - Suomi NPP. This composite image uses a number of swaths of the Earth's surface taken on January 4, 2012. The NPP satellite was renamed 'Suomi NPP' on January 24, 2012 to honor the late Verner E. Suomi of the University of Wisconsin. Image Credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring

Originally posted at Forbes!

You very nearly got there! Let’s run with your example of a small room at the center of the Earth, but for my sanity, I’m going to make your room a sphere instead of a square, because everything else involved in this example is going to be round, and it makes the example easier.

So; gravity pulls on any object with a force that’s related to the masses of the two objects involved, and inversely related to the distance between them. This tells us that the more massive the two objects, the greater the pull, and the greater the distance between them, the weaker the influence. For us, on the surface of the Earth, we can work out how strong gravity is. All you need is the mass of the Earth, the mass of a human, and the distance between the center of the Earth and the surface of the Earth.

The mass of the Earth in kilograms is 5.972 × 10^24. 10^24 is a septillion, which is a number so outrageously large that it might be more manageable to think about as a trillion trillion kilograms. (In SI units, which most physicists use, this is gives you the prefix yotta. 5972 yottagrams! It’s fun to say.)  The mass of a human, in comparison, is negligible. The radius of the Earth is 3,959 miles - 6,371 km. If you plug these numbers in, you pull out the gravitational acceleration at the surface of the Earth; 9.81 meters per second every second. This pulls you in towards the surface of the planet.

Photographed from a shuttle training aircraft, space shuttle Endeavour and its six-member STS-134 crew head toward Earth orbit and rendezvous with the International Space Station. Liftoff was at 8:56 a.m. (EDT) on May 16, 2011, from Launch Pad 39A at NASA's Kennedy Space Center. Onboard are NASA astronauts Mark Kelly, commander; Greg H. Johnson, pilot; Michael Fincke, Andrew Feustel, Greg Chamitoff and European Space Agency astronaut Roberto Vittori, all mission specialists. STS-134 will deliver the Alpha Magnetic Spectrometer-2 (AMS), Express Logistics Carrier-3, a high-pressure gas tank and additional spare parts for the Dextre robotic helper to the International Space Station. STS-134 is the final spaceflight for Endeavour. Image credit: NASA

Photographed from a shuttle training aircraft, space shuttle Endeavour and its six-member STS-134 crew head toward Earth orbit and rendezvous with the International Space Station. Liftoff was at 8:56 a.m. (EDT) on May 16, 2011, from Launch Pad 39A at NASA's Kennedy Space Center. Onboard are NASA astronauts Mark Kelly, commander; Greg H. Johnson, pilot; Michael Fincke, Andrew Feustel, Greg Chamitoff and European Space Agency astronaut Roberto Vittori, all mission specialists. STS-134 will deliver the Alpha Magnetic Spectrometer-2 (AMS), Express Logistics Carrier-3, a high-pressure gas tank and additional spare parts for the Dextre robotic helper to the International Space Station. STS-134 is the final spaceflight for Endeavour. Image credit: NASA

Now, we’ve done something sneaky here, which is to assume that we can place the entire mass of the Earth at the very center of the Earth, and consider ourselves simultaneously at the surface, and six thousand kilometers away from the Earth’s mass. We can do this because the Earth is a sphere, and that means that there's an awful lot of symmetry to work with. You can also do the math very carefully, considering the pull of the Earth’s mass to the left of you, which will pull you slightly to the left, and the pull of the Earth’s mass to the right of you, which pulls you equally strongly to the right. There’s no net force going sideways, because you’re standing on a symmetric planet, and all the left and right directions will cancel out. All that’s left is the 'downward' direction.

Again, if you do it carefully, you have to consider the gravitational pull from the ground directly beneath your feet (which is quite close) and the ground on the opposite side of the planet, which is 7918 miles away. There’s the same amount of planet closer to you and farther from you (relative to the center of our planet), so on average, the force is the same as if it came from a point at the center. Mathematically, our trick of assuming that the entire mass of the planet is contained at the core of the Earth is identical to doing it all very carefully, and it is much easier to do.

ESA’s Swarm satellites have led the discovery of a jet stream in the liquid iron part of Earth’s core 3000 km beneath the surface. In addition, Swarm satellite data show that this jet stream is speeding up. Launched in 2013, the Swarm trio is dedicated to identifying and measuring precisely the different magnetic signals that make up Earth’s magnetic field. Image credit: ESA CC BY-SA 3.0 IGO

ESA’s Swarm satellites have led the discovery of a jet stream in the liquid iron part of Earth’s core 3000 km beneath the surface. In addition, Swarm satellite data show that this jet stream is speeding up. Launched in 2013, the Swarm trio is dedicated to identifying and measuring precisely the different magnetic signals that make up Earth’s magnetic field. Image credit: ESA CC BY-SA 3.0 IGO

What does this mean for your spherical room at the core of the planet? Well, this principle of canceling out forces if they’re pulling on you in different directions still holds, and so you’re absolutely on the money to say that you should be weightless in there. You absolutely could float at the center of the planet; the entire mass of one half of the planet pulling you to the left would cancel the remaining mass of the planet pulling you to the right. And the same is true of being pulled upwards/downwards, or any direction that you care to slice the planet in half. There would be no 'down'.

What does a depiction of space time look like in the middle of the planet? Let’s remember that our depictions of space time usually depict divots surrounding massive objects, where gravity pulls you “down” into the gravitational well. With no gravitational force, and no 'down', your room in the core of the Earth would have a space-time curve that was very flat. No bending, no vortex. If there’s no net gravitational force, there can be no slant or directionality to space-time.

It’s flat because you’re at the very very bottom of the gravitational well. To leave your room at the center of the Earth, you’d have to climb your way all the way back up to the surface of the Earth, and as soon as you left, there would be a net force, pulling you back down. As you climb out, more and more of the Earth is left below you, and the downward dragging force you would feel would increase almost continuously until you reached the surface. The surface is a much more hospitable place, in any case; it’s got all my favorite things on it.

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Why Does The Earth Pull On One Side Of The Moon More? Is The Moon Lopsided?

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How Many Rockets Would We Need To Launch Into Space To Feel Lighter On Earth?

How many rockets and space equipment would we need to send up before making a change in the earth’s gravity?
Orbits of current Earth-orbiting geophysics satellites. In magenta: TIM (Thermosphere, Ionosphere, Mesosphere) observations; in yellow: solar observations and imagery; in cyan: Geospace and magnetosphere; in violet: Heliospheric observations. At geostationary orbit, GOES and SDO keep watch on the Sun. Image credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

Orbits of current Earth-orbiting geophysics satellites. In magenta: TIM (Thermosphere, Ionosphere, Mesosphere) observations; in yellow: solar observations and imagery; in cyan: Geospace and magnetosphere; in violet: Heliospheric observations. At geostationary orbit, GOES and SDO keep watch on the Sun. Image credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

Originally posed at Forbes!

A lot. The strength of Earth’s gravity is controlled by two fundamental properties of our planet; the distance from the very core of the earth to the surface, and how much mass is held within that space. If our planet were the same size, but made out of packing peanuts instead of rock, the force of gravity at the surface would be much less than it currently is. Similarly, if we took the same amount of material – the same mass – but changed how densely packed it is, you could reduce the pull of gravity on the surface. However, neither of these things is easily changed. The Earth is Earth-sized because it’s mostly made of molten rock and metal, with a bit of liquid water and solid rock on the surface. Molten rock and metal are both pretty hard to compress beyond a certain density, and difficult to fluff up to make it more styrofoam-like (unless you fill it with pockets of gas).

The equation to figure out the strength of gravity on the Earth is pretty simple: g = GM/r^2. M is the mass of the planet, r is the distance from the center to the surface, and G is the gravitational constant, which is a constant feature of our Universe. It’s also a very small number, so it winds up canceling out the very large numbers the Earth is going to dump into this equation. Once we plug in the radius of the Earth and the mass of the earth, we find that gravity on the surface of the Earth is pulling you towards the ground at 9.81 m/s every second.

If we wanted to change the force of gravity, we’d have to reduce this number, which means either increasing the size of the planet (basically impossible), or removing some of its mass (more possible). We have our method of mass removal given in the question, so we’re going to build rocket ships, load them up with stuff, and launch them out into space. How much stuff would we need to remove before the Earth’s gravity changes? Technically, everything we send up removes some mass from the Earth, but it’s such a minuscule fraction of the Earth’s mass that we will never notice the difference. So how much material would we have to send up before we’d notice the difference?

Let’s try and change gravity by ten percent.

This means everyone will feel 10% lighter on the surface, and with the same amount of force, you’d be able to jump higher, and falling would hurt less.

This means we need to reduce the Earth’s gravitational pull from 9.81 meters per second per second to 8.83. If we’re not expanding the distance to the Earth’s surface, the only thing left to change is the mass of the earth, so we’ll have to reduce the Earth’s mass by ten percent. Pretty straightforward.

But the Earth is pretty big. 5.972 × 10^24 kg big. This is a number so outrageously huge that it basically doesn’t make sense to write it in kilograms anymore. We typically write it in “Earth masses” instead, but that’s even less useful for getting a sense of scale. In any case, let’s divide this number to find 10% – there’s some useful scale coming ahead. 10% of the Earth is 5.972 × 10^23 big – one less zero, but twenty three zeros is still a pretty big number.

A comparison of the sizes of Earth and Mars. Image credit: NASA

A comparison of the sizes of Earth and Mars. Image credit: NASA

In fact, it is the mass of Mars. Mars is only slightly more massive than this – with a mass of 6.39 × 10^23 kg, it’s just under 11 percent of the mass of the Earth. So in order to change the gravity of the Earth by a noticeable but not incredibly dramatic ten percent, we would have to extract from the surface of the earth, One Whole Mars worth of material. This, as you can probably guess, would be grossly unwise. If we were to peel off the entire crust of the earth, which is some 3-30 miles deep, and throw in the entire mass of all of the oceans for kicks, we’re still only looking at about a half a percent of the earth’s mass, and we’ve made our planet into Lava Planet. (Never mind the mechanics of peeling off the crust of the Earth, which I can only imagine would go spectacularly poorly.) In fact, in order to get our ten percent, we’d have to extract pretty much the entire upper mantle and jet it into space in order to reduce gravity by 10 percent, and our surface relies on that upper mantle for stability. When the mantle moves, our crust moves with it- which is part of the reason we get earthquakes. Removing that structure from underneath us would be a Grade A Bad Plan.

A NASA/university study of data on Earth’s rotation, movements in Earth’s molten core and global surface air temperatures has uncovered interesting correlations. Image credit: NASA/JPL-Université Paris Diderot – Institut de Physique du Globe de Paris

A NASA/university study of data on Earth’s rotation, movements in Earth’s molten core and global surface air temperatures has uncovered interesting correlations. Image credit: NASA/JPL-Université Paris Diderot – Institut de Physique du Globe de Paris

Of course, beyond the matter of extracting a Mars-worth of magma from the innards of the planet, there’s the slight issue of where to put it.  Mars is not exactly our smallest planetary body, so if we reassembled all of our Earth-extractions into a planet again we might run into some minor orbital disturbances, suddenly having a second Mars hanging around. If we don’t leave it as a single object, but leave it scattered in small pieces, then we have created our very own Asteroid belt.  I would recommend putting your asteroid belt very far away from Earth, or holy space junk Batman, we have created a very hazardous near-Earth environment, which already needs some cleaning.

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