Does The Earth's Magnetic Field Go Past The ISS?

Does the Earth’s magnetosphere encompass the ISS and does it offer the same protection as it does our atmosphere and planet?
A profile view of the magnetic field and density data. Image Credit: NASA's Scientific Visualization Studio, the Space Weather Research Center (SWRC), the Community-Coordinated Modeling Center (CCMC) and the Space Weather Modeling Framework (SWMF).

A profile view of the magnetic field and density data. Image Credit: NASA's Scientific Visualization Studio, the Space Weather Research Center (SWRC), the Community-Coordinated Modeling Center (CCMC) and the Space Weather Modeling Framework (SWMF).

Originally posted on Forbes!

The International Space Station, or ISS, orbits our planet once every 90 minutes at the lofty height of 400 kilometers (about 248 miles) above the surface of our planet. This altitude puts it pretty well above the vast majority of the atmosphere, but it doesn’t place it outside the reaches of the magnetic field which surrounds our Earth.

The International Space Station, in orbit around Earth. Image credit: Science@NASA and NASA’s Goddard Space Flight Center, International Space Station image courtesy of NASA

The International Space Station, in orbit around Earth. Image credit: Science@NASA and NASA’s Goddard Space Flight Center, International Space Station image courtesy of NASA

The magnetic field of our planet — otherwise known as the magnetosphere — extends out to about 65,000 kilometers (40,000 mi) above the surface of the planet. However, that “about” part is pretty critical — the magnetosphere isn’t a fixed boundary, which always remains at exactly 40,000 miles from the surface. This surface is a little more flexible, and if you’ve ever held two opposing magnet ends against each other, you’ve felt this exact flexibility. The resistance between two magnets isn’t a wall, where suddenly you can’t move them any closer to each other. The pressure there is a little more like pressing on a slightly under-inflated balloon.

In the case of the Earth’s magnetic field, the pressure on our magnetic field comes from the Sun. The Sun is constantly battering everything surrounding it with a solar wind, made up of charged particles. If you’re a planet without a protective magnetic field, this solar wind will slam into your atmosphere, and can destroy it over time. This is roughly what we believe happened to Mars’ atmosphere. The Earth has a very fortunate protective shield, but this constant pressure on the Sun’s side of our planet means that this magnetic protection is pressed back, closer to the planet. That 40,000 mile number I gave is this: the typical, Sun-compressed, Sun-facing side of our magnetosphere. The nighttime side of our planet, facing away from the Sun, has a long magnetic tail drifting out beyond it, extending several times farther out than the Sun-side.

A profile view of the magnetic field and density data during a solar outburst. Image Credit: NASA’s Scientific Visualization Studio, the Space Weather Research Center (SWRC), the Community-Coordinated Modeling Center (CCMC) and the Space Weather Modeling Framework (SWMF).

A profile view of the magnetic field and density data during a solar outburst. Image Credit: NASA’s Scientific Visualization Studio, the Space Weather Research Center (SWRC), the Community-Coordinated Modeling Center (CCMC) and the Space Weather Modeling Framework (SWMF).

If the Sun has a particularly strong outburst – a coronal mass ejection or any kind of solar flare — the pressure on our magnetic field gets much stronger, but nothing the Sun typically does will press the magnetic field down close enough to the Earth’s atmosphere so that the ISS would exit the magnetic field. Some of our highest orbiting satellites do exit the magnetic field of the Earth, as of course all craft going to other planets must also do. However, these satellites and spacecraft must be constructed to protect their inner workings from the charged particles in the solar wind. Satellites are effectively very elaborate electronics, and electronics do not like being exposed to charged particles. It shorts their circuits.

All this really means that the ISS is in a much safer region of space than it could be – not that it’s totally safe. Our magnetic field is not a perfect blocker of high energy particles, and so things like gamma rays, cosmic rays, and other damaging radiation can still appear in higher quantities than they would if the astronauts were still safely on  the ground. Our atmosphere is pretty good at blocking a lot of these high energy particles, so on the ground you’d never get exposed to them. But the ISS is above the atmosphere, and doesn’t have this extra layer of shielding, so there are radiation monitors on the space station to keep track of how much of a radiation dose they’re getting. If a solar flare is on its way, the astronauts usually have a few days’ warning, and can take shelter in more strongly shielded section of the ISS if they need to. (Not all solar storm are aimed in their direction, and not all storms are strong enough to require this precaution.)

So yes, the ISS is firmly embedded in the Earth’s magnetosphere, making it — for a space based outpost — a relatively safe haven for our astronauts.

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How Are Astronomical Distances Measured?

How is astronomical distance determined? Just cannot get my head around cepheid variables, parallax, etc. How is it possible to tell how far away something is when you cannot bounce a radar beam off the object and time its return?
RS Puppis rhythmically brightens and dims over a six-week cycle. It is one of the most luminous in the class of so-called Cepheid variable stars. Its average intrinsic brightness is 15,000 times greater than our Sun’s luminosity. Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration

RS Puppis rhythmically brightens and dims over a six-week cycle. It is one of the most luminous in the class of so-called Cepheid variable stars. Its average intrinsic brightness is 15,000 times greater than our Sun’s luminosity. Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration

Originally posted at Forbes!

Well, you’re absolutely right that radar is an ideal way of measuring distances to objects; with radar you bounce a radio or microwave pulse off of the other object (a planet, for instance) and wait for it to come back. The Arecibo Observatory in Puerto Rico is one of the best observatories for doing this kind of work, and it’s not limited to planets, though objects that are large and nearby are easiest. Asteroids and comets are also good targets for radar observations, and the radar allows us to not only get great distances but a general idea of the shape of the object.

But radar has limited usefulness once the object you’re interested in gets too far away, and when we need to get a distance from an object in the outer regions of our solar system, or for the nearest stars, we have to find another option. That option is parallax, which is also pretty straightforward as astronomy distance measurements go, because it’s mostly just geometry.

Parallax illustration. Image credit wikimedia user Abeshenkov, public domain.

Parallax illustration. Image credit wikimedia user Abeshenkov, public domain.

We’re well familiar with parallax as a phenomenon, even if we’ve never had the name to apply to it. Parallax is simply that objects which are close to you will appear to move, relative to some distant object, if you move. It’s why, when you take a picture out of a moving car’s window, the scenery along the roadside will be blurred out, whereas the distant scenery is captured without any motions. The roadside has a large parallax effect relative to the background. You can do this at home, too – close one eye and hold a finger out at arm’s length. Get your finger to cover up some object on your wall – a light switch or something. Now close that eye and open the other one. Your finger will appear to jump sideways away from the object it was covering. That’s parallax.

With a little math you can figure out how far away that apparently moving object is. All you need to know is how far apart your vantage points were (in the finger example, the distance between your eyes), and how far the object (your finger) appeared to move. With that information, you can work out how far away the object must have been in order for the angles to work out. If the object were closer, it would appear to have moved more. If it were farther, it would move less.

The other thing you can change is how far apart your viewing positions are. The farther apart, the more obvious the effect. For astronomical distances, we can make use of this by measuring the positions of stars when our planet is at opposite ends of our orbit around the Sun. Six months apart gives us viewing positions which are 186 million miles apart, instead of the few inches between your eyes. That allows you to see even the tiniest changes in a star’s position, relative to even more distant stars.

However, once you get beyond a few hundred parsecs, this kind of measurement gets really hard to do, and even with the best telescopes, parallax is only measurable out to about 1000 parsecs. Considering that we’re sitting 8,000 parsecs from the center of our galaxy, that doesn’t get very far. We’re going to need another method to get even farther away.

The NASA/ESA Hubble Space Telescope quashed the possibility that what was previously believed to be a toddler galaxy in the nearby universe may actually be considered an adult. Called I Zwicky 18, this galaxy has a youthful appearance that resembles galaxies typically found only in the early universe. Hubble has now found faint, older stars within this galaxy, suggesting that the galaxy may have formed at the same time as most other galaxies. Hubble data also allowed astronomers for the first time to identify Cepheid variable stars in I Zwicky 18, marked by the red circles. These flashing stellar mile-markers were used to determine that I Zwicky 18 is 59 million light-years from Earth, almost 10 million light-years more distant than previously believed. Image Credit: NASA/ ESA/ STScI (A. Aloisi)

The NASA/ESA Hubble Space Telescope quashed the possibility that what was previously believed to be a toddler galaxy in the nearby universe may actually be considered an adult. Called I Zwicky 18, this galaxy has a youthful appearance that resembles galaxies typically found only in the early universe. Hubble has now found faint, older stars within this galaxy, suggesting that the galaxy may have formed at the same time as most other galaxies. Hubble data also allowed astronomers for the first time to identify Cepheid variable stars in I Zwicky 18, marked by the red circles. These flashing stellar mile-markers were used to determine that I Zwicky 18 is 59 million light-years from Earth, almost 10 million light-years more distant than previously believed. Image Credit: NASA/ ESA/ STScI (A. Aloisi)

This is where Cepheid Variables come in. Cepheids are an interesting class of star which change their brightness over time in a predictable, repeating pattern. And, very usefully for distance measurements, that repeating pattern changes depending on how intrinsically bright the star is, a discovery made by Henrietta Swan Leavitt in 1902. We can therefore use the speed of the Cepheid’s pulse to tell if it’s faint in our skies because it’s intrinsically dim, or because it’s faint because it’s far away.

We know how bright the Cepheid should be, because of its pulse, so any fainter means it’s farther away. We know how brightness fades with distance – twice as far away means eight times as faint. The mismatch between how bright the Cepheid appears in the sky, and how bright it should be gives us this distance. This method works well throughout our galaxy and out to the nearest galaxies beyond us. To go even farther out in the universe, we need an even brighter tracer – supernovae.

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What Direction Should You Change Gravity To Change Earth's Distance From The Sun?

If we want to change the distance to the sun, in which direction do we have to change the gravity?
The diagram shows the Earth’s orbit in comparison with a circle centered on the Sun. The foci of the orbital ellipse and the positions of aphelion and perihelion are indicated. The count of ecliptical longitude starts at the First Point of Aries (the direction of the vernal equinox). Image credit: wikimedia user Sch, CC BY 3.0 A-SA

The diagram shows the Earth’s orbit in comparison with a circle centered on the Sun. The foci of the orbital ellipse and the positions of aphelion and perihelion are indicated. The count of ecliptical longitude starts at the First Point of Aries (the direction of the vernal equinox). Image credit: wikimedia user Sch, CC BY 3.0 A-SA

Originally posted at Forbes!

In any direction!  The distance between the Earth and the Sun is a careful balance between the mass of the Earth, the mass of the Sun, the speed with which the Earth orbits the Sun, and the strength of gravity.  If any of these things change in any direction, the orbit of the Earth would change.  If the orbit of the Earth changes, then the distance between the Earth and the Sun changes.

If we kept the strength of gravity the same, and increased the mass of the Sun, the Sun would exert a stronger gravitational force on everything that orbits it, and in the absence of any other changes in the solar system, would disturb the orbits of all of the planets, pulling them closer for at least part of their orbits. Our Earth has an almost perfectly circular orbit around the sun, but that’s not required to be the case – just look at the long, looping orbits of comets around our sun.  If you tug on an object once (as you would, if you suddenly increased the mass of the sun), you’ll likely pull a circular orbit into a more oval, looping orbit.

If the Earth’s mass suddenly increased, we’d have a few problems here on earth, as our bodies are calibrated to work best when we’re dealing with exactly the amount of gravitational force that we have. But on an orbital sense, the gravitational force between two objects depends on the mass of both the larger and the smaller object. So if we increase the mass of the smaller object (the Earth), that will increase the gravitational force between the Sun and the Earth, probably still pulling the Earth off of its circular orbit, but also pulling more strongly on the Sun, making the Sun wobble very slightly more as the Earth orbits around it.

Orbits of 3 periodic comets: Halley, Borrelly and Ikeya-Zhang. Image credit: wikimedia user Morgan Phoenix , CC BY 3.0 A-SA

Orbits of 3 periodic comets: Halley, Borrelly and Ikeya-Zhang. Image credit: wikimedia user Morgan Phoenix , CC BY 3.0 A-SA

It’s not hard to imagine ways of changing the distance between two objects before you get around to playing with the strength of gravity. However, changing the strength of gravity does the exact same kind of things to an orbit as changing the masses of the objects you’re interested in.  Let’s say we increase the strength of gravity; that’s equivalent to making everything in the solar system more massive at once.  That change would, in turn, mean that the strength of the gravitational pull between all objects gets stronger.  The exact response of the orbits will depend on how different the masses are, but we can safely say that the Sun would be pulled into a more wobbly rotation around its own axis, and that the Earth would wind up closer to the Sun for at least part of the year.

If, on the other hand, we let gravity get fainter, the opposite happens.  The pull between Earth and Sun grows weaker, and the planets would drift farther from the Sun, spending much more time at much greater distances from the Sun. If you’d like to play around with this more directly you can try out this solar system simulator (there are a lot of other setups you can tinker with).

So really, any change to the strength of gravity, or any change to the masses of the objects involved, would change the distance between the Earth and the Sun. However, changing the strength of gravity would have relatively catastrophic consequences for a whole series of physical processes that we currently rely on, so I wouldn’t recommend it as a scenario.

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Is Cause & Effect Limited By The Speed Of Light?

Imagine lots of dominoes all lined up and ready to go over the length of a light-year. Am I right to assume that the speed of light is also the speed of causality (how fast things lead to one another in the universe) and that no matter what I do, the last domino will fall at best one year after the first?
A narrow wedge showing the evolution of the clustering across cosmic time. Image credit: Gus Evrard and Andrzej Kudlicki, Galaxy clusters in Hubble Volume Simulations, Evrard et al., 2002, ApJ, 573, 7

A narrow wedge showing the evolution of the clustering across cosmic time. Image credit: Gus Evrard and Andrzej Kudlicki, Galaxy clusters in Hubble Volume Simulations, Evrard et al., 2002, ApJ, 573, 7

Originally posted on Forbes!

The speed of light is usually thought of as a speed limit for how fast an object can travel, but you’re right to also think of it as a speed limit on the transfer of information.  If you want an object to influence another object, you first have to transfer some information around.  In the case of light, this influence can come in the form of light arriving on a detector, or perhaps a burst of radio waves, and this light unsurprisingly traverses the cosmos at precisely the speed of light.

The definition of causality from a physics perspective goes beyond a simple cause & effect link.  It’s more than just tying an event to the thing that caused it, though this is a critical component.  If I knock a glass over onto the floor, we can reasonably blame me for being the cause of that glass tipping over – that’s the cause and effect part.

However, say I was in the other room, and I just heard the glass fall, and don’t know what caused it. The physical principle of causality imposes limits on the number of things which could have caused the glass to fall.  The first rule of causality is that the order of time must be kept. Nothing you can do now will influence events that have already happened, earlier in time.  The second rule is that to influence anything in the universe later in time, the first event or object must transfer information across space and time.  Bu we already know that we have a maximal speed of information transfer – a maximum speed that can link two things causally; the speed of light.

The speed of light seems quick, and on human frames of reference, it is. But a lot of information travels much slower than the speed of light.  Sound, for instance, travels significantly slower than the speed of light. You’ll catch the flash of light from a lightning flash, but it’s the rolling thunder that will rattle the windows. The causal link between lightning strike and rattled windows travels slower than the speed of light, and so you have to wait for the sound wave to arrive for it to influence your windows.

Example of a light cone. Image credit: Wikimedia user Stib, CC 3.0 A-SA.

Example of a light cone. Image credit: Wikimedia user Stib, CC 3.0 A-SA.

You can draw out the regions of space which can possibly affect anything around you, and the regions of space which you can affect in the future, and it looks a bit like an hourglass.  This hourglass is called a light cone, with the point at the very centre as the present. In your domino example, you start the dominos tumbling at “now”, at this central point.  Your cone of influence extends out through time, as information about your push of the first domino extends outwards in space and time.  Someone watching you pushing over the domino sits along the edge of the light cone, as the information they need to see (light) travels at the speed of light. Of course, as time progresses, the “now” point progresses, and the light cones travel with it. In your example, the domino hits the next domino at some point in the future, which in turn hits the domino after that, all of which must be contained within this cone-space.

So yes – your dominos are bound to fall one after another within this light cone, which limits us to communicating at the fastest, at the speed of light.  So assuming that the dominos fell exactly instantaneously (unlikely to impossible), and no time was taken in transferring the energy from domino to domino (also unlikely to impossible), the fastest the furthest one could fall, while having the first domino be the cause, is one year after the first.  By the same token, if you see the last dominos fall before a year has gone by, it guarantees that something other than the domino stack is responsible for its fall.

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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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