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Mapping Gravity
overThruster writes: "No, you don't need to drink the water... Gravity is less strong in India--enough so that you weigh almost 1% less there. See BBC story about NASA's gravity map." Here's another story about the mission, and the GRACE home page (or NASA's less-informative page).
This was Astronomy Picture of the Day last week.
Plenty of depth/background available from there, as always!
"If you create user accounts, by default, they will have an account type of Administrator with no password." KB Q293834
Mapping the geoid is one of the most fundamental problems in oceanography. Ocean currents are all basically caused by water running downhill. The problem is that "downhill" in this case is relative to the geoid, which is a bumpy, not-nice surface. With this kind of map, we should be able to map surface currents from space; their velocity, their position, everything you want to know about how the surface currents are moving. This is important for climate studies of global warming, since the ocean currents are one of the main transporters of heat from the equator to the poles. This will allow us to get a much better idea of where the heat in the world is going, and how long it takes to get there, which in turn will give us a better handle on global warming.
Oceanographers have been trying to figure out a way to remove the geoid from their equations for a hundred years. Now we can just measure the damn thing. Crazy.
Weight != mass, dude. 1 tonne of mass takes just as much energy to move 1 kilometer across the earths surface, whether it exerts 1 "tonne of force" or 0.9 "tonnes of force" due to gravity. Basic physics. UPS hasn't ripped you off (not like they did to this guy, anyway). It's only when you go vertical that you have to counter gravity - and that's when weight becomes significant.
Oh, and the SI unit of force is a Newton (N), which is a kilogram-meter per second squared (k-m/s²). One tonne (1000 kilograms) of mass would exert 9.8 KN (KiloNewtons) of force at mean gravity on the earth. Weight apprears to be the same mass since we use gravity to comapre masses, but they are not the same. As well, in the US and Imperial systems, 1 lb of mass exerts 1 lb of force - just to be confusing.
That concludes tonight's lecture. (My Physics teacher would be so proud. *snif*)
Soko
"Depression is merely anger without enthusiasm." - Anonymous
>>As well, in the US and Imperial systems, 1 lb of mass exerts 1 lb of force - just to be confusing.
<<
Actually, in metric land (precisely, in the land of the metre), the kilogram-force (kgf) has been in widespread use, about a century ago. It was more or less equal to the gravity force exerced by earth on a piece of matter with a mass of 1 kg. It took several decades to get rid of that unit (you can still sometimes see indications like "max 2000 kgf" on cranes in old workshops).
Nowadays, low-level mechanics are taught to use the decanewton (daN) as their primary unit of force (be it weight or any other force)... no wonder why !
...the fact that moving at speeds approaching the speed of light will cause you to move faster through time, so that if you left Earth, travelled at near light speeds, and then came back shortly afterwards, 100 years might have elapsed on Earth in what you perceived as about 10 minutes.
I think that physical laws like this have a very significant effect on the lumpiness of the Earth, and therefore, on the variations in gravitational pull.
Imagine that you're running down a square field, from one side to the side parallel to it, and it takes you 10 minutes to run across this field. Ok, now imagine that you're running across the same field, but instead of running "straight," you're running at an angle, so that you're not perpendicular to the edges of the field that you're running from and to. It will take you a bit longer to get to the other side of the field, even though you're running at the same speed, because by going at an angle, you've increased the distance you have to go to get from one edge to the other.
Now suppose we call the field a 2-dimensional surface, like a piece of paper. You could say that the first time you ran across the field, you travelled along one axis, or dimension--let's say the X axis. But on the way back, you ran at an angle, which means that you've gone along two axes, the X and Y axes. But you went the same speed. This means that you have split the same speed across two dimensions.
We say that time is a fourth dimension. Now picture this: No matter what's happening, you're ALWAYS moving through the 4 axes (the three "space" dimensions and the one "time" dimension) at exactly the speed of light. It's just that you're splitting that speed (the speed of light) across some combination of the 4 dimensions. You're doing one of the following:
I think all of these physical laws have a very significant effect on the lumpiness of the Earth, and therefore, on the variations in gravitational pull.
And, of course, the obligatory OH WELL.
As well, in the US and Imperial systems, 1 lb of mass exerts 1 lb of force
The pound is never a measure of mass, the "imperial" mass unit is the slug.
"Should we expect to see more US companies building launch facilities in SE Asia after this report has been out a while?"
Not the least bit likely. India has it's own launching agency, Indian Space Research Organisation. They currently launch satellites using Polar Satellite Launch Vehicles, and have actually launched a Geosynchronous Satellite using an indigenously designed Launch Vehicle. They're close to breakthrough on indigenous cryogenic engines for the GSLV which they had to develope from scratch because of the US of A feeling threatened by the transfer of technology from Russia. (The launch used Russian engines).
The fact is, the Indian government is a launching agency, and getting permission to set up a private launch facility is not possible.
Besides, ISRO are said to provide the cheapest (money wise) launches compaired to Airaine and NASA.
-Shaunak.
As I recall, the debate is between an asteroid/comet impact in the Yucatan vs a violent and prolonged period of volcanic activity in India causing the mass extinctions 65 MYears ago. Both would produce huge amounts of dust and ash and lay waste to whole continents. Problem is, geology can't quite pin down which one caused it. Hell, it could be both that pushed them over the edge, though the timing for that would be rather amazing.
Dyolf Knip
gravity
n 1: the force of attraction between all masses in the universe;
especially the attraction of the earth's mass for bodies
near its surface; "gravitation cannot be held
responsible for people falling in love"--Albert Einstein
[syn: {gravitation}, {gravitational attraction}, {gravitational
force}]
Since I don't have any karma I can't lose it
The worst terrorist attack in recorded history occurred on September 11th, and now we're involved in a WAR against Islam and you people have the gall to be discussing mapping gravity????
Yes, we have the gall.
Ask NAVO (the Naval Oceanographic Office) just how much gall they have, mapping gravity over the surface of the seas! In the Old Days, before nifty toys like Satellite Gravity, we used to grid the earth's field by taking in situ measurements all over; *much* of which was done by oceanographic research vessels
Now, a good portion of that gravity grid was done for nice oceanographic or geologic reasons; if you know the density of the stuff below you, you can get a pretty good guess at the shape and contents of the seafloor below, but curiously, the more sensitive and more accurate gravity meters were owned and operated by the USN.
Why is that? Because a good map of the gravity patterns of the sea floor can help with navigating around it, when you *haven't* the luxuries of GPS or loran or other positioning systems.
Submarines!
Gravity maps done by NAVO ships in the Indian Ocean (which have greater detail and precision than the NASA maps, even if they are much narrower and smaller region of coverage) are quite possibly as we speak, helping guide USN subs in the vicinity, as they prepare for any lurking regional threats.
For a quick glimpse of grav fluctuations in the south pacific, as recorded on a Navy Gravimeter (aboard a civilian research ship) try at the bottom
Anyway, most everyone in the Oceanographic community is really excited about satellite gravity, since its coverage is just about universal (except for the poles) but we still lug out the Bell Aerospace meters (ugly black things) from port to port.
If anyone were interested, I could post descriptions of how some or any of these things work, except this is slashdot and this post will probably end up as (Score:-1, TrollFood)
First, nothing begins if not opening
It's huge. It's only hidden because it's under water. Check here for pictures of said hole in the ground.
One satellite that was launched and is busy producing data, is CHAMP. It too is mapping the Earth's gravity field, by virtue of GPS tracking and a three-axes accelerometer. And it is sounding the atmosphere by GPS limb sounding, just like GRACE will do and METSAT has been doing some time ago already.
For those not aware of the importance of GPS limb sounding, it is a method to determine indirectly the scale height and thus temperature around the tropopause, a useful indicator for global warming.
Actually one thing nobody here has pointed out is that GRACE aims at studying changes in the gravity field, mostly due to movements of ocean water, ground water, ice, air etc. etc. (in short, the 'blue film' we see in space photographs of Earth!). The sensitivity of the GRACE mission, which will consist of a satellite pair tracking each other, to such changes is quite unbelievable.
Good explanation. Thanks!
;-)
Two more things though.
1) The gravity field of the oceans can be mapped from space by using satellite radar altimetry. One Navy satellite especially launched for this was GEOSAT. The detailed altimetry data from this mission was long classified, until equivalent data became available from the ERS-1 mission...
It works by mapping the precise shape of the ocean surface from space, from a known orbit. Assuming that the ocean surface is in hydrostatic equilibrium, this gives you the geoid ("mean sea level"). The assumption is wrong, of course, which is where the "real" satellite gravity missions -- and in-situ measurements -- come in.
2) The reason the Navy wants to have the precise gravity field is not only to be able to use inertial navigation themselves (for the submarines), but also, and especially, to know the direction of the vertical at the precise location where those Poseidon missiles take off. They too use inertial guidance, and the platform aligns with local gravity before launch! If they take off in a direction that is 5'' wrong, due to an erroneous local vertical deflection, that translates into a 150 m targeting error at 6000 km.
For some reason this is not considered good enough