Mars
Mission
India's Mars orbiter mission has left Earth's orbit after
performing a manoeuvre to put it on its way to orbit the red planet.
The spacecraft fired its main engine for more than
20 minutes to reach the correct velocity to leave the Earth's orbit, the
Bangalore-based Indian Space Research Organisation said. It said that all systems
on board the spacecraft were performing normally.
India
launched its first spacecraft bound for Mars on 5 November, a complex mission
that it hopes will demonstrate and advance technologies for space travel.
The 1.3-tonne orbiter Mangalyaan, which means
"Mars craft" in Hindi, must travel 485m miles over 300 days to reach
an orbit around Mars next September.
If the mission is successful, India will become the
fourth space programme to visit the red planet after the Soviet Union, the US
and Europe.
Some have questioned the price tag for a country of
1.2 billion people still dealing with widespread hunger and poverty. But the
government defended the Mars mission, and its $1bn space programme in general,
by noting its importance in providing hi-tech jobs for scientists and engineers
and practical applications in solving problems on Earth.
Decades of space research have allowed India to
develop satellite, communications and remote sensing technologies that are
helping to solve everyday problems at home, from forecasting where fish can be
caught by fishermen to predicting storms and floods.
The orbiter will gather images and data that will
help in determining how Martian weather systems work and what happened to the
large quantities of water that may have once existed on Mars.
Experts say the data will improve understanding
about how planets form, what conditions might make life possible and where else
in the universe it might exist.
The orbiter is expected to have at least six months
to investigate the planet's landscape and atmosphere. At its closest point, it
will be 227 miles from the planet's surface, and its furthest point will be
nearly 50,000 miles away.
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* How
can a spacecraft leave orbit?
In order to leave orbit, a spacecraft needs to be going fast
enough to break free of gravity. A huge push is needed to do that. Either that
push was given to a ship as it was launched or it is given to a ship already in
orbit. To push a ship that is already in orbit farther from the planet,
thrusters should be fired at the highest point in the orbit. Generally, a ship
will go into higher and higher orbits until it intersects with its destination.
Spacecraft can go from planet to planet that way. Even if a ship
from the Earth leaves Earth orbit, it is still in orbit around the Sun. Huge
amounts of energy are needed to push a ship fast enough to break free from the
Sun's gravitational pull.
To leave orbit and travel towards the body it is orbiting, a ship
only needs to slow down (by retroburning or aerobraking) and wait for gravity
to pull the ship in.
**
Matter
Invisible
Bypass
the light wave in such a way that the Matter in between doesnot become an
obstacle and light doesnot appear on the matter.
Speed
Increase of a space craft
Make
free a matter or space craft from Gravitational force of The Earth and create
pressure to create force.
· If gravity
isn't a force, how does it accelerate objects? (Advanced)
Einstein said there is no such thing as a gravitational force.
Mass is not attracting mass over a distance. Instead, it's curving spacetime.
If there's no force, then how do you explain acceleration due to gravity?
Objects should accelerate only when acted upon by a force; otherwise they
should maintain a constant velocity. A few of the explanations I've found
online refer to equivalence and the thought experiment of a man standing on
Earth experiencing the same g-force as a man in a rocket being accelerated in
space. I understand why those conditions are the same, but I fail to see how
that explains a brick falling from a building accelerating at 9.8 m/s2. Also, in that thought experiment a force is
being exerted (the thrust of the rocket).
This is perhaps the most common question about general relativity.
If gravity isn't a force, how does it accelerate objects?
General relativity says that energy (in the form of mass, light,
and whatever other forms it comes in) tells spacetime how to bend, and the
bending of spacetime tells that energy how to move. The concept of "gravity"
is then that objects are falling along the bending of spacetime. The path that
objects follow is called a "geodesic". Let's begin by looking at the
bending side of things, and then we'll come back to look at geodesics.
The amount of bending that is induced by an object is directly
related to that object's energy (typically, the most important part of its
energy is its mass energy, but there can be exceptions). The Sun's mass is the
biggest contribution to bending in our solar system. So much so, that it dwarfs
the bending of spacetime by the Earth to the extent that to a very good
approximation, we can just consider the Earth to be massless as it travels
around the Sun (we call this the test particle limit). Similarly, when you're
standing on the Earth, the Earth's mass dominates the bending of spacetime over
your own, and so you can treat yourself as a massless test particle for all
intents and purposes. However, truth be told, you warp the spacetime around you
just a teensy tiny bit, and that does have an impact upon the earth in
response.
Now, let's get back to those geodesics. A body undergoing geodesic
motion feels no forces acting upon itself. It is just following what it feels
to be a "downward slope through spacetime" (this is how the bending
affects the motion of an object). The particular geodesic an object wants to
follow is dependent upon its velocity, but perhaps surprisingly, not its mass
(unless it is massless, in which case its velocity is exactly the speed of
light). There are no forces acting upon that body; we say this body is in
freefall. Gravity is not acting as a force. (Technically, if the body is larger
than a point, it can have tidal forces acting upon it, which are forces that
occur because of a differential in the gravitational effect between the two
ends of the body, but we'll ignore those.)
OK, so let's look a little deeper into these geodesic things. What
do they look like? Standing on the surface of the Earth, if we throw a ball
into the air, it will trace out a parabola through space as it rises and then
falls back down to Earth. This is the geodesic that it follows. It turns out
that given the appropriate definition, this path is the equivalent of a
straight line through four-dimensional spacetime, given the bending of spacetime.
How does this relate to what we think of as the acceleration due to gravity?
Let us choose a coordinate system based on our location on the
Earth. We'll say that I'm at the origin, and define that we throw the ball up
in the air at time t = 0 (this is essentially giving a name to the location,
nothing more). We can describe the position of the ball in spacetime in this
coordinate system using an appropriate parameter (that we call an "affine
parameter"). As the ball moves through spacetime, its position in
spacetime is given by appropriate functions of this parameter. We can rewrite
things slightly, to relate its position in space to its position in time. Then,
when we look at this trajectory, it appears that the object is accelerating
towards the earth, giving rise to the idea that gravity is acting as a force.
What is really happening, however, is that the object's motion in
our coordinate system is described by the geodesic equation. If you want some
maths, this equation looks like the following:
(geodesic equation) (image courtesy of
http://en.wikipedia.org/wiki/Geodesic_equation#Affine_geodesics)
Here, x (with superscript Greek indices) describes the position of
the ball in our coordinate system. The indices indicate whether we're talking
about the x,y,z or time coordinate. The parameter t that the derivatives are
being taken with respect to is the affine parameter; in this case, it is known
as the "proper time" of the object (for slowly moving objects, we can
think of t as the time coordinate in our coordinate system). The first term in
this equation is the acceleration of the object in our coordinate system. The
second term describes the effect of gravity. The thing that looks like part of
a hangman's game is called a connection symbol. It encodes all of the effects
of the bending of space time (as well as information about our choice of
coordinate system). There are actually sixteen terms here: it's written in a
convention called Einstein summation convention. This shows that the effects of
the bending of spacetime change the acceleration of an object, based on its
velocity through not only space but also through time.
If there is no curvature to spacetime, then the connection symbols
are all zero, and we see that an object moves with zero acceleration (constant
velocity) unless acted upon by an external force (which would replace the zero
on the right-hand side of this equation). (Again, there are some
technicalities: this is only true in a Cartesian coordinate system; in
something like polar coordinates, the connection symbols may not be vanishing,
but they're just describing the vagaries of the coordinate system in that
case.)
If there is some bending to spacetime, then the connection symbols
are not zero, and all of a sudden, there appears to be an acceleration. It is
this curvature of spacetime that gives rise to what we interpret as
gravitational acceleration. Note that there is no mass in this equation - it
doesn't matter what the mass of the object is, they all follow the same
geodesic (so long as it's not massless, in which case things are a little
different).
So, what good is this geodesic description of the force of
gravity? Can't we just think of gravity as a force and be done with it?
It turns out that there are two cases where this description of
the effect of gravity gives vastly different results compared to the concept of
gravity as a force. The first is for objects moving very very fast, close to
the speed of light. Newtonian gravity doesn't correctly account for the effect
of the energy of the object in this case. A particularly important example is
for exactly massless particles, such as photons (light). One of the first
experimental confirmations of general relativity was that light can be
deflected by a mass, such as the sun. Another effect related to light is that
as light travels up through the earth's gravitational field, it loses energy.
This was actually predicted before general relativity, by considering
conservation of energy with a radioactive particle in the earth's gravitational
field. However, although the effect was discovered, it had no description in
terms of Newtonian gravity.
The second case in which the effect of gravity vastly differs is
in the realm of extremely strong gravitational fields, such as those around
black holes. Here, the effect of gravity is so severe that not even light can
escape from the gravitational pull of such an object. Again, this effect was
calculated in Newtonian gravity by thinking about the escape velocity of a
body, and contemplating what happens when it gets larger than the speed of
light. Surprisingly, the answer you arrive at is exactly the same as in general
relativity. However, as light is massless, you once again do not have a good
description of this effect in terms of Newtonian gravity, which tells you that
there has to be a more complete theory.
So, to summarize, general relativity says that matter bends
spacetime, and the effect of that bending of spacetime is to create a
generalized kind of force that acts on objects. However, it isn't a force as
such that acts on the object, but rather just the object following its geodesic
path through spacetime.
**** **** ****
On
a dark night, we can often see a band of light stretching across the sky. This
band is the Milky Way galaxy -- a gigantic collection of stars, gas and dust.
Far beyond the Milky Way, there are billions of other galaxies -- some similar
to our own and some very different -- scattered throughout space to the very
limits of the observable universe.
Types
of Galaxies
Astronomers
classify galaxies into three major categories. Spiral galaxies look like flat
disks with bulges in their centers and beautiful spiral arms. Elliptical
galaxies are redder, more rounded, and often longer in one direction than in
the other, like a football. Galaxies that appear neither disk-like nor rounded
are classified as irregular galaxies.
Spiral
Galaxies
Spiral
galaxies usually consist of three components: a flat disk, an ellipsoidally
formed bulge and a halo. The disk contains a lot of interstellar gas and dust,
and most of the stars in the galaxy. The gas, dust and stars in the disk rotate
in the same direction around the galactic center at hundreds of kilometres per
second and are often arranged in striking spiral patterns. The bulge is located
at the centre of the disk and consists of an older stellar population with
little interstellar matter. The near-spherical halo surrounds the disk, and is
thought to contain copious amounts of dark matter: matter that acts
gravitationally like "normal" matter but that can't be seen!
Astronomers infer the presence of this dark matter by the motions of stars and
gas in the disk of the galaxy, as well as older stellar populations in the halo
like globular clusters. The young stars in the disk are classified as stellar
population I, and the old bulge and halo stars as population II.
Astronomers
classify spiral galaxies according to their appearance by using the Hubble
scheme. Those with pronounced bar structures in their centers are called
"barred spirals" and are classified "SB" (examples are
given in brackets). Galaxies with conspicuous bulges and tightly wound spiral
arms are called "Sa" (Sombrero galaxy) or "SBa" (NGC 3185).
Galaxies with prominent bulges and pronounced spiral arms are classified as
"Sb" (M31, M81) or "SBb" (M95, NGC 4725). Other spirals
with loose spiral arms and a small bulge are classified as "Sc" (M33,
M74, M100) or "SBc" (M83, M109).
There
are some galaxies like M84, M85 and NGC 5866 that are disk galaxies without any
spiral structure. These galaxies are called "S0" or lenticular
galaxies. Though the origin of lenticular galaxies is still debated the most
plausible explanation to date is that the gas and stars that would reside in
the galaxy disk have been stripped by interactions with the hot gas in clusters
and groups of galaxies. From their appearance and their stellar contents, they
look more like ellipticals rather than spirals and have often been
misclassified due to this fact. For instance, misclassification has occured for
both the Messier object examples listed above.
* Elliptical Galaxies
Elliptical
galaxies are ellipsoidal agglomerations of stars, which usually do not contain
much interstellar matter. Photometric studies of elliptical galaxies suggest
that they are triaxial (all the three axes of the ellipsoid are of different
sizes). Unlike spiral galaxies, ellipticals have little or no global angular
momentum, so that different stars orbit the center in different directions and
there is no pattern of orderly rotation. Normally, elliptical galaxies contain
very little or no interstellar gas and dust and consist of old population II
stars only. Elliptical galaxies are classified according to the Hubble scheme
into classes "E0" to "E7", in increasing order of
ellipticity. Thus E0 galaxies appear round like M89 while E6 galaxies like M110
and NGC 3377 are almost cigar shaped.
The
largest galaxies in the universe are giant elliptical galaxies. They contain a
trillion stars or more and span as much as two million light years - about 20
times the width of the Milky Way. These giant ellipticals are often found in
the hearts of galaxy clusters. For example the giant elliptical galaxy M87 is
found in the heart of the Virgo Cluster.
Elliptical
galaxies also constitute some of the smallest galaxies in the universe. These
galaxies are called dwarf elliptical galaxies and dwarf spheroids. Relative to
normal ellipticals they are very faint, and are often found in galaxy clusters
or near large spiral galaxies. For instance, there are 9 dwarf spheroids like
Leo I which are satellites of our Milky Way galaxy.
Irregular Galaxies
A
small percentage of the large galaxies we see nearby fall into neither of the
two major categories. This irregular class of galaxies is a miscellaneous
class, comprising small galaxies with no identifiable form like the Magellanic
clouds (the Large Magellanic Cloud and Small Magellanic Cloud are two satellite
galaxies of the Milky Way) and "peculiar" galaxies that appear to be
in disarray like NGC 1313. There is no discernable disk in these systems,
although they often have copious amounts of gas as well as high rates of star
formation. Irregular galaxies are often found to be gravitationally interacting
with galaxies nearby, which often accounts for their ragged appearance.
Galaxy
Evolution, Interactions and Mergers
Galaxies were once thought of as "island
universes" evolving slowly in complete isolation. Today we think that just
the opposite is true: gravitational interactions of galaxies with each other,
and even the coalescence of two galaxies into one, or mergers, are commonplace
in the Universe! We see striking examples of merging galaxies in the local
Universe, such as NGC 2207 and its companion IC 2163 and the Mice. These
interacting systems often sport long tidal tails of gas and stars, a result of
the mutual gravitational pull of each system. During the merger, the gas in
each galaxy disk flow to the galaxy centers, becomes very dense, and forms
stars at an alarming rate. This inflowing material also feeds the supermassive
black holes at the galaxy centres, which heat up the infalling material to
millions of degrees and eject some of it along powerful jets. All of these
mechanisms make merging galaxies very bright, such as Arp 220, are among the
most luminous objects in the local Universe.
What do mergers leave behind? Both observations of actual
systems and simulations of merging galaxies on a computer suggest that merging
spirals create elliptical galaxies. The gas in the progenitor spiral galaxies
is used up in making stars which subsequently eject heavier elements and dust
from the system, and the collision is forceful enough to randomize the orbits
of the stars in the incoming disks into a spheroidal shape. Different types of
galaxies are therefore intimately linked by galaxy evolution and mergers:
spiral galaxies evolve into elliptical galaxies, and irregular galaxies are
galaxies in the process of becoming one or the other!
Galaxy coalescence doesn't only happen between two large
galaxies: in fact, most large galaxies are constantly swallowing up the
smaller, dwarf galaxies that surround them. Our Milky Way is no exception to
this rule: it is currently ripping appart our nearest neighbor, the Sagittarius
Dwarf!
Supernovae
In a sense, stars are like people: they are born, they live and they die. A
star "lives" by fusing lighter elements into heavier ones in its
central regions.
The
pressure generated by this "combustion" holds the star up against the
enormous gravitational force that its outer layers extert on the stellar core.
The supply of elements that the star can fuse is limited, and when this runs
out the star "dies": its properties change rapidly and violently, and
a new astronomical object is created. Supernovae represent the most catastrophic
(and picturesque!) of these stellar deaths.
Anatomy of a Supernova
Stars
of all masses spend the majority of their lives fusing hydrogen nuclei into
helium nuclei: we call this stage the main sequence. When all of the hydrogen
in the central regions of a star is converted into helium, the star will begin
to "burn" helium into carbon. However, the helium in the stellar core
will eventually run out as well; so in order to survive, a star must be hot
enough to fuse progressively heavier elements, as the lighter ones become
exhausted one by one. Stars heavier than about 5 times the mass of the Sun can
do this with no problem: they burn hydrogen, and then helium, and then carbon,
oxygen, silicon, and so on... until they attempt to fuse iron. Iron is special
in that it is the lightest element in the periodic table that doesn't release
energy when you attempt to fuse it together. In fact, instead of giving you
energy, you end up with less energy than you started with! This means that
instead of generating additional pressure to hold up the now extended outer
layers of the aging star, the iron fusion actually takes thermal energy from
the stellar core. Thus, there is nothing left to combat the ever-present force
of gravity from these outer layers. The result: collapse! The lack of radiation
pressure generated by the iron-fusing core causes the outer layers to fall
towards the centre of the star. This implosion happens very, very quickly: it
takes about 15 seconds to complete. During the collapse, the nuclei in the
outer parts of the star are pushedWhat happens next depends on the mass of the
star. Stars with masses between about 5 and 8 times the mass of our Sun form
neutron stars during the implosion: the nuclei in the central regions are
pushed close enough together to form a very dense neutron core. The outer
layers bounce off this core, and a catastrophic explosion ensues: this is the
visible part of the supernova. Stars with masses greater than about 10 times
the mass of the Sun meet a very different fate. The collapse of the outer
regions of the star is so forceful that not even a neutron star can support
itself against the pressure of the infalling material. In fact, no physical
force is strong enough to counter the collapse: the supernova creates a black
hole, or a region of spacetime that is so small and so dense that not even
light can escape from its clutches. In this case, the details of how the
ensuing explosion actually occurs have still to be worked out. Observationally,
supernovae are found by patiently observing the sky and looking for bright
objects where there were none before. At its peak luminosity, the supernova
resulting from a single star may be bright enough to outshine an entire galaxy.
very close together, so close that elements heavier than iron are formed.
A Cosmic Cycle...
Supernovae
play a fundamental role in a great cosmic recycling program. We believe that
almost all of the elements in the Universe that are heavier than hydrogen and
helium are created either in the centres of stars during their lifetimes or in
the supernova explosions that mark the demise of larger stars. Supernovae then
disperse this newly synthesized material in the interstellar neighbourhood.
From this material a new, enriched generation of stars will form, and the cycle
begins anew. This is how we think that the heavy elements in the Sun came to
be. Since the planets in the solar system formed from leftover material in a
disk around the proto-Sun, all of the heavy elements in the Earth (including
those in humans!) must have come from the same source. This means that in the
most literal sense, we are stardust!
From Wikipedia, the
free encyclopedia
This article is about the fictional propulsion system. For the actual model
of spacetime, see Alcubierre drive. For the street in Virginia, see Warp Drive.
Warp drive is a faster-than-light
(FTL) spacecraft propulsion system in many science
fiction works, most notably Star Trek. A spacecraft equipped with a warp drive
may travel at speeds greater than that
of light by many orders of magnitude. In contrast to some other FTL
technologies such as a jump drive or hyper
drive, the warp drive does not permit instantaneous travel between two
points but involves a measurable passage of time which is pertinent to the
concept. Spacecraft at warp velocity theoretically continue to interact with
objects in "normal space". The general concept of "warp
drive" was introduced by John
W. Campbell in his 1931 novel Islands
of Space.[1]
Einstein's
theory of special relativity states that energy and mass are interchangeable,
thus, speed of light travel is impossible for material objects that weigh more
than photons. The problem of a material object exceeding light speed is that an
infinitely increasing amount of kinetic energy is required to attempt moving as
fast as a massless photon. This problem can theoretically be solved by warping
space to move an object instead of increasing the kinetic energy of the object
to do so.[2]
Contents
Star Trek
The Original Series:
Establishing a background
Warp drive is one of the fundamental features
of the Star Trek franchise; in the first pilot episode of Star Trek: The Original Series,
"The Cage", it is referred to as a
"hyperdrive"/"time warp" drive combination, and it is
stated that the "time barrier" has been broken, allowing a group of
stranded interstellar travelers to return to Earth far sooner than would have
otherwise been possible. The light speed time barrier shouldn't be confused
with time
dilation which occurs when approaching very fast speeds. Warp drive
technology avoids time dilation.
The episode "Metamorphosis", also from The
Original Series, establishes a backstory for the invention of warp drive on
Earth, in which Zefram Cochrane discovered the "space
warp". Cochrane is repeatedly referred to afterwards, but the exact
details of the first warp trials were not shown until the second Star Trek: The Next Generation
movie, Star Trek: First Contact. The movie
depicts Cochrane as having first operated warp drive on Earth in 2063 (two
years after the date speculated by the first edition of the Star Trek Chronology). By using a
matter/antimatter reactor to create plasma, and by sending this plasma through warp
coils, he created a warp bubble which he could use to move a craft into subspace, thus allowing it to exceed the speed
of light. This successful first trial led directly to first contact with the Vulcans.
Enterprise: Leading
up to The Original Series
Later on, a prequel series titled Star Trek: Enterprise describes the warp
engine technology as a "Gravimetric Field Displacement Manifold" (Commander
Tucker's tour, "Cold Front"), and describes
the device as being powered by a matter/anti-matter reaction which powers the
two separate nacelles (one on each side of the ship) to create a displacement
field.[citation
needed]
The episode also firmly establishes that many
other civilizations had warp drive before humans; First Contact
co-writer Ronald D. Moore suggested Cochrane's drive was in
some way superior to forms which existed beforehand, and was gradually adopted
by the galaxy at large.[3]
Enterprise, set in 2151 and onwards, follows the voyages of the first human ship capable of traveling at warp factor 5.2,
which under the old warp table formula (the cube of the warp factor times the
speed of light), is about 140 times the speed of light (i.e., 5.2 cubed). In
the series pilot episode "Broken Bow", Capt.
Archer equates warp 4.5 to "Neptune and back
[from Earth] in six minutes" (which would correspond to a distance of 547
light-minutes or 66 au, consistent with Neptune being a minimum of 29
au distant from Earth).[citation
needed]
The Next
Generation onwards
Only three stories in the original Star
Trek series involved the Enterprise traveling beyond Warp 10 ( Warp
11, briefly, as a result of Nomad's "correction of inefficiencies" in
the antimatter control system in "The Changeling";
Warp 11 again in "By Any Other Name" after the Kelvans modify
the Enterprise's engines for greater sustained speed to make the trip
from the Milky Way Galaxy to the Andromeda Galaxy; and Warp 14.1 in "That Which Survives" after the ship was
put through a Kalandan transporter, beamed parsecs away from where it had been,
and reassembled slightly out of phase). In The Next Generation, such
stories were rare, and usually involved a malfunction in (or alien interference
with) a starship's engines. A new warp scale was drawn up, with Warp Factor 10
set as an unattainable maximum (according to the new scale, reaching or
exceeding Warp 10 required an infinite amount of energy). This is described in
some technical manuals as "Eugene's limit", in homage to
creator/producer Gene Roddenberry. Warp 8 in the original series
was the "Never Exceed" speed for the hulls and engines of Constitution-class
starships, equivalent to the aircraft VNE V-speed.
Warp 6 was the VNO "Normal Operation" maximum safe
cruising speed for that vessel class.[4]
The Warp 14.1 incident was the result of runaway engines which brought the hull
within seconds of structural failure before power was disengaged.[5]
The limit of 10 did not entirely stop warp
inflation. By the mid-24th century, the Enterprise-D could travel at
Warp 9.8 at "extreme risk", while normal maximum operating speed was
Warp 9.6 and the maximum rated cruise was Warp 9.2. According to the Deep Space Nine Tech Manual,
during the Dominion War, Galaxy-class starships were refitted with newer
technology including modifications which increased their maximum speed to Warp
9.9.
In the episode "Where No One Has Gone Before" the
Enterprise-D was shown to exceed Warp 10, traveling 2.7 million
light-years from their home galaxy in a matter of minutes (though the ship's
extreme velocity was due to the influence of an alien being and could not be
achieved by starship engines). The Intrepid-class starship Voyager has a maximum sustainable
cruising speed of Warp 9.975; the Enterprise-E can go even faster,
with a maximum velocity of Warp 9.999[citation
needed]. In the alternative future depicted in "All Good Things...",
the series finale of The Next Generation, the "future" Enterprise-D
travels at Warp 13, although it is never established whether this is truly
"above" Warp 10, or simply the result of another reconfiguration of
the warp scale.
Warp velocities
Warp drive velocity in Star Trek is
generally expressed in "warp factor" units, which—according to the Star Trek Technical Manuals—correspond
to the magnitude of the warp field. Achieving warp factor 1 is equal to
breaking the light barrier, while the actual velocity corresponding to higher
factors is determined using an ambiguous formula. Several episodes of the
original series placed the Enterprise in peril by having it travel at
high warp factors; at one point in "That Which Survives" the Enterprise
traveled at a warp factor of 14.1. In the Star Trek: The Next Generation
episode "The Most Toys" the crew of Enterprise-D
discovers that the android Data may have been stolen while on board another
ship, Jovis. At this point the Jovis, which has a maximum warp
factor of 3, has had a 23-hour head start, which the Enterprise-D figures puts
her anywhere within a 0.102 light year radius of her last known position.
However, the velocity (in present dimensional units) of any given warp factor
is rarely the subject of explicit expression, and travel times for specific
interstellar distances are not consistent through the various series.
According to the Star Trek episode
writer's guide for The Original Series, warp factors are converted to
multiples of c with the cubic
function v = w3c,
where w is the warp factor, v is the velocity, and c is
the speed of light. Accordingly, "warp 1" is
equivalent to the speed of light, "warp 2" is 8 times the speed of
light, "warp 3" is 27 times the speed of light, etc.
Michael Okuda's new warp scale
For Star Trek: The Next Generation and
the subsequent series, Star Trek artist Michael
Okuda devised a formula based on the original one but with important
differences; for warp factors 1 through 9, v = w10/3c.
In the half-open interval from warp 9 to warp 10, the
exponent of w increases toward infinity. Thus, in the Okuda scale, warp
velocities approach warp 10 asymptotically.
There is no exact formula for this interval
because the quoted velocities are based on a hand-drawn curve; what can be said
is that at velocities greater than warp 9, the form of the warp function
changes because of an increase in the exponent of the warp factor w. Due
to the resultant increase in the derivative,
even minor changes in the warp factor eventually correspond to a greater than
exponential change in velocity. In the episode "Threshold", Tom Paris breaks
the warp 10 threshold, but travel beyond the threshold is later discovered to
be unacceptably hazardous to biological life.
Slingshot effect
The "slingshot
effect" is first depicted in "Tomorrow Is Yesterday" (1967) as a
method of time travel. The procedure involves traveling at a high
warp velocity in the direction of a star, on a precisely calculated
"slingshot" path; if successful, the ship is caused to travel to a
desired point, past or future. The same technique is used in the episode "Assignment:
Earth" (1968) for historic research. The term "time warp"
was first used in "The Naked Time" (1966) when a
previously untried cold-start intermix of matter and antimatter threw the Enterprise
back three days in time. The term was later used in Star Trek IV in
describing the slingshot effect. The technique was mentioned as a viable method
of time travel in the TNG episode "Time Squared"
(1989).
This "slingshot" effect has been
explored in theoretical physics: it is hypothetically possible to slingshot
oneself "around" the event
horizon of a black hole. As a result of the black hole's extreme
gravitation, time would pass at a slower rate near the event horizon, relative
to the outside universe; the traveler would experience the passage of only
several minutes or hours, while hundreds of years would pass in 'normal' space.
Warp core
A primary component of the warp drive method
of propulsion in the Star Trek universe is the "gravimetric field
displacement manifold", more commonly referred to as a warp core.
It is a fictional reactor that taps the energy released in a matter-antimatter
annihilation to provide the energy necessary to power a starship's warp drive,
allowing faster-than-light travel. Starship warp cores
generally also serve as powerplants for other primary ship systems.
When matter and antimatter come into contact,
they annihilate—both
matter and antimatter are converted directly and entirely into enormous
quantities of energy, in the form of subnuclear particles and electromagnetic radiation
(specifically, mesons
and gamma
rays). In the Star Trek universe, fictional "dilithium crystals" are used to regulate
this reaction. These crystals are described as being non-reactive to
anti-matter when bombarded with high levels of radiation.
Usually, the reactants are deuterium,
which is an isotope
of hydrogen,
and antideuterium (its antimatter
counterpart). In The Original Series and in-universe chronologically
subsequent series, the warp core reaction chamber is often referred to as the
"dilithium intermix chamber" or the "matter/antimatter reaction
chamber", depending upon the ship's intermix type. The reaction chamber is
surrounded by powerful magnetic fields to contain the anti-matter. If the
containment fields ever fail, the subsequent interaction of the antimatter fuel
with the container walls would result in a catastrophic release of energy, with
the resultant explosion capable of utterly destroying the ship. Such "warp
core breaches" are used as plot devices in many Star Trek episodes. An intentional
warp core breach can also be deliberately created, as one of the methods by
which a starship can be made to self-destruct.
The mechanisms that provide a starship's propulsive
force are the "warp nacelles", one (or more) cylindrical pods that
are offset from the hull of the ship by large pylons; the nacelles generate the
actual 'warp bubble' that surrounds the ship, and destruction of one or both
nacelles will cripple the ship, and possibly cause a warp-core breach.
Real-world theories and science
Warp requirements for 10m OD sphere.
In 1994, physicist Miguel
Alcubierre formulated a theoretical solution, called the Alcubierre
drive, for faster-than-light travel which models the warp drive concept.
Subsequent calculations found that such a model would require prohibitive
amounts of negative energy or mass.[6]
In 2012, NASA researcher Harold White
hypothesized that by changing the shape of the warp drive, much less negative
mass and energy could be used, though the energy required ranges from the
mass of Voyager
1 to the mass of the observable universe, or many orders of magnitude
greater than anything currently possible by modern technology. NASA engineers have
begun preliminary research into such technology.[7]
See
also
- When Stephen Hawking guest starred on the Star
Trek: The Next Generation episode "Descent", he was
taken on a guided tour of the set. Pausing in front of the warp core set
piece, he remarked: "I'm working on that."[8]
References
1.
Gene Roddenberry: The Making of Star Trek
From Wikipedia, the
free encyclopedia
Anti-gravity also known as
non-gravitational field is an idea of creating a place or object that is free
from the force of gravity. It does not refer to
the lack of weight under gravity experienced in free
fall or orbit,
or to balancing the force of gravity with some other force, such as
electromagnetism or aerodynamic lift. Anti-gravity is a recurring concept in
science fiction, particularly in the context of spacecraft propulsion.
Examples are the gravity blocking substance "Cavorite" in H.
G. Wells' The First Men in the Moon
and the Spindizzy
machines in James Blish's Cities
in Flight.
In Newton's law of
universal gravitation, gravity was an external force transmitted by
unknown means. In the 20th century, Newton's model was replaced by general relativity where
gravity is not a force but the result of the geometry of spacetime. Under
general relativity, anti-gravity is impossible except under contrived
circumstances.[1][2][3]
Quantum physicists have
postulated the existence of gravitons, massless elementary particles that
transmit gravitational force, but the possibility of creating or destroying
these is unclear.
"Anti-gravity"
is often used colloquially to refer to devices that look as if they reverse
gravity even though they operate through other means, such as lifters,
which fly in the air by moving air with electromagnetic fields.[4][5]
Contents
Hypothetical solutions
Gravity
shields
In 1948 successful businessman Roger
Babson (founder of Babson
College) formed the Gravity Research Foundation to
study ways to reduce the effects of gravity.[6]
Their efforts were initially somewhat "crankish",
but they held occasional conferences that drew such people as Clarence
Birdseye known for his frozen-food products and Igor
Sikorsky, inventor of the helicopter. Over time the Foundation
turned its attention away from trying to control gravity, to simply better
understanding it. The Foundation nearly disappeared after Babson's death in
1967. However, it continues to run an essay award, offering prizes of up to
$4,000. As of 2017, it is still administered out of Wellesley, Massachusetts, by
George Rideout, Jr., son of the foundation's original director.[7]
Winners include California astrophysicist George
F. Smoot, who later won the 2006 Nobel Prize in physics.
General relativity
research in the 1950s
General relativity was introduced in the
1910s, but development of the theory was greatly slowed by a lack of suitable
mathematical tools. Although it appeared that anti-gravity was outlawed under
general relativity.
It is claimed the US
Air Force also ran a study effort throughout the 1950s and into the
1960s.[8]
Former Lieutenant Colonel Ansel Talbert wrote two series
of newspaper articles claiming that most of the major aviation firms had
started gravity control propulsion research in the 1950s. However, there is
little outside confirmation of these stories, and since they take place in the
midst of the policy by press release era,
it is not clear how much weight these stories should be given.
It is known that there were serious efforts
underway at the Glenn L. Martin Company, who
formed the Research Institute for Advance Study.[9][10]
Major newspapers announced the contract that had been made between theoretical
physicist Burkhard Heim and the Glenn L.
Martin Company. Another effort in the private sector to master understanding of
gravitation was the creation of the Institute for Field Physics, University of North
Carolina at Chapel Hill in 1956, by Gravity Research Foundation
trustee, Agnew H. Bahnson.
Military support for anti-gravity projects
was terminated by the Mansfield Amendment of 1973,
which restricted Department of Defense
spending to only the areas of scientific research with explicit military
applications. The Mansfield Amendment was passed specifically to end
long-running projects that had little to show for their efforts.
Under general relativity, gravity is the
result of following spatial geometry (change in the normal shape of space)
caused by local mass-energy. This theory holds that it is the altered shape of
space, deformed by massive objects, that causes gravity, which is actually a
property of deformed space rather than being a true force. Although the
equations cannot normally produce a "negative geometry", it is possible
to do so by using "negative mass". The same
equations do not, of themselves, rule out the existence of negative mass.
Both general relativity and Newtonian gravity
appear to predict that negative mass would produce a repulsive gravitational
field. In particular, Sir Hermann Bondi proposed in 1957
that negative gravitational mass, combined with negative inertial mass, would
comply with the strong equivalence principle
of general relativity theory and the Newtonian laws of conservation of linear
momentum and energy. Bondi's proof yielded singularity free solutions for the
relativity equations.[11]
In July 1988, Robert L. Forward presented a
paper at the AIAA/ASME/SAE/ASEE 24th Joint Propulsion Conference that proposed
a Bondi negative gravitational mass propulsion system.[12]
Bondi pointed out that a negative mass will
fall toward (and not away from) "normal" matter, since although the
gravitational force is repulsive, the negative mass (according to Newton's law,
F=ma) responds by accelerating in the opposite of the direction of the force.
Normal mass, on the other hand, will fall away from the negative matter. He
noted that two identical masses, one positive and one negative, placed near
each other will therefore self-accelerate in the direction of the line between
them, with the negative mass chasing after the positive mass.[11]
Notice that because the negative mass acquires negative kinetic
energy, the total energy of the accelerating masses remains at zero.
Forward pointed out that the self-acceleration effect is due to the negative inertial
mass, and could be seen induced without the gravitational forces between the
particles.[12]
The Standard
Model of particle physics, which describes all presently known forms
of matter, does not include negative mass. Although cosmological dark
matter may consist of particles outside the Standard Model whose
nature is unknown, their mass is ostensibly known – since they were postulated
from their gravitational effects on surrounding objects, which implies their
mass is positive. The proposed cosmological dark
energy, on the other hand, is more complicated, since according to
general relativity the effects of both its energy density and its negative
pressure contribute to its gravitational effect.
Fifth
force
Under general relativity any form of energy
couples with spacetime to create the geometries that cause gravity. A
longstanding question was whether or not these same equations applied to antimatter.
The issue was considered solved in 1960 with the development of CPT
symmetry, which demonstrated that antimatter follows the same laws
of physics as "normal" matter, and therefore has positive energy
content and also causes (and reacts to) gravity like normal matter (see gravitational
interaction of antimatter).
For much of the last quarter of the 20th
century, the physics community was involved in attempts to produce a unified field theory, a single
physical theory that explains the four fundamental forces: gravity,
electromagnetism, and the strong and weak nuclear forces. Scientists have made
progress in unifying the three quantum forces,
but gravity has remained "the problem" in every attempt. This has not
stopped any number of such attempts from being made, however.
Generally these attempts tried to
"quantize gravity" by positing a particle, the graviton,
that carried gravity in the same way that photons
(light) carry electromagnetism. Simple attempts along this direction all
failed, however, leading to more complex examples that attempted to account for
these problems. Two of these, supersymmetry
and the relativity related supergravity, both required
the existence of an extremely weak "fifth force" carried by a graviphoton,
which coupled together several "loose ends" in quantum field theory,
in an organized manner. As a side effect, both theories also all but required
that antimatter be affected by this fifth force in a way similar to
anti-gravity, dictating repulsion away from mass. Several experiments were
carried out in the 1990s to measure this effect, but none yielded positive
results.[13]
In 2013 CERN looked
for an antigravity effect in an experiment designed to study the energy levels
within antihydrogen. The antigravity measurement was just an "interesting
sideshow" and was inconclusive.[14]
General-relativistic "warp
drives"
There are solutions of the field equations of
general relativity which describe "warp drives" (such as the Alcubierre
metric) and stable, traversable wormholes.
This by itself is not significant, since any spacetime geometry is a
solution of the field equations for some configuration of the stress–energy tensor field
(see exact solutions in
general relativity). General relativity does not constrain the
geometry of spacetime unless outside constraints are placed on the
stress–energy tensor. Warp-drive and traversable-wormhole geometries are
well-behaved in most areas, but require regions of exotic
matter; thus they are excluded as solutions if the stress–energy
tensor is limited to known forms of matter. Dark matter and dark energy are not
understood enough at this present time to make general statements regarding
their applicability to a warp-drive.
Breakthrough Propulsion Physics Program
During the close of the twentieth century NASA
provided funding for the Breakthrough Propulsion
Physics Program (BPP) from 1996 through 2002. This program studied a
number of "far out" designs for space propulsion that were not
receiving funding through normal university or commercial channels.
Anti-gravity-like concepts were investigated under the name "diametric
drive". The work of the BPP program continues in the independent, non-NASA
affiliated Tau Zero Foundation.[15]
Empirical claims and commercial efforts
There have been a number of attempts to build
anti-gravity devices, and a small number of reports of anti-gravity-like
effects in the scientific literature. None of the examples that follow are
accepted as reproducible examples of anti-gravity.
Gyroscopic devices
A "kinemassic
field" generator from U.S.
Patent 3,626,605: Method and apparatus for generating a secondary
gravitational force field
Gyroscopes
produce a force when twisted that operates "out of plane" and can
appear to lift themselves against gravity. Although this force is well
understood to be illusory, even under Newtonian models, it has nevertheless
generated numerous claims of anti-gravity devices and any number of patented
devices. None of these devices have ever been demonstrated to work under
controlled conditions, and have often become the subject of conspiracy theories as a
result. A famous example is that of Professor Eric
Laithwaite of Imperial College, London, in the 1974 address to the
Royal Institution.[16]
Another "rotating device" example
is shown in a series of patents granted to Henry Wallace between 1968 and 1974.
His devices consist of rapidly spinning disks of brass, a
material made up largely of elements with a total half-integer nuclear spin. He
claimed that by rapidly rotating a disk of such material, the nuclear
spin became aligned, and as a result created a "gravitomagnetic"
field in a fashion similar to the magnetic field created by the Barnett
effect.[17][18][19]
No independent testing or public demonstration of these devices is known.
In 1989, it was reported that a weight
decreases along the axis of a right spinning gyroscope.[20]
A test of this claim a year later yielded null results.[21]
A recommendation was made to conduct further tests at a 1999 AIP conference.[22]
Thomas Townsend Brown's gravitator
In 1921, while still in high school, Thomas Townsend Brown found
that a high-voltage Coolidge tube seemed to change
mass depending on its orientation on a balance scale. Through the 1920s Brown
developed this into devices that combined high voltages with materials with
high dielectric
constants (essentially large capacitors); he called such a
device a "gravitator". Brown made the claim to observers and in the
media that his experiments were showing anti-gravity effects. Brown would
continue his work and produced a series of high-voltage devices in the
following years in attempts to sell his ideas to aircraft companies and the
military. He coined the names Biefeld–Brown effect and electrogravitics
in conjunction with his devices. Brown tested his asymmetrical capacitor
devices in a vacuum, supposedly showing it was not a more down to earth electrohydrodynamic effect
generated by high voltage ion flow in air.
Electrogravitics is a popular topic in ufology,
anti-gravity, free energy, with government
conspiracy theorists and related websites, in books and publications with
claims that the technology became highly classified in the early 1960s and that
it is used to power UFOs and the B-2 bomber.[23]
There is also research and videos on the internet purported to show
lifter-style capacitor devices working in a vacuum, therefore not receiving
propulsion from ion drift or ion wind being generated in
air.[23][24]
Follow-up studies on Brown's work and other
claims have been conducted by R. L. Talley in a 1990 US Air Force study, NASA
scientist Jonathan Campbell in a 2003 experiment,[25]
and Martin
Tajmar in a 2004 paper.[26]
They have found that no thrust could be observed in a vacuum and that Brown's
and other ion
lifter devices produce thrust along their axis regardless of the
direction of gravity consistent with electrohydrodynamic effects.
Gravitoelectric coupling
In 1992, the Russian researcher Eugene
Podkletnov claimed to have discovered, whilst experimenting with superconductors,
that a fast rotating superconductor reduces the gravitational effect.[27]
Many studies have attempted to reproduce Podkletnov's experiment, always to
negative results.[28][29][30][31]
Ning Li and Douglas Torr,
of the University of Alabama
in Huntsville proposed how a time dependent magnetic field could
cause the spins of the lattice ions in a superconductor to generate detectable gravitomagnetic
and gravitoelectric fields in a series of papers published between 1991 and
1993.[32][33][34]
In 1999, Li and her team appeared in Popular
Mechanics, claiming to have constructed a working prototype
to generate what she described as "AC Gravity." No further evidence
of this prototype has been offered.[35][36]
Douglas Torr and Timir
Datta were involved in the development of a "gravity
generator" at the University of South Carolina.[37]
According to a leaked document from the Office of Technology Transfer at the
University of South Carolina and confirmed to Wired reporter Charles
Platt in 1998, the device would create a "force beam" in any desired
direction and that the university planned to patent and license this device. No
further information about this university research project or the "Gravity
Generator" device was ever made public.[38]
Göde Award
The Institute for Gravity Research of the
Göde Scientific Foundation has tried to reproduce many of the different
experiments which claim any "anti-gravity" effects. All attempts by
this group to observe an anti-gravity effect by reproducing past experiments
have been unsuccessful thus far. The foundation has offered a reward of one
million euros for a reproducible anti-gravity experiment.[39]
See
also
References
1.
Peskin, M and Schroeder, D.; An Introduction to Quantum
Field Theory (Westview Press, 1995) ISBN 0-201-50397-2
Polchinski, Joseph (1998). String Theory,
Cambridge University Press. A modern textbook
Mooallem,
J. (October 2007). "A curious attraction". Harper's Magazine. 315
(1889): 84–91.
Goldberg, J. M. (1992). US air force support of general
relativity: 1956–1972. In, J. Eisenstaedt & A. J. Kox (Ed.), Studies in
the History of General Relativity, Volume 3 Boston, Massachusetts: Center
for Einstein Studies. ISBN 0-8176-3479-7
Mallan, L. (1958). Space satellites (How to book
364). Greenwich, CT: Fawcett Publications, pp. 9–10, 137, 139. LCCN 58-001060
Clarke,
A. C. (1957). "The conquest of gravity". Holiday. 22 (6): 62.
Forward,
R. L. (1990). "Negative matter propulsion". Journal of Propulsion and
Power. 6 (1): 28–37. doi:10.2514/3.23219.; see also commentary Landis, G.A.
(1991). "Comments on Negative Mass Propulsion". Journal of Propulsion
and Power. 7 (2): 304. doi:10.2514/3.23327.
Supergravity and the Unification of the Laws of
Physics, by Daniel Z. Freedman and Peter van Nieuwenhuizen, Scientific
American, February 1978
Iwanaga,
N. (1999). "Reviews of some field propulsion methods from the general
relativistic standpoint". AIP Conference Proceedings. 458:
1015–1059..
Thomas Valone, Electrogravitics II: Validating Reports on
a New Propulsion Methodology, Integrity Research Institute, page 52-58
Woods, C., Cooke, S., Helme, J., and Caldwell, C.,
"Gravity Modification by High Temperature Superconductors," Joint
Propulsion Conference, AIAA 2001–3363, (2001).
Hathaway, G., Cleveland, B., and Bao, Y., "Gravity
Modification Experiment using a Rotating Superconducting Disc and Radio
Frequency Fields," Physica C, 385, 488–500, (2003).
Tajmar, M., and de Matos, C.J., "Gravitomagnetic Field
of a Rotating Superconductor and of a Rotating Superfluid," Physica C,
385(4), 551–554, (2003).
Wilson,
Jim (1 October 2000). "Taming
Gravity". Popular Mechanics. HighBeam Reseatch. Retrieved 5
January 2014.
Further
reading
- Cady,
W. M. (15 September 1952). "Thomas Townsend Brown: Electro-Gravity
Device" (File 24-185). Pasadena, CA: Office of Naval Research. Public
access to the report was authorized on 1 October 1952.
From Wikipedia, the
free encyclopedia
Spacecraft propulsion is any method used to
accelerate spacecraft and artificial satellites.
There are many different methods. Each method has drawbacks and advantages, and
spacecraft propulsion is an active area of research. However, most spacecraft
today are propelled by forcing a gas from the back/rear of the vehicle at very
high speed through a supersonic de Laval nozzle. This sort of engine is called a
rocket
engine.
All current spacecraft use chemical rockets (bipropellant or solid-fuel)
for launch, though some (such as the Pegasus
rocket and SpaceShipOne) have used air-breathing engines on their first
stage. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet
rockets for orbital station-keeping and some use momentum
wheels for attitude control. Soviet bloc satellites have used
electric propulsion for
decades, and newer Western geo-orbiting spacecraft are starting to use them for
north-south stationkeeping and orbit raising. Interplanetary vehicles mostly
use chemical rockets as well, although a few have used ion
thrusters and Hall effect thrusters (two different types of
electric propulsion) to great success.
Contents
Requirements
Artificial satellites must be launched
into orbit and
once there they must be placed in their nominal orbit. Once in the desired
orbit, they often need some form of attitude
control so that they are correctly pointed with respect to Earth, the Sun, and possibly some astronomical
object of interest.[1]
They are also subject to drag
from the thin atmosphere, so that to stay in orbit for a long
period of time some form of propulsion is occasionally necessary to make small
corrections (orbital stationkeeping).[2]
Many satellites need to be moved from one orbit to another from time to time,
and this also requires propulsion.[3]
A satellite's useful life is usually over once it has exhausted its ability to
adjust its orbit.
Spacecraft designed to travel further also
need propulsion methods. They need to be launched out of the Earth's atmosphere
just as satellites do. Once there, they need to leave orbit and move around.
For interplanetary travel, a spacecraft must use
its engines to leave Earth orbit. Once it has done so, it must somehow make its
way to its destination. Current interplanetary spacecraft do this with a series
of short-term trajectory adjustments.[4]
In between these adjustments, the spacecraft simply falls freely
along its trajectory. The most fuel-efficient means to move from one circular
orbit to another is with a Hohmann transfer orbit: the spacecraft
begins in a roughly circular orbit around the Sun. A short period of thrust in the
direction of motion accelerates or decelerates the spacecraft into an
elliptical orbit around the Sun which is tangential to its previous orbit and
also to the orbit of its destination. The spacecraft falls freely along this
elliptical orbit until it reaches its destination, where another short period
of thrust accelerates or decelerates it to match the orbit of its destination.[5]
Special methods such as aerobraking or aerocapture are sometimes used for this
final orbital adjustment.[6]
Artist's concept of a
solar sail
Some spacecraft propulsion methods such as solar sails
provide very low but inexhaustible thrust;[7]
an interplanetary vehicle using one of these methods would follow a rather
different trajectory, either constantly thrusting against its direction of motion
in order to decrease its distance from the Sun or constantly thrusting along
its direction of motion to increase its distance from the Sun. The concept has
been successfully tested by the Japanese IKAROS solar sail
spacecraft.
Spacecraft for interstellar travel also need propulsion
methods. No such spacecraft has yet been built, but many designs have been
discussed. Because interstellar distances are very great, a tremendous velocity
is needed to get a spacecraft to its destination in a reasonable amount of
time. Acquiring such a velocity on launch and getting rid of it on arrival will
be a formidable challenge for spacecraft designers.[8]
Effectiveness
When in space, the purpose of a propulsion
system is to change the velocity, or v, of a spacecraft. Because
this is more difficult for more massive spacecraft, designers generally discuss
momentum, mv.
The amount of change in momentum is called impulse.[9]
So the goal of a propulsion method in space is to create an impulse.
When launching a spacecraft from Earth, a
propulsion method must overcome a higher gravitational
pull to provide a positive net acceleration.[10]
In orbit, any additional impulse, even very tiny, will result in a change in
the orbit path.
The rate of change of velocity is
called acceleration,
and the rate of change of momentum is called force. To reach a
given velocity, one can apply a small acceleration over a long period of time,
or one can apply a large acceleration over a short time. Similarly, one can
achieve a given impulse with a large force over a short time or a small force
over a long time. This means that for maneuvering in space, a propulsion method
that produces tiny accelerations but runs for a long time can produce the same
impulse as a propulsion method that produces large accelerations for a short
time. When launching from a planet, tiny accelerations cannot overcome the
planet's gravitational pull and so cannot be used.
Earth's surface is situated fairly deep in a gravity
well. The escape velocity required to get out of it is 11.2
kilometers/second. As human beings evolved in a gravitational field of 1g
(9.8 m/s²), an ideal propulsion system would be one that provides a
continuous acceleration of 1g (though human bodies can tolerate much
larger accelerations over short periods). The occupants of a rocket or
spaceship having such a propulsion system would be free from all the ill
effects of free
fall, such as nausea, muscular weakness, reduced sense of taste, or leaching of calcium from their bones.
The law of conservation of momentum means that in
order for a propulsion method to change the momentum of a space craft it must
change the momentum of something else as well. A few designs take advantage of
things like magnetic fields or light pressure in order to change the
spacecraft's momentum, but in free space the rocket must bring along some mass
to accelerate away in order to push itself forward. Such mass is called reaction
mass.
In order for a rocket to work, it needs two
things: reaction mass and energy. The impulse provided by launching a particle
of reaction mass having mass m at velocity v is mv. But
this particle has kinetic energy mv²/2, which must come from somewhere.
In a conventional solid, liquid,
or hybrid
rocket, the fuel is burned, providing the energy, and the reaction products
are allowed to flow out the back, providing the reaction mass. In an ion
thruster, electricity is used to accelerate ions out the back. Here some
other source must provide the electrical energy (perhaps a solar panel or a nuclear
reactor), whereas the ions provide the reaction mass.[10]
When discussing the efficiency of a
propulsion system, designers often focus on effectively using the reaction
mass. Reaction mass must be carried along with the rocket and is irretrievably
consumed when used. One way of measuring the amount of impulse that can be
obtained from a fixed amount of reaction mass is the specific
impulse, the impulse per unit weight-on-Earth (typically designated by
