The Universe-4.
These two elements have had a profound influence on human history.
They sit just a couple of spaces apart in the periodic table.
Iron is element number 26 and copper is at 29.
What is the fundamental difference between them?
The atoms are composed of the building blocks, protons, neutrons, and electrons.
We don’t need to consider the quarks inside the protons and neutrons,
because at the temperatures on Earth they stay locked away.
So when discussing Earth’s chemistry, we can ignore them.
Hydrogen has a nuclei with a single proton.
The proton has a positive electric charge.
This allows it to trap an electron to orbit it around it, to form a hydrogen atom.
The electron carries a negative electric charge, equal and opposite to that of the proton.
This means that the hydrogen atoms are electrically neutral.
The reason why the electron has exactly the equal and opposite charge,
of the proton is not known.
This is even more surprising when we look at the quarks that build up the proton.
The proton is made of three quarks - two up quarks and one down quark.
The up quark has an electric charge of + 2 by 3.
The down quark has a charge of minus 1 by 3.
The electron has a charge of minus 1.
The neutron consists of two down quarks and one up quark.
This means it has no electric charge.
This cannot be a coincidence.
It is one of the greatest challenges of the 21st century to explain it.
Chemical elements differ because of varying number of protons in their atomic nucleus.
But the number of neutrons make no difference to their chemical properties.
Chemistry is down to the way electrons that orbit around the nucleus, behave.
The number of electrons is equal to the number of protons.
Hydrogen has 1 proton and 1 electron.
There is another form of hydrogen called deuterium.
Deuterium has a neutron attached to the proton.
But that does not change the chemical properties, as there is only one electron.
Technically, deuterium and hydrogen are two different isotopes of the same element.
Helium atoms have two protons and two electrons.
It also forms with one or two neutrons, known as helium-3 and helium-4.
Next come lithium with 3 protons, 3 electrons and either 3 or 4 neutrons, and so on.
The rule is that each successful element has one more proton in its nucleus,
although the number of neutrons varies.
The neutrons help the nucleus to stick together,
which is bound tightly by the strong nuclear force.
Electric charge is a bad thing for the nucleus, because the protons are positively charged,
they repel each other and try to blow the nucleus apart.
The neutrons don’t have this problem.
This is one of the reasons why heavier nuclei tend to have more neutrons than protons.
So the construction of chemical elements is simple.
If we want to turn iron into copper, we need to add 3 protons and a few neutrons.
This of course is easier said than done.
Yet nature can do it, because when the Universe is a few minutes old,
the first four chemical element existed.
The building blocks were present, but the heavier elements were assembled later.
Edward Teller and Enrico Fermi were friends and colleagues,
who would both go to become members of the Manhattan team, which built the nuclear bomb.
These bombs would later be dropped on Hiroshima and Nagasaki with devastating effect.
But in 1941, before any nuclear bomb was assembled, their minds started wandering,
beyond the nuclear bomb.
The Manhattan nuclear bombs were fission bomb,
which worked by splitting the nuclei of very heavy elements.
Uranium was used in the Hiroshima bomb.
Plutonium was used in the Nagasaki bomb.
Nuclear fission splits these elements into lighter elements, such as strontium and caesium.
This is the assembly of the elements in reverse.
Each time a nucleus of uranium or plutonium splits, neutrons were released,
which trigger the splitting of other nuclei.
In this way a nuclear chain reaction ensues.
Each time a heavy nucleus splits, a large amount of energy is liberated.
This ‘nuclear binding energy’ is stored in the strong nuclear force field,
that sticks the protons and neutrons together inside the nucleus.
In the very early stages of the Manhattan project, Enrico Fermi postulated,
that there was a possibility of creating a far more powerful type of bomb.
Edward Teller became obsessed with his friend’s idea, and spent the next decade,
designing and building a device that would create the most powerful explosion,
ever made on Earth.
It earned Teller the title ‘Father of the hydrogen bomb’.
In 1952, the fruits of Fermi’s conversation with Teller was realised.
The first successful testing of a hydrogen bomb took place in an atoll, in the pacific ocean.
Ivy Mike was the code name given to the project.
The explosion was estimated to be 450 times more powerful,
than the bomb dropped on Nagasaki.
It produced a fireball over 5 kilometers wide, and a crater 2 kilometers wide.
It wiped the tiny atoll off the map.
Teller had collaborated with another Manhattan scientist, Stanislaw Ulam.
Stanislaw was not present for the explosion.
Instead he sat watching a seismometer, thousands of kilometres away in Berkley, California.
He could clearly see the shockwaves in his office.
’Its a boy!’ he cryptically told his colleagues to inform them of the success.
The Ivy Mike test was the first man made nuclear fusion reaction.
Nuclear fusion is the direct opposite of nuclear fission.
Nuclear fusion is the process by which two atomic nuclei,
are fused to form a single heavier element.
The hydrogen bomb reproduces the process that occurred in the first seconds,
of the evolution of the Universe - the assembly of hydrogen into helium.
The Teller-Ulan designed for the hydrogen bomb, is the basic design employed by all 5,
of the major nuclear weapon states today.
Although the fusion element of the design, is only part of the explosive power,
combined with the other stages, contained within the bomb, it creates destruction,
on an unparalleled scale.
Here are two different ways of creating new elements, and releasing vast amounts of energy.
The first, fission, involves taking a heavier element and splitting it.
The second, fusion, involves taking lighter elements and sticking them together.
But how can both the processes result in energy being released.
This is how nature works.
Its all down to the delicate balance between the electric repulsion of the protons in the nucleus,
and the power of the strong nuclear force to stick the protons and neutrons together.
Since there are two competing forces, one trying to blow the nucleus apart,
and one trying to glue it together.
We might think that there must be some kind of balancing point -
an ideal mixture of protons and neutrons, that is perfectly poised between,
attraction and repulsion.
There are in fact two elements that are very close to the mixture of optimal stability.
These are iron and nickel.
Elements lighter than these can be made more stable releasing energy in the process,
by fusing them together.
Elements heavier than these can be made more stable, releasing energy in the process,
by breaking them apart.
To be completely accurate, we should mention that there are other factors,
then just the balance between electromagnet and nuclear forces,
that feed into the stability of elements.
These are to do with the shape of the nucleus itself, and that the balance between,
protons and neutrons is favoured for quantum and mechanical reasons.
Here on Earth, fusion may seem the ultimate human technological achievement.
Actually it is the most natural thing in the Universe.
It didn’t only happen only in the Big Bang.
It is the process that can be found occurring across the Universe all the time.
Fusion is the process that powers every star in the Universe, including our Sun.
Deep in the Sun’s core, 800,000 kilometres below the surface, temperatures reach 15 million degrees.
Here the Sun is fusing hydrogen into helium, at a furious rate.
Its just one second, the Sun converts 600 million tons of hydrogen into helium,
releasing as much energy as the human race will use in the next million years.
This is the energy that makes the stars shine.
It is the process of turning hydrogen into helium, that creates the energy,
that allows all life on Earth to exist
This process is repeated in every star in the Universe.
The assembly of the second simplest element, helium, is well understood.
The stars do it all the time.
We know it happened in the early Universe.
We can do it ourselves on Earth.
But this doesn’t help to explain the origin of the other 94 naturally occurring elements.
Clearly, somewhere in the Universe there must be a plentiful source of the other elements,
because they are everywhere.
Our whole planet is made from them.
We are made up of trillions of atoms, like magnesium, zinc, iron, and of course,
the one atom that life is more dependent on than any other - carbon.
Every human being is made up of 10 to the power of 27 carbon atoms.
That is an unimaginable number of carbon atoms, that simply didn’t exist in the early Universe.
Where did they come from?
The answer is in nuclear fusion, and the natural place is within the stars themselves.
The first stars formed around 100 million years after the Big Bang.
The rate at which they burn the hydrogen fuels essentially depends on their mass.
The more massive the star, the brighter it shines, and shorter its lifetime.
The key to understanding how the heavier elements came into being,
lies in what happens to stars when they have exhausted their hydrogen fuel.
For the most massive known stars, this may take a few million years.
For stars like our Sun, it may take 10 billion years.
The Universe has been around for plenty of time to allow generations of stars to live and die.
As a star exhausts its hydrogen stores, we might expect it to slowly flicker away.
But, for stars like our Sun, the opposite happens.
Having spent millions or billions of years, with the core as its beating heart,
a star that is running out of hydrogen, in fact swells up to potentially hundreds of times,
its original size.
One of the closest red giants to Earth is the star Alpha Orionis, better known as Betelgeuse.
It is the ninth brightest star in the night sky, and is 500 light years away.
When astronomers tried to measure its diameter, they realised it was no ordinary star.
They used a specially designed telescope to measure the scale of this red star,
using a technique known as interferometry.
They came up with a number that revealed something profound.
Betelgeuse is a true giant in every sense.
The star is about 20 times the mass of our Sun, but its size is very impressive.
If we put Betelgeuse at the centre of our solar system, it would extend past the Earth’s orbit,
encompassing everything out to Jupiter.
Current estimates suggest it is around 800 million kilometres in diameter.
Due to its immense size and relative proximity, we can study Betelgeuse in incredible detail.
In 1996, the Hubble space telescope took a picture of Betelgeuse,
that was the first direct image of another star to reveal its disk and surface features.
We have even imaged sunspots on surface and being able to study its atmosphere,
in ever increasing detail.
However its not the surface of the red giant that holds the clue,
to where the heavy elements are made.
To understand that, we need to journey into its dying heart.
Stars exists in an uneasy equilibrium.
There gravity acts to compress them, which heats them up until the electromagnetic repulsion,
between the hydrogen atoms is overcome and they fuse together to make helium.
This releases energy, which keeps the star up.
When the hydrogen runs out, the outward pressure disappears.
Gravity regains the upper hand, and the structure of the star changes dramatically.
The core collapses rapidly, leaving a shell of hydrogen and helium behind.
Within the shrinking core the temperature raises until, at 100 million degrees,
a new fusion process is triggered.
At these temperatures helium nuclei can overcome their mutual electromagnetic repulsion,
and wander close enough together to fuse.
The star begins to burn helium.
This transfer from hydrogen to helium fusion has two profound effects.
Firstly, sufficient energy is released to halt the stellar collapse.
So the star stabilises and rapidly swells.
This is the beginning of its life as a red giant.
Secondly, it fuses into existence, the element vital for life.
At first sight the fusion of two helium nuclei, with two protons and two neutrons,
should only be able to produce the isotopes beryllium-8.
This unstable isotope of beryllium that quickly breaks down,
but in the intense temperatures of a dying star, as the core exceeds 100 million degrees,
these nuclei live just long enough to fuse with a third helium nucleus.
This creates the precious element carbon12.
This is where all the carbon in the Universe comes from.
Every carbon atom on the planet was produced in the heart of a dying star.
The Helium burning phase doesn’t end with the synthesis of carbon.
During the same intensely hot phase in the star’s life,
the conditions allow a nucleus of helium to fuse with a carbon nucleus to create oxygen.
Oxygen makes up 21% of the air we breathe.
It is prerequisite for water, the solvent of life.
It is the third most common element in the Universe after hydrogen and helium.
We breathe in two and half grams of oxygen every minute, which was created in a dying star.
Compared with a life time of a star, this stellar production of carbon and oxygen,
is over in the blink of an eye.
Within a million years the helium supply in the core is used up,
and for many stars that is where fusion stops .
Any average size star, like our Sun, has by now reached the end of its productive life.
When our Sun reaches this stage, in a few billion years time,
there won’t be enough gravitational energy,
to compress the core any further and restart fusion.
Instead the star becomes more and more unstable, huge pressure points will build up,
until eventually the whole stellar atmosphere explodes.
It hurls the precious cargo of oxygen, carbon, and hydrogen, and all on its journey into space.
During this time of a few tens of thousands of years, a dying star will create,
one of the most beautiful structures in our Universe: a planetary nebula.
Once this stage is over, an average size star will shrink to an object no bigger than Earth.
A white dwarf is the fate of such stars, and billions are like it.
For massive stars like Betelgeuse, the action is far from over.
In these cases the chemical production line will continue.
As helium fusion slowly comes to an end, gravity takes over,
and the collapse of the core restarts.
The temperature rises, launching the third stage in the birth of our Universe’s elements.
Temperature rise to 100s of millions degrees.
Carbon fuses with helium to make neon.
Neon fuses with more helium to make magnesium.
Two carbon atoms fuse to make sodium.
With more and more elemental ingredients entering the cooking pot, and temperature rising,
the heavier elements are produced one after another.
The core continues to collapse, the temperature continues to rise,
and the next stage of fusion begins, leaving layers of newly minted elements behind.
With the first 25 elements now created within the star, the runaway production line hits a block,
at the 26th element, iron created from a complex cascade of fusion reactions fuelled by silicon.
At this stage, the temperature of the star is 2.5 million degrees, but it has nowhere to go.
The peak of nuclear stability has been reached, and no more energy can be released by adding,
more protons or neutrons to iron.
The final stage of iron production last only a couple of days.
It transforms the heart of the star into almost pure iron,
in a desperate bid to release every last gasp of nuclear binding energy and stave of gravity.
This is where the fusion process stops.
Once the star’s core has been fused into iron, it has only a few seconds to live.
Gravity must now win.
A star like the red giant Betelgeuse collapses in a few seconds,
under its own weight forming a planetary nebula.
The first 26 of the elements are forged in the cores of stars.
They are distributed throughout the Universe when the stars collapse.
But what about the rest of the elements?
There are over 60 elements heavier than iron in the Universe.
Some are valuable, such as gold, silver and platinum.
Some are vital for life, such as copper, and zinc.
Some are just useful, like tin, lead, and uranium.
Very massive stars can produce very tiny amounts of the heavier elements,
upto bismuth, element number 89, in their cores, by a process called neutron capture.
But it is known that this makes nowhere near enough,
to account for the abundance we observe today.
There simply haven’t been enough massive stars in the Universe.
The conditions necessary to produce large amounts of the elements beyond iron,
are only found in the most rare of all celestial events.
In a galaxy of 100 billion stars the conditions violent enough to form,
substantial amounts of these elements,
will exists on an average for less than 2 minutes in every century.
All the stars are born from clouds of gas.
The length of their life and their eventual fate, are governed by their mass.
Stars dozens of times heavier than the sun live for only a few million years.
They finally swell up into super giants and explode as supernova.
Stars like the sun live longer and die more gently.
They shine steadily for billions of years before swelling into red giants,
and losing their outer layers as a planetary nebula.
The core of the star, exposed as a white dwarf, then continues to glow for billions of years more,
before gradually fading out.
The least massive stars, the red dwarfs simply fade out, over 10’s of billions of years.
High mass stars, eventually become red super giants,
with a size of about 1000 million kilometres.
It becomes more red as it starts to cool.
These red giants eventually explode as a supernova.
Its outer layers are blown off, producing elements heavier than iron.
If a star’s remnant mass is over 1.4 solar masses, it will collapse and become a neutron star,
with the size of about 15 kilometres.
If the remnant mass is above three solar masses, it will collapse and become a black hole.
These are regions of space in which gravity is so strong that not even light can escape.
Their size is around 50 kilometres.
They are surrounded by swirling discs of captured gas and dust.
Sun like stars go through a different cycle.
They start out as dense nebula, about 200,000 billion kilometres.
It begins to contract.
Gravitational pull will cause it collapse.
Central temperatures rises to 15 million degrees.
This size of this protostar is about 100 million kilometres.
The protostar becomes a main sequence star.
It emits heat and light caused by nuclear fusion.
Its size is now about 1 million kilometres.
Eventually it becomes a red giant as hydrogen shell burning begins.
The red giants outer layers start to form a planetary nebula.
After burning its helium shell, it collapses to form a white dwarf.
This will eventually become a black dwarf, with a size of 10000 kilometres.
It stops glowing at this stage.
Low mass stars or red dwarfs, shines for trillions of years.
It never becomes a red giant.
Its size is around 100 thousand kilometres.
These stars starts to collapse as hydrogen is used up.
The star becomes extremely dense with a faint core.
It will turn into a white dwarf.
Eventually it will stop glowing.
After a few millions years of life, the destiny of the largest stars in our Universe,
is a dramatic one.
Having run out of hydrogen, and burnt through the elements all the way to iron,
giant stars teeter on the edge of collapse.
Yet even in this dilapidated state these stars have one last violent act.
It occurs with such intensity, that it allows the creation of heavier elements.
If we could gaze deep into the heart of one of these dying giants,
we could see the core finally succumb to gravity.
As fusion grinds to a halt, this giant ball of iron, falls in on itself with enormous speed,
contracting at upto quarter of the speed of light.
This dramatic collapse causes a rapid increase in temperature and density,
as the core shrinks to a fraction of its original size.
The inner core might shrink to 30 kilometres in diameter.
At this point with temperatures reaching 100 billion degrees,
and densities comparable to those inside an atomic nucleus,
quantum mechanics steps in to halt the collapse.
By now most of the electrons and protons in the core have been literally forced,
to merge into neutrons.
Neutrons, in common with protons and electrons,
obey something called the Pauli exclusion principle.
This effectively prevents them from getting too close to one and another.
In more technical terms, no two neutrons can be in the same quantum state.
This has the effect of making a ball of neutrons, the most rigid material in the Universe.
They are 10 to the power of 20 times as hard as a diamond.
When the neutrons can be compressed no more, the contraction must stop,
and all the super heated collapsing matter rebounds with colossal force.
A shock wave shoots out through the star and as this blast wave,
runs into the outer layers of the star, it generates the highest temperature in the Universe,
100 billion degrees.
The precise mechanism for the rapid heating is not fully understood,
but it is known that for a matter of seconds, the conditions are intense enough,
to form all the heaviest elements in our Universe, from gold to plutonium.
This is a type II supernova, the most powerful explosion we know of.
Supernovae are so rare that since the birth of modern science,
we never had the chance to see one, close up.
The last supernova explosion in our galaxy was in 1604.
This was a few years before the invention of the astronomical telescope.
On average, it is expected that there should be around one supernova explosion,
in the Milky way per century.
But for the last 400 years we had no luck.
Astronomers are always searching for stars,
which they think might be the most likely candidate to go supernova.
One of the prime candidates is Orion’s shining red jewel, Betelgeuse.
With many telescopes trained on this nearby star,
we have been able to follow its every move for decades.
Charting its brightness, we have discovered that it is extremely unstable.
It has dimmed by about 15% in the past decade.
As supernova candidates go, Betelgeuse is top of the list.
It is generally thought that Betelgeuse could go supernova at anytime.
It is relatively young star, perhaps only 10 million years old.
It has sped through its life cycle so rapidly because it so massive.
However when you are 10million years old, the end of your life cycle can be quite drawn out.
When we say ‘any time soon’ in stellar terms,
it could be at some point in the next million years.
It could also be tomorrow.
What we do know is that, when it does go, it will provide us with quite a show.
Betelgeuse is just 500 light years away, almost uncomfortably close.
It means that the supernova will be incredibly bright.
It will be by far the brightest star in the sky.
It may even shine as brightly as a full moon at night,
and fill the sky as a second sun during the day.
In a single instant, Betelgeuse will release more energy,
than our Sun will produce in its entire lifetime.
As the explosion tears the star apart, it will fling out into space all the elements,
the star has created through its life.
Over millions of years the newly minted elements will spread out to become a nebula.
It will be a rich chemical cloud drifting in space.
At the heart of it, all that will remain will be the super dense core of neutrons.
This is the remnants of stars that was once a billion kilometres across.
It will have been squashed out of recognition by gravity.
This is a neutron star, the ultimate destiny of Betelgeuse.
It becomes a dense, hot ball of matter, which is the same mass as our Sun,
but only 30 kilometres across.
We may not have seen neutron stars close up, but we have seen them from afar.
X-ray images have been taken that give us vital information about these stars.
In particular we have seen recent pictures of RCW103,
the 2000 year old remnant of a supernova explosion, that occurred 10000 light years from Earth.
This may sound like a cosmic graveyard, but it is in the death of old stars,
that new stars are born.
This is the earthly cycle of death and rebirth, played out on a cosmic scale.
We can see that beautiful cycle in the constellation of Orion.
In an area known as the sword handle lies the Orion nebula.
To the naked eye it appears to be a misty patch of light in the night sky.
But through a telescope it is a majestic wonder of the Universe.
Hidden in its clouds are bright points of light.
These are new stars forming from the clouds of elements blown out by supernova explosions.
These are new borns from the death of the old.
It is from such a cycle that we emerged, within a nebula just like this, 5 billion years ago,
when our Sun was formed.
Around that star a network of planets condensed from the ashes.
Amongst them was Earth: a planet whose ingredients originated from the nebula,
a cloud of elements formed in the death of stars, drifting through space.
This is not the end of the story.
It is now thought that the chemical elements are not the most complex pieces of ‘us’,
that were assembled in the depth of space.
If we look at the graph, of the spectrum of light from the Orion nebula,
we find that the information that it contains is fascinating.
It reveals that the orion nebula is not just a cloud of elements.
There is complex chemistry happening out there deep in space.
The graph reveals there are complex molecules like water and sulphur dioxide in the nebula.
More surprisingly there are also complex carbon compounds, methanol, hydrogen cyanide,
formaldehyde and dimethyl ether.
This is direct evidence for complex carbon chemistry occurring in deep space.
This is exciting because it means, we are seeing the beginning of chemistry of life,
in a vast cloud of interstellar gas.
The connection doesn’t end there.
We may be connected to the chemistry out there in space even more directly.
There are thousands of astroids in our solar system.
They are mostly in the astroid belt, that formed 4.568 billion years ago.
On an average one meteorite falls to Earth once a month.
These astroid pieces give us a real insight into what forms the building blocks of life.
Some meteorites are almost certainly older than any rock on Earth,
because it was formed from the primordial dust cloud, in the nebula that collapsed to form,
the Sun and planets 5 billion years ago.
These meteorites were found to contain amino acids, which are the building blocks of life.
This strongly suggest that there was very complex carbon chemistry happening out there in space,
forming the building blocks of life, over four and half billion years ago.
It raises the intriguing prospect that the first amnio acids on Earth may have formed,
in the depths of space and delivered to our planet by meteorites.
We are truly children of the stars.
Written into every atom and molecules of our bodies, is the history of the Universe,
from the Big Bang to the present day.
For all its scale and grandeur, the Universe is shaped by the action of just 4 forces of nature.
Two of these the weak and strong nuclear forces, remain hidden from every day experience,
inside the atomic nucleus.
The third force electromagnetism, is perhaps most familiar to us.
It is the one we marshal to power our lives.
Electric current flow because of the action of this force.
Finally, there is gravity, the force that acts between the stars.
Gravity shapes the cosmos on the largest distant scales.
From the formless clouds of hydrogen and helium that once filled our Universe,
gravity forged the first stars.
It sculpted the first planets and arranged them into the exquisite shapes of the galaxies.
Having assembled countless billions of solar systems, gravity drives their cycles and rythmes.
It is the invisible string behind the revolution of every moon around every planet,
and every planet around every star.
Gravity keeps our feet on the ground, and the Universe ticking over.
Gravity is both the creator and destroyer of stars.
Stars shine in temporary resistance to gravitational collapse.
When they run out of nuclear fuel, and the other three forces,
can no longer rearrange the matter in the cores,
in order to release energy and resist its inward pull,
gravity crushes the most massive of them out of existence.
Yuri Gagarin and Gherman Titov were selected for the first soviet manned space program.
In April 1961, Gagarin became the first human to travel into space, in Vostok 1.
He completed his single orbit in 108 minutes, before returning to Earth.
Titov was the second man to orbit our planet in August 1961.
He remains the youngest man ever to make the journey into space, at under 26 years old.
He piloted Vostok 2, which completed 17 orbits of Earth.
He became the first person to sleep in space.
He also was the first person to suffer from space sickness.
This conditions include a variety of symptoms such as nausea, vomiting, vertigo, and headaches.
To train for experience in weightlessness, space agencies create weightless conditions,
by falling towards Earth.
The American space program was called project Mercury, a series of six manned launches.
Alan Shepherd was the first American in space in May 1961.
John Glenn was the first American to orbit Earth.
John Glenn went on his final space shuttle mission in 1998, at the age of 77.
During training for project Mercury, NASA developed a way of flying regular military aircraft,
to create weightless conditions.
Using a C-131 aircraft, and flying a unconventional parabolic flight path,
a brief period of 25 seconds of weightlessness was created.
This was used for training astronauts by flying them in this 20 or 30 times.
The C-131 was named as the Vomit Comet.
It is a perfect place to experience the two related aspects of the force of gravity,
that hold key to understanding what gravity actually is .
Firstly, it is possible to completely cancel out the effects of gravity,
by simply falling towards the ground.
This sets gravity apart from all the other forces of nature.
It is not possible to negate the effect of electric charge,
other than by adding more electric charge of the opposite side.
The Comet achieves the removal of gravity, by accelerating towards the ground,
at 9.81 metres per second squared, to cancel out the force of gravity.
It is also possible to add to Earth’s gravitational pull by accelerating away from it.
Astronauts in space are weightless and float around inside there spacecraft.
It is not because they are a long way from Earth, that gravity is absent.
In fact they are only few hundred kilometres above Earth’s surface.
The strength of Earth’s gravitational field in near-Earth orbit is not too different,
to the strength on this surface.
It is that the effects of gravity are removed by falling.
This is important point number one.