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Learning objectives

By the end of this section, you will be able to:

  • Explain how an object (such as a satellite) can be put into orbit around Earth
  • Explain how an object (such as a planetary probe) can escape from orbit

Newton’s universal law of gravitation and Kepler’s laws describe the motions of Earth satellite    s and interplanetary spacecraft as well as the planets. Sputnik, the first artificial Earth satellite, was launched by what was then called the Soviet Union on October 4, 1957. Since that time, thousands of satellites have been placed into orbit around Earth, and spacecraft have also orbited the Moon, Venus, Mars, Jupiter, Saturn, and a number of asteroids and comets.

Once an artificial satellite is in orbit, its behavior is no different from that of a natural satellite, such as our Moon. If the satellite is high enough to be free of atmospheric friction, it will remain in orbit forever. However, although there is no difficulty in maintaining a satellite once it is in orbit, a great deal of energy is required to lift the spacecraft off Earth and accelerate it to orbital speed.

To illustrate how a satellite is launched, imagine a gun firing a bullet horizontally from the top of a high mountain, as in [link] , which has been adapted from a similar diagram by Newton. Imagine, further, that the friction of the air could be removed and that nothing gets in the bullet’s way. Then the only force that acts on the bullet after it leaves the muzzle is the gravitational force between the bullet and Earth.

Firing a bullet into orbit.

Firing a Bullet into Orbit. In panel (a), at left, a figure is drawn at the “top” of the Earth and is pointing a pistol to the left. The first bullet is fired with velocity Va (illustrated with a short arrow at top) and travels only about 1/8 of the way around the globe before hitting the ground. The second bullet is fired with velocity Vb (illustrated with a longer arrow at top) and travels about 1/4 of the way around the globe before hitting the ground. Finally, the final bullet is fired with velocity Vc (illustrated with a long arrow at top) and travels all the way around the globe, striking the unfortunate marksman in the back. Panel (b), at right, shown a similar illustration by Isaac Newton.
(a) For paths a and b , the velocity is not enough to prevent gravity from pulling the bullet back to Earth; in case c , the velocity allows the bullet to fall completely around Earth. (b) This diagram by Newton in his De Mundi Systemate , 1731 edition, illustrates the same concept shown in (a).

If the bullet is fired with a velocity we can call v a , the gravitational force acting upon it pulls it downward toward Earth, where it strikes the ground at point a . However, if it is given a higher muzzle velocity, v b , its higher speed carries it farther before it hits the ground at point b .

If our bullet is given a high enough muzzle velocity, v c , the curved surface of Earth causes the ground to remain the same distance from the bullet so that the bullet falls around Earth in a complete circle. The speed needed to do this—called the circular satellite velocity—is about 8 kilometers per second, or about 17,500 miles per hour in more familiar units.

Each year, more than 50 new satellites are launched into orbit by such nations as Russia, the United States, China, Japan, India, and Israel, as well as by the European Space Agency (ESA), a consortium of European nations ( [link] ). Today, these satellites are used for weather tracking, ecology, global positioning systems, communications, and military purposes, to name a few uses. Most satellites are launched into low Earth orbit, since this requires the minimum launch energy. At the orbital speed of 8 kilometers per second, they circle the planet in about 90 minutes. Some of the very low Earth orbits are not indefinitely stable because, as Earth’s atmosphere swells from time to time, a frictional drag is generated by the atmosphere on these satellites, eventually leading to a loss of energy and “decay” of the orbit.

Satellites in earth orbit.

Satellites in Earth Orbit. The thousands of objects orbiting the Earth are represented as white dots surrounding the planet in this illustration. Most of the objects are in low Earth orbit, roughly between about 100 to 1000 miles.
This figure shows the larger pieces of orbital debris that are being tracked by NASA in Earth’s orbit. (credit: NASA/JSC)

Interplanetary spacecraft

The exploration of the solar system has been carried out largely by robot spacecraft sent to the other planets. To escape Earth, these craft must achieve escape speed    , the speed needed to move away from Earth forever, which is about 11 kilometers per second (about 25,000 miles per hour). After escaping Earth, these craft coast to their targets, subject only to minor trajectory adjustments provided by small thruster rockets on board. In interplanetary flight, these spacecraft follow orbits around the Sun that are modified only when they pass near one of the planets.

As it comes close to its target, a spacecraft is deflected by the planet’s gravitational force into a modified orbit, either gaining or losing energy in the process. Spacecraft controllers have actually been able to use a planet’s gravity to redirect a flyby spacecraft to a second target. For example, Voyager 2 used a series of gravity-assisted encounters to yield successive flybys of Jupiter (1979), Saturn (1980), Uranus (1986), and Neptune (1989). The Galileo spacecraft, launched in 1989, flew past Venus once and Earth twice to gain the energy required to reach its ultimate goal of orbiting Jupiter.

If we wish to orbit a planet, we must slow the spacecraft with a rocket when the spacecraft is near its destination, allowing it to be captured into an elliptical orbit. Additional rocket thrust is required to bring a vehicle down from orbit for a landing on the surface. Finally, if a return trip to Earth is planned, the landed payload must include enough propulsive power to repeat the entire process in reverse.

Key concepts and summary

The orbit of an artificial satellite depends on the circumstances of its launch. The circular satellite velocity needed to orbit Earth’s surface is 8 kilometers per second, and the escape speed from our planet is 11 kilometers per second. There are many possible interplanetary trajectories, including those that use gravity-assisted flybys of one object to redirect the spacecraft toward its next target.

Questions & Answers

If the atmosphere blocks the view of Venus' surface, what am I seeing that looks like craters?
Michael Reply
clouds?
Michele
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters when view the planet?
Michael Reply
I don't Know! perhaps the mountains?
Michele
my question is if the surface of Venus is shining y not our earth
Tahir
when we look at venus we can't see any crater like things... check it once again
RIEM
and venus shines because of the clouds that are made up sulpher dioxide and sulphuric acid droplets. and the clouds are so dense. the case of earth is different
RIEM
also because of Venus's 70% albedo phenamenon
Shivam
and plus it reflects 70 percent of its light back into space earth don't shine because it's not a cloudy planet and its farther than the sun
americantuber
can I become an astronaut without taking mathematics as a subject in 11th class
UNIVERSAL Reply
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters? Are these atmospheric storms?
Michael
can a death of a massive star be the new big bang
Neeta Reply
No it might be a supernova. The big bang was much more massive.
Nick
It created the universe as we know it
Nick
the big bang is just a new beginning
Neeta
the death of a gigantic star makes it possible for a new begging
Neeta
Only in the local area of space
Nick
so many smaller universes are created and destroyed
Neeta
i think it created a supernova!
Michele
you still think? I am sure I created one supernova of my own
Neeta
I'm sure you did
Nick
neeta the death of a massive star could lead to an explosion that leads to a super nova and when the supernova explodes it becomes a nebula like messier 1
americantuber
a supernova remnant
americantuber
what is means by earthbound
Satyam Reply
you are earthbound, arnt we all earthbound? except the ISS etc.
Collins
I don't agree we may be earthbound because of our gross body/physical body...but there is also a subtle body which does not limit us to earthly existence
Neeta
good questions, as humans our habitat is earth, we are bound to this and have to alter ourselves to stay alive off land and outside the atmosphere, do you not agree? I'd guess you could also say the moon is earthbound. anything earthbound is restricted to earth. hope this helps, love your question
Collins
how do you describe astral projection
Neeta
Astral projection is spiritual. I'm assuming Satyam was talking about the physical world.
TheDirtyGhost
ah yes spiritually we are bound only to what we chose, like nothing, I like to think that way
Collins
anyone here also participating in seti@home?
dreamer
what is seti?
Mahesh
what has extraterrestrial entity got to do with this why @seti project..we are discussing human existence on earth and beyond earth
Neeta
please join me at the cosmic Diner where all your intergalactic dreams come true.
Alba
I was just thinking what if somebody ignorantly mistaken a cosmic microwave Background with a conventional microwave we be in a lot of trouble.
Alba
don't kno
dreamer
You mean the Restaurant at the end of the Universe...right we can come back and visit again but not sure of the mode of payment
Neeta
yeah I'm already heading there
dreamer
how to read stars
Amresh Reply
you don't
Max
astrology i guess
Uttam
astrologer is the study of star systems and constellations...but astronomy is the study of formation of universes multiverses birth and evolu of stars
Neeta
cool
Uttam
what is astronomy
babul Reply
study of formation evolution and the death of star star systems and galaxies
Neeta
astronomy is the study of universe
UNIVERSAL
astronomy and physics are basically the same except physics is the study of the motion and behavior of the universe
americantuber
What is 12 constellation of zodiac and why it is important to study astrology
MUHIBULLAH
And also what is main purpose of these 12 constellation of zodiac in astronomy
MUHIBULLAH
astronomy and astrology with two different things
Deja
it Is the study of universe and of life speaking generally! i think!
Michele
universe is born 13,7 miliardi of years with big Bang if i remember well! but there are also other theories for the universe, speaking generally! i remember so!
Michele
universe is actually about 13.7 billion years old
Nick
is astrology like astronomy
debjani Reply
Astrology is a lower dimension.. astronomy is much more vast and multidimensional
Neeta
yes.. astrology is about constellations only, and astronomy is about all stars, galaxies, gravity, dark matter, dark energy.. etc everything including astrology
Mahesh
can you become an astronaut without taking mathematics as a subject in 11th class
UNIVERSAL
I was reading the chapter on Cosmic Microwave Background. And, I can not seem to find it now. If anyone could help me find that portion of the Astronomy Textbook I would really appreciate it. AZ
Alba Reply
Where or which chapter discusses Cosmic Microwave Background?
Alba
29.4
Andrew
29 the big bang ch 29.4
Andrew
yes
pratham
interesting
Orlando
what's cold dark matter?
pratham Reply
How are Roche Worlds formed?
AlteredEdge Reply
In need to read some books about Astronomy so how can in get it actually leg live in Ethiopia can uh help me with that?
Mom Reply
I am sri Sharan .m .what is my best favourable numbers
Madhesh Reply
how can we know it
sruthi
😂😂
Rango
bahut hard
Rango
Why no object can travel with speed greater than light?
geometry Reply
Because the mass of light particles is zero
Asim
Can mass be negative?
geometry
No
Asim
anybody stated that the light is the only thing is speed
sruthi
geometry are you a korean or kpop fan
sruthi
in this universe anything can be faster than light because we simply can't get to an conclusion
sruthi
I am a an Army but lets not bring spam here.
geometry
But why mass can't be negative. What I mean is how can we claim an object made of strange matter has positive mass. In other words, theoretically can negative mass exist?
geometry
in our observable universe nothing is faster than the speed of light
Rango
Thanks
geometry
But theoretically, is it possible? Sorry I'm being very curious..
geometry
I don't know about dark matter but may be you should think about dark matter because dark matter are really strange
Rango
Exactly so theoretically, an dark matter lazer can be greater in speed. Also maybe quartz..
geometry
may be but i am not sure about quartz and btw techyons travel faster than light but techyons are hypothesized
Rango
there are many objects which are perceived to be faster than light
Ganeshan
The object have their own declination rate in speed,the air friction can develop higher temperature resulting the object to be disappear in molecules,not considering the Physical conditions in space.The object should be unaffected with the air friction and also speed should proportionate with fuel.
Near
the early universe expanded faster than the speed of light. Consider, at the speed of light in vacuum, time stops.
Robert
Does this mean something moving beyond the limit experiences negative time?
Robert
Under such conditions, would matter exist as wave-fronts?
Robert
When examining a cosmology it is possible to find quite unbelievable conditions. I like to imagine life at the tip of an asymptote, but to surpass it seems perilously remote.
Robert
what is Deja Vu theory?
rockstar Reply
Have you ever felt like something has ever happened before? That's Deja Vu. The Deja Vu theory says that once you have those visions, you are actually experiencing what an alternate reality would have been. Basically it's the collision of both our universe and another.
AlteredEdge
Yeah it gives us an idea about the existence of a parallel universe
Debabrata
is this a hypothesis? or theory? isn't there something with your brain/memory processing the image or info twice in a few seconds?
Collins
I do also love this experience, seems so unreal but real all at once.
Collins
this can be real if we will solve the mystery of quantum physics .
YOG
I was giving just giving my own hypothesis of the general topic. While it would be interesting to find that we transitioned through dimensions, I think it just boils down to a heightened ability to predict future outcomes and a series of subconscious decisions to enact one of those possible outcomes
AlteredEdge
What I'm saying is that it's all in your head. 😂
AlteredEdge
yeah its all in our heads for sure, the brain is amazing, all of the above hypothesis really interesting. one day hopefully we might figure it out.
Collins
I hope that this hypothesis might be proved true
Debabrata
I feel deja vu is kind of a wormhole which connects us to a parallel universe.
geometry
I like what you said 'alterededge' "dena vu" is a vision of a past memory reenacting itself at an unexpected time and place. traveling and transitioning through dimensions.
Alba
opps, "deja vu"***
Alba
you are are right Geometry Lover.
Alba
Deja vu might even prove the existence of future time travel.
geometry
Well I'm not exactly saying it's a memory, it still hasn't happened yet, more like subconscious pre-cognitive thinking, where your mind eventually leads you down a path that makes the "deja vu" experience reality true after you had it.
AlteredEdge
We says time as a 3rd dimension and space as a fourth dim, so as come to 5 dimension what we say? I hope you got my point
Asim
According to string theory there are 10 dimensions
Debabrata
Yes you are right Mishra
Asim
But Tesseract is said to be 5 dim
Asim
How?
Asim
How do giants planets remain gaseous when they are located in cold regions of the solar system? Do they generate enough heat and if they do, are they not able to also heat their moons?
Kuda
Kuda just to answer part of your question. The gas inside Saturn for example is extremely dense not like gas in a room. The definition of gas is that the particles have little to no interaction between each other meaning there's no chemical bonds.
Sandra
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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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