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Though entirely predictable over times of a few centuries, the orbits of Earth-approaching asteroids are unstable over long time spans as they are tugged by the gravitational attractions of the planets. These objects will eventually meet one of two fates: either they will impact one of the terrestrial planets or the Sun, or they will be ejected gravitationally from the inner solar system due to a near-encounter with a planet. The probabilities of these two outcomes are about the same. The timescale for impact or ejection is only about a hundred million years, very short compared with the 4-billion-year age of the solar system. Calculations show that only approximately one quarter of the current Earth-approaching asteroids will eventually end up colliding with Earth itself.

If most of the current population of Earth-approaching asteroids will be removed by impact or ejection in a hundred million years, there must be a continuing source of new objects to replenish our supply of NEAs. Most of them come from the asteroid belt between Mars and Jupiter, where collisions between asteroids can eject fragments into Earth-crossing orbits (see [link] ). Others may be “dead” comets that have exhausted their volatile materials (which we’ll discuss in the next section).

Near-earth asteroid.

Rotation of NEA Toutatis. The rotation of this elongated asteroid can be seen in this four panel image.
Toutatis is a 5-kilometer long NEA that approached within 3 million kilometers of Earth in 1992. This series of images is a reconstruction its size and shape obtained from bouncing radar waves off the asteroid during its close flyby. Toutatis appears to consist of two irregular, lumpy bodies rotating in contact with each other. (Note that the color has been artificially added.) (credit: modification of work by NASA)

One reason scientists are interested in the composition and interior structure of NEAs is that humans will probably need to defend themselves against an asteroid impact someday. If we ever found one of these asteroids on a collision course with us, we would need to deflect it so it would miss Earth. The most straightforward way to deflect it would be to crash a spacecraft into it, either slowing it or speeding it up, slightly changing its orbital period. If this were done several years before the predicted collision, the asteroid would miss the planet entirely—making an asteroid impact the only natural hazard that we could eliminate completely by the application of technology. Alternatively, such deflection could be done by exploding a nuclear bomb near the asteroid to nudge it off course.

To achieve a successful deflection by either technique, we need to know more about the density and interior structure of the asteroid. A spacecraft impact or a nearby explosion would have a greater effect on a solid rocky asteroid such as Eros than on a loose rubble pile. Think of climbing a sand dune compared to climbing a rocky hill with the same slope. On the dune, much of our energy is absorbed in the slipping sand, so the climb is much more difficult and takes more energy.

There is increasing international interest in the problem of asteroid impacts. The United Nations has formed two technical committees on planetary defense, recognizing that the entire planet is at risk from asteroid impacts. However, the fundamental problem remains one of finding NEAs in time for defensive measures to be taken. We must be able to find the next impactor before it finds us. And that’s a job for the astronomers.

Key concepts and summary

Near-Earth asteroids (NEAs), and near-Earth objects (NEOs) in general, are of interest in part because of their potential to hit Earth. They are on unstable orbits, and on timescales of 100 million years, they will either impact one of the terrestrial planets or the Sun, or be ejected. Most of them probably come from the asteroid belt, but some may be dead comets. NASA’s Spaceguard Survey has found 90% of the NEAs larger than 1 kilometer, and none of the ones found so far are on a collision course with Earth. Scientists are actively working on possible technologies for planetary defense in case any NEOs are found on a collision course with Earth years in advance. For now, the most important task is to continue our surveys, so we can find the next Earth impactor before it finds us.

Questions & Answers

Does any body know about Quantum Battery ?
Yabtsega Reply
How to get into the field of astronomy after completing B.tech in department of ECE?
Lakshmi Reply
Well my option is to do your job while getting a distant education from a reptued university for B.Sc and try for M.Sc in IIA and remaining astrophysics universities in india
Mahesh
Do I require GATE score for that?
Lakshmi
Yeah I meañ get a better score in gate.I think the university considers it and would help you to direct enroll into a masters progarmme
Mahesh
But the score should be good.
Mahesh
okay. I will do my best.
Lakshmi
Thank you
Lakshmi
Wanna join conversation about time travel?
Pratiksha Reply
Of course
Yabtsega
From where do we start?
Yabtsega
is there a competition formed by NASA in this month
Scientific Reply
nooo
Kygo
don't know
Pratiksha
It's because of presence of iron oxide in its surface.
Info Reply
That point is known as Singularity
Tushar Reply
Indian culture talks about black holes very well
Tushar
The word "MAHAKAAL" means black hole
Tushar
I love Quantum mechanics
Tushar
I love particle physics
Tushar
wow do you know about some maths in quantum physics?
Tanmay
Hi! Anybody can tell me that what is the exactly the black hole?
sach Reply
l no
Jee
OK then tell me
sach
A black hole is a space where gravity pulls so much that even light cannot get out.
Pratiksha
This can happen when a star is dying
Pratiksha
ok👍
Pratiksha
Explain more
sach
ok wait
Pratiksha
when a star of mass greater than 20 times the mass of sun dies it forms a stellar mass black hole
Tanmay
it bends space time so much that you need to travel faster than speed of light to get out of it
Tanmay
the innermost stable orbit of a black hole is that orbit where only light speed particles can orbit it
Tanmay
accretion disk of black holes are the matter that it gained from the surrounding orbiting the black holes
Tanmay
black hole forms when the center of very massive star collapsed it upon it self
Pratiksha
relativistic jets come out of black holes when the angle between accretion disk is not 90 degrees(this might be wrong but I know that it has to do something with magnetic field)
Tanmay
@pratiksha you took a lot of time to type this I thought you are Going to type an essay
Tanmay
@Gurjinder you can ask it here
Tanmay
sorry, but the text was not going 😅
Pratiksha
OK thanks
sach
yr most welcome sis
Pratiksha
I am a student class 12
Tanmay
there are 3 types of black holes stellar mass intermediate mass and super massive black holes
Tanmay
yeah!
Pratiksha
super massive black holes are found at the center of most galaxies
Tanmay
My hometown is at Odisha but I study and live in up
Tanmay
I still don't know a lot of stuff especially the maths involved in astrophysics
Tanmay
becoming an astrophysics thanks for your words
Tanmay
oh cool what all subjects are you studying
Tanmay
oh it happens sometimes
Tanmay
sadly chemistry XD
Tanmay
are there any chapters only for astrophysics
Tanmay
thank you same to you
Tanmay
oh that's a lot of hard work I hope you get great achievements can you tell me what all you are currently studying and which channel
Tanmay
oh that's a good strategy thank you
Tanmay
super
Jee
positional astronomy?
Tanmay
can you tell what positional astronomy is
Tanmay
I am fairly good in trigonometry but really bad at graphs
Tanmay
Hello everyone.Looks a cool discussion
Mahesh
ok I will practice thanks
Tanmay
where are you studying @Gurjinder
Mahesh
ok I will learn more
Jee
At which college I meañ.
Mahesh
Good
Mahesh
I want to be a astronomer,astrophysicist,theortical physicist and a engineer
Mahesh
I am not that good at self study
Mahesh
@Gurjinder that's a lot of hardwork I have high respect for you
Tanmay
I am always curious
Mahesh
nothing about astrophysics?
Tanmay
Lol
Mahesh
is college better than school?
Pratiksha
l am in school
Jee
Mainly from feymann if you want a good grip of basics
Mahesh
me too
Pratiksha
I just completed so what.Learning has no age limit.
Mahesh
which standard
Jee
at least your physics teacher should know something about astrophysics
Tanmay
l no
Jee
I love the book on quantum mechanics
Mahesh
though I hate classical mechanics
Mahesh
yeah the education system in India just cares about Mark's I don't know what will happen to the new system
Tanmay
yeah! yr right
Pratiksha
I love the new system.
Mahesh
They are going to introduce
Mahesh
but I am happy to know that people around the world are destroying it
Tanmay
the old system
Tanmay
Hmmm
Mahesh
what is black hole is something and what can do a black hole is another thing
Kygo
A black hole is a singularity where matter is condensed and a gravity well that light cannot escape from.
Micheal
what is blue marble
Jee Reply
it is the another name of earth.
Sadman
hey friends how are you?
Sadia
why Mars is known as red planet?
Sadia
blue marble not blue planet,so it z not earth but the image of earth taken in 19's
Ethical
you know when I was small, I had seen a comet ☄️ in the sky but I have a doubt that was that really a comet or meteors.. what is the difference between these two?
Jugal Reply
Do any one observe the moon every night?
Pratiksha Reply
I do look at it but with naked eyes
Tanmay
what do you see?
Shivam
sometime not every night as clouds are often there in the sky
Jugal
I look stars,Venus and moon by naked eye as I have no telescope yet
Jugal
how much does a telescope cost
Shivam
depending upon the resolution.....good resolution cost more
Jugal
I can some big dark spots from my naked eyes
Tanmay
A good telescope would cost 400$
Tushar
I have 16 inch dobsonian(truss tube).I'm observing Orion nebula and studying it.
Tushar
wow I am jealous.
Mahesh
why
Tushar
I have made this telescope by my own efforts
Tushar
16 inch is big enough to see Galaxies and star clusters
Tushar
wow you made it on your own
Mahesh
Yes
Tushar
But for that you need Some engineering skills and some proper guidance
Tushar
My Uncles helped me
Tushar
I'm from middle class family This project was funded by Uncles
Tushar
I am from a middle class family too
Mahesh
Uncommon For Me
Anti-Capitalism
What do you think will happen if we find humans rather than aliens on other planets?
Tanmay Reply
Hmmm
Mahesh
let's assume that those humans are just like us physically and mentally. Then what do you think will happen?
Tanmay
We and those humans form a conference like meeting and talk to improve the connection from both sides
Mahesh
But I think that the connection won't be trustworthy
Tanmay
In the same they think about us...
Mahesh
yeah you are correct they might send imposters to collect more data about us
Tanmay
The same way trump and Kim are acting in front of media as they sorted it out...
Mahesh
this would be a really tough situation
Tanmay
You think so...
Mahesh
If you go to Europe there are high chances of underwater creatures what about that..
Mahesh
underwater creatures what? I didn't understand what you are saying
Tanmay
You know europa
Mahesh
yes
Tanmay
The water on europa is more than the water in our earth that is able to sustain lige
Mahesh
So scientists predicted that there maybe underwater life in europa
Mahesh
oh yes I heard it
Tanmay
So in the time of crisis we can go to europa and bulid underwater colonies.
Mahesh
ok but a lot of people would have to be left behind
Tanmay
it is smaller than our moon
Tanmay
But we can bulid underwater floating cities . scientists said that not me..
Mahesh
Yeah and I have been wanting to ask you. Are you a girl or boy..
Mahesh
I am a boy
Tanmay
ok
Mahesh
yeah I don't know who they will leave behind and who will go to that
Tanmay
what do you think who all will be left
Tanmay
I didn't quite catch the question.Say it again..
Mahesh
what do you think, who will be left behind in earth in the time of disaster and who all would go to europa
Tanmay
I don't think all humans can be able to travel.So some humans and animals
Mahesh
yeah the astronauts would have select people so what do you think that on what basis they will select us
Tanmay
I think all the productive people would be allowed in the ship
Tanmay
Hmm I don't know.Maybe the people who are on the verge of dying and who are incapable for space travel are left behind I think.
Mahesh
yeah all the people capable of continuing our generation would be allowed. I hope that time shall not come
Tanmay
It will come.Time is inevitable
Mahesh
Do you know that Newton predicted that the world will end in 2060
Mahesh
some were saying that world would end in 2016
Tanmay
but I never heard Newton about 2060 one
Tanmay
But this Newton we are talking about not a astrologer or civilization calaender
Mahesh
I mean I never heard Newton saying that
Tanmay
He said it is one of his works I think..
Mahesh
but according to sanskrit calendar the earth will be doomed is a million years or so
Tanmay
as told by my teacher that the "kal yug" will end in a million years
Tanmay
kaliyug..
Mahesh
Not kal you
Mahesh
oh yes that
Tanmay
Hmm let's see.Did you watch meowise
Mahesh
Neowise
Mahesh
no I don't know what that is
Tanmay
comet neowise
Mahesh
no I never saw or heard about it
Tanmay
Really.It appeared in last month for a period of days
Mahesh
did it come at the end of August?
Tanmay
or somewhere in middle?
Tanmay
Yeah
Mahesh
Middle it is..
Mahesh
oh then I would have been able to see that but I didn't
Tanmay
I just searched it and it's orbital period is ginormous 6766 years
Tanmay
Yeah
Mahesh
That's why so many star grazers have clicked many photos of it all around the world
Mahesh
yeah I saw the photos
Tanmay
have you heard about doomsday clock?
Tanmay
kind of..
Mahesh
Btw what is the logic of a doomsday clock
Debabrata
some people have made a community which based on political stuff change the time of this clock
Tanmay
for me this is worthless to be honest
Tanmay
Are we living in a simulation?
Vedant Reply
it could be true.
Shivam
at this time there is no way for us to find out that we are in a simulation or not
Tanmay
We can say our universe is a simulation if we were able to do time travel and find out that no matter what you do future will remain same
Tanmay
It could be true because in every 100 years a pandemic comes and tests us,how are we doing in management against pandemic.There are some people that are extraordinary and they are discovering things that are essential for us to know,.There are alot of ufo sightings on earth every year.
Shivam
and there was a rumour that NASA had 2 two alien bodies in area51.Why NASA is not sending more missions on moon and not going there.They are just focusing on Mars but not on moon,when every one is focusing on lunar research.
Shivam
what is the basic idea I need to genarate astronomy?
SCIBASED Reply
I didn't understand the question
Mahesh
as per my opinion, the basic ideas that you need to generate the interest in Astronomy can only be said by looking at your interested topics in Astronomy..There are different topics like steller astronomy,planetary astronomy,etc
Jugal
Astronomy is only a branch for which you don't have to go somewhere to get something.. Just you have to turn your head towards the night sky in a pollution less area and then you can start your observations 👍👍
Jugal
how many types of astronomy branches are the?
Shivam
if I take astronomy as my career.Would I study all of them or I have to study them separately.
Shivam
what is cosmology?
Shivam
you have to choose a particular topic in astronomy (not all) and have to do research on it...👍👍
Jugal
ok
Shivam
cosmology is also a branch of physics in which one have to do a deep study about the past, present and future condition of the Universe. It includes the questions like how our Universe formed,what were the condition at that time,how life formed. A cosmologist can also predicted the future condition
Jugal
ok
Shivam
but which branch of astronomy is the best?
Shivam
tell me meaning of all branches? please
Shivam
there are many branches like planetary astronomy, astrometry,solar Astronomy, stellar Astronomy, galactic Astronomy,radio astronomy,etc
Jugal
you have to do research on a particular topic and also have to write a thesis or book on your topic in PhD to complete your research degrees...after that you would become a successful Astronomer 🔭🔭
Jugal
which do you think is the best?
Shivam
What does type I, II, III civilization mean? How do we calculate it?
Tanmay Reply
Type 1,2,3 civilization is the three categories of kardashvev scale which is a method of mesuring the tecnological advancement based on the amoumt of energy they can use
Sadman
Type 1 civilization can use the full energy which was came from there host star and their host planet
Sadman
type 2 civilization can use the hole energy of their host star
Sadman
type 3 civilization can use the energy of their entire host galaxy
Sadman
Ok but how can we calculate this is there any formula or some kind of energy output value or something?
Tanmay
A Type I civilization would have complete and total mastery over all the energy of its planet. A Type II civilization would be masters of all the energy produced by their home star, and a Type III civilization would own all the energy in their home galaxy.
Astronomy
The Kardashev scale is a method of measuring a civilization.
Astronomy
our civilization is 0.72
Mahesh
So the readings in Kardashev scale is based upon total energy consumption of a civilisation?
Tanmay
What is the lowest and highest value possible in this scale?
Tanmay
is time travel possible?
Shivam
Yes time travel is possible (in theory)...
Taj
Time is a physical property that can and IS affected by gravity... This has been PROVEN through experimentation via seismic clocks.
Taj
thanks for the answer
Shivam
no problem
Taj
The study of astronomy is really interesting, hard and full of amazing events
Pratiksha Reply
It Is The Oldest Of All The Science's.....
Adam
yeah!
Pratiksha
Do You Observe The Night Sky?
Adam
What Is Your Favorite Object Or Object's To Observe?
Adam
For Me, I Love Observing The Moon, It Allows You To Get Up Close And Personal, Like No Other Object Does.....
Adam
definitely have to agree with that (especially living near centre of the city), such clarity with very little effort. Jupiter is always a beautiful one to observe due to the positions of the moons and bands of colour, but would definitely agree that the moon is my fav 🙌
ben
yeah moon is my favourite as it easy to see and have many variations like supermoon red/blue moon etc. they look very cool
Tanmay
i also post images of saturn and Jupiter that i capture with my telescope.
Stardust
where?
Tanmay
do you post?
Tanmay
yeah, I also can't see the post
Pratiksha
my fav. Is Orion in night sky❤
Joker
my fav. is Orion In night sky
Joker
my fav. is Orion In night sky
Joker
i post photos of moons...of eartg Jupiter..and also saturn and mars if i get lucky
Stardust
stardust. you name is maverick in insta
Joker
yeah orion is the easiest to identify
Tanmay
my favorite is Saturn
John
hmm some real identity is being leaked X)
Tanmay
I just love looking at the night sky full of stars
Mahesh
yes night sky full of stars are also amazing thought it is rare now
Tanmay
Yes it's like a dream for some people to look up and observe the full potential of the night sky with their naked eye
Debabrata
hyy everyone I have a question that can we produce antigravity if yes how it works in space stations sucha nasa ,isro etc
Tahir
anti-gravity is the opposite of gravity and we are unable to detect or see it.They space station is accelerating so fast that they people there don't feel their weight because it's balanced by a opposing force caused by acceleration,those weight and opposing force cancel each other and that's why
Shivam
they feel weightlessness meaning no weight or our weight is=0
Shivam
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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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