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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

what hubbel's law. and examples on it.?
Kavyanjali Reply
Evil space aliens drop you and your fellow astronomy student 1 km apart out in space, very far from any star or planet. Discuss the effects of gravity on each of you.
Shell Reply
are you doing a test?
donot
what is quantum physics
sanjeev Reply
Quantum mechanic or physics is a branch of physics which deals with the dynamics of smallest things of all like atoms subatomic particles and elementary particles.
Ronaldo
Correct.....
Adam
Yes
ashish
what is circumstellar matter
sakshi Reply
where the Pluto has been gone
sanjeev Reply
pluto hasnt gone,its stiĺl on kuiper belt with others
1412
it have not gone anywhere it is still present on it's position in kuipler belt
YOG
Pluto is present at its old postion in solar system.But now it is not in a list of planet because it is too much small in size so it is called as 'Dwarf Planet '.Now it is present on Kupier belt.
mrunal
does anyone know the smallest thing in the whole universe I know I'm taking a general test
BHUMI Reply
Quarks... Think so.. 🤔
Pavan
higgs Bo particle .. think so.
sakshi
higgs boson particle
sakshi
A.) Your brain...
Andy
i guess andy should go and check his brain
Amritansh
can anybody think that what is present before big bang?
YOG Reply
I Don't Think Time Existed Before The Big Bang, So Talking About Anything Before The Big Bang Is Useless.....
Adam
but we have to think that what is present before big bang by which this whole universe is formed
YOG
because something can't create from nothing
YOG
it is thought by the scientists that 2 parallel universe collided with each other and got mixed in a point which exploded and formed the big bang It is thought that this process of big bang is continuous which means it will be repeated after a certain time interval
BHUMI
2 parallel universe collided with each other and got mixed in a point which exploded and formed the big bang, it is thought that this process of big bang is continuous which means it will be repeated after a certain time interval
BHUMI
before the big bang 2 parallel universe collided with each other
BHUMI
We Don't Know For Sure If That's The Case, We Need To Establish What Is Nothing?
Adam
One big crunch before big bang..just like now.. Our universe will end with big crunch or by expand.. More expand.. Much more since planets get separated 🖖 from its own core... I don't know more.. 😉
Pavan
how those two parallel universes came into existence
sakshi
no one knows what came before. The theory exists only because the numbers for expansion (calculated via redshift, measuring the CMB, etc) add up.
Jacie
We used to think that there were 3 possible endings to the universe - expanding , oscillating (cycle of crunch, then expand, the crunch again), and the steady-state theory (multiple places where matter enters the universe.
Jacie
However, the expanding model is the only valid one left. Dark energy and dark matter are speeding the process of expansion up exponentially. the process had been slowing down up till now. this means that there's no going back (or big crunch) and disproves the oscillating universe theory
Jacie
👍👍
Ashish
we also measured the density of objects and that is decreasing, meaning that all space objects (predominantly referring to galaxies) are moving farther away from each other. this means that the universe is not staying the same and disproves the steady-state theory.
Jacie
Oh yeah, and the steady-state theory stated that the universe has stayed the same and will not change Fun fact: the steady-state theory was first disproved when we discovered a quasar from really far away (meaning that the light reaching us was old and we saw into the past). It was so different
Jacie
and therefore disproved the theory
Jacie
Well Said.....
Adam
As for before the big bang, I find the application of entropy to a multiverse an interesting hypothesis. From the Law of Entropy, systems, left alone, will devolve to their least organized state. In the multiverse hypothesis, universes are popping in and out of existence all the time.
Damon
Time is relative, just frequency of events relative to other events. With no events, there is no time. If the big bang was an event that happened, then there must have been time before the big bang and something must have caused it to happen.
Toby
That's Pre Bang Physics, Strictly Speculation At This Point.....
Adam
Time is relative and critical to this question. As we don't really know what went before, we will need to make some assumptions. Before the big bang, I believe that there was dinner and a movie.....
Andy
before Big Bang there were matters and anti matters so this is a Theory of matters and anti matters it is believed that there was a battle between matter and antimatter in which matter won and that is certainly the reason that why we study States of matter all forms of matter
Amritansh
but if antimatter would have won what would be the nature of universe
sakshi
If Anti Matter Would've Won, The Universe Would've Look Exactly The Same As It Does Now.....
Adam
bc antimatter did win
Jacie
very slightly tho
Jacie
we can actually explain how universe came into existence by the theory of Quantum gravity. matter won over Anti matter because of neutrino there is present research going on neutrino or sterile neutrino it could be the answer I think once we find the cnu background radiation or cosmic neutrino bac
Ronaldo
can we make quantum universe by quantum level bigg bang explosion.
Prabhakar Reply
what do u mean by quantum universe?
rutuja
see quantum mechanics is a theory like we use general relativity for the study and characteristics of big objects in the same way we use quantum mechanics for the small objects study. This is called quantum mechanics and the object or thing in the universe on which we apply it is quantum universe
BHUMI
string theory is more appropriate for universe
YOG
pls explain about her hoags galaxy
sakshi Reply
Hoags Galaxy Is A Non Typical Galaxy, Of A Type Known As Ring Galaxy.....
Adam
o wow
Rohit
what are theories for the formation of asteroids. kindly someone tell
shamaun
what is nebula
sakshi
its a stellar nursury
Ashish
nebulae is a cloud of dust and various other gases . All the stars are born from nebulae and several stars die and form nebulae for eg, cats eye nebulae or Orion nebulae
BHUMI
how does it far between moon and earth?
Nyan Reply
About 240,000 Miles From Earth Or 380,000 Kilometers.....
Adam
The distance is about 3,84,400 km.
mrunal
do anyone know about the new celestial phenomenon called STEVE
sakshi Reply
I don't believe i have. Can you elaborate?
josh
No, never heard. Thank you for the information!
donot
scientists have observed mysterious lights in sky , they call them STEVE . They assume that it has to do with electrons and protons from a region around the earth called magnetosphere
sakshi
ohooo
Mad
explain about space weather pls?
sakshi
Atmospheric Optical Phenomenon.....
Adam
?
sakshi
Hot Gas In The Sky..... Strong Thermal Emission Velocity Enhancement Or STEVE Also Called "Sky Glow".....
Adam
pls tell me briefly about multiuniverse theory
sakshi Reply
we doesn't know what is beyond this world or how is space time actually..so there are many ideas put forward by scientists. 1.infinite universe 2.bubble universes 3.daughter universes 4.mathematical universes 5.parallel universes these are just different concepts ..
Mad
No Evidence Or Data Exist For Multiverse Hypothesis, That's Why They're About 9 Different Interpretations Of It.....
Adam
Theories are formed on imaginations and evidences which can be theoritical and logical. yes there are evidences for Multiverse theory. thsts why many Scientists bekieve in it.
Udit
yeah
Mad
evidence?
Mad
Then Why Are They Called Hypothesis?
Adam
according to the super position law everything have equal chance that it can be present but we can't see every state of it so it is possible that multiverse are present but we can only see one universe but it is true that our universe have many copies if it called multiverse
YOG
Can we done biggbang explosion at quantam level
Prabhakar Reply
Is bigbang really exist?
donot
that's just a concept na?
Mad
i mean that's the thing we believe had the beginning..but no solid proofs
Mad
big bang is just a theory or idea of some scientists no body precisely knows how it occurred it may be correct or wrong...... but it is now just an idea not any experiment is conducted to support it....
Zunain
yeah yeah
Mad
The Big Bang Theory Is The Best Explanation We Have For The Early Stages Of The Universe, All Our Data Supports The Big Bang Theory, And The Hypothesis Was Proposed By George Lemaitre A Belgian Catholic Priest And Astronomer, It Did Not Become A Theory Until Edwin Hubble The Astronomer Observed The
Adam
Expansion Of The Universe In 1929, Hypothesis Is An Educated Guess, Theory Has Data And Experiments To Back Up It's Claim..... So Any Time You See The Word Theory, That Means Their Is Evidence To Support The Theory.....
Adam
ooh
Mad
thanks martinez it really help us
1412
your explanation
1412
You're Very Welcome.....
Adam
A Couple Of Examples Would Be, Germ Theory, Atomic Theory, Theory Of Relativity, Theory Of Evolution And Plate Tectonics Theory And Quantum Theory And Cell Theory Are All Backed By Data And Evidence.....
Adam
oohoo
Mad
The real question is who programned the big bang, why are the laws of physics perfectly balanced to allow existence of matter. Is our universe just 1 possible universe in an infinite multiverse, or is it the only universe, but with deliberate nature?
Toby
god made our unirverse
1412
If That's What You Believe, That's Fine And Dandy..... But Proving A God Exist Or Doesn't Exist Is Impossible, Many Great Scientist Believed In A Personal Creator, But Not Even One Was Able To Prove The God Hypothesis..... Me Personally, I Believe That Beliefs Are Way Overrated, Even Mine.....
Adam
well you have the name of the first human being.
donot
and you say it's overrated. ofcourse scientist can never prove the God hypothesis. It's God made, non-human made.
donot
you all know about blackhole right? It's first said in the Holy Quran that given to Prophet Muhammad SAW. and that was on at-Takwir: 15-16, you can search it at google. and "black hole" is called Khunnas.
donot
I am not trying to make you all moeslem. But, I can't leave you while you denying God. Allah SWT. and many top-rated astronomers were change they religion to Islam.
donot
You Say We Can't Prove God, But In Your Next Sentence You're Saying Your Holy Book Proves Just That..... If A Creator Does Exist It's Of No Use To Us..... I Will Say This Before Islam Took Over That Region Of The World, They Were The Leaders Of The World When It Came To Math And Science.....
Adam
@Donot texteme relax im a muslim and astronomer too (well still on college)
1412
@1412 can you teach me... I want to particpate in National Astronomy Olimpiad next spring. btw Im Indonesian
donot
@adam what happen before you alive? what makes you, you? what happen if you dead?
donot
@Adam I don't want to argue, believe me. My heart, hurt painfully seeing you deny God. I pray for you, so you can believe the One and Only God. ALLAH SWT. aamiin.
donot
chill out guys. it's not a compitition. it's growing together. do not force upon others.
Brandon
I just want to clarify that I'm a native american. it's a humanity thing to help each other into a better understanding.
Brandon
Yes, you are absolutely correct @Brandon
donot
How I believe in Astronaut predicting my future by my natal date? Does it really work?
Dennis
no. absolutely. the future is God secret
donot
Have you ever experienced God works in your life? And how should I start to believe in God works?
Dennis
look around you... everything is God work. Including you and me and everyone.
donot
I have experienced it. It is me, I'm one of God works the smallest one. and by me, I mean human.
donot
No Arguing, Just Dialogue, If You're Going To Deal With Beliefs, Be Ready To Defend Said Beliefs, No Idea Is Safe From Scrutiny..... This Is Science Not Religion.....
Adam
well @Adam religion and science are connected.
donot
How Are They Connected?
Adam
Could the big bang have been programned by a sophisticated and advanced intelligence that emerged from a previous universe at the end of its lifespan, to create a new universe with the correct laws to allow physical reality to continue and intelligence to emerge again in a never ending cycle?
Toby
In An Uncertain Universe, Anything Is Possible, But For Some Superior Intelligence To Create The Universe We Find Ourselves In, It Would Have To Be More Complex Than It's Creation, And Who Created That Superior Intelligence?
Adam
i would say the cycle goes on beyond the superior intelligence
Tankia
thats the reason theories are possible in science because there are so many vast possibilities
Tankia
Science without religion is lame,religion without science is blind." So said Albert Einstein
donot
Complexity emerges from a simple code. Intelligence is an emergent property of the universe and the basic laws that govern it, and evolves over time.
Toby
I Don't Agree With That, Science Does Not Need Religion To Be Exciting..... But A Lot Of Amazing Scientist Believed In A Personal God.....
Adam
Well Said.....
Adam
I also don't agree. Some amazing scientist didn't even believe in a religion. For example, Stephen Hawking, he instead took his own beliefs and made them theories into his own. Some people think God is what started this world but what if it was something else?
Роза
A person in Boulder Colorado observes the planet Jupiter on the Eastern horizon right after sunset. Six hours later where will Jupiter be located?
Tig Reply
would the answer be high in the south
Tig
does all the multiverses should same particles I mean same feilds
Nanda Reply
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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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