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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

Currently, I'm studying in 10th grade. What should I do after my 10th so that I can get an astronomy career?
Chaya Reply
try jee exam and achieve a good score in jee advance to join in IIST(Space research institute)
okay.. thanks
this famous person was the first to recognise earthshine on the moin
Rajan Reply
Leonardo Da Vinci.....
what is string theory?
sakshi Reply
A Cosmological Theory Based On The Exsistence Of Cosmic Vibrating Strings.....
Does MIT have good astrophysics courses ?
priyanshu Reply
Probably The Best In The United States.....
But Don't Get Caught Up On Who's The Best, You Can Be The Next Noble Winning Astrophysicist.....
hmmm..........that's nice one....Adam had said.....
How did ASTRONOMY helped you understand yourself ? 🤔
Hussain Reply
why do you want to study ASTRONOMY?
Hussain Reply
It is one of the natural sciences and therefore worth exploring. You are a part of the universe and it is a part of you. The sky is my classroom. (student of cosmology, Oxford Uni)
because everything in the space is just beautiful...interesting and worth studying and exploring...
I have been studying astrophysics an love it
I love to learn what makes up our lives an heavens an how it works there us so much more out there than books an our veiw of thw heavens can reveal
How did ASTRONOMY helped you understand yourself ?
Because it proves the impossibles and the limitless.
It helped me realize that there are like countless possibilities in life and get all through it.
It honestly showed me to view the world in a balanced way. Because space beautiful and calm yet violent, and so is the world, and we should still help, but there will always be violent people much like violent astrophysical jets.
Does anyone know where can I study astronomy in Spain? My budget is too low for traveling out there...
Valerian Reply
How long the duration was when the laws of physics became active after the big bang?And why the time is called plank's time?
Mostak Reply
how many galaxies in universe?
Shaikh Reply
10 to the power 11
if u can count all the sands on the beaches and on the deserts then u will know how much it will be.......some said more than 1,000,000,000 galaxies are there in our universe...
it's ok bro
I hope you will become a member of nasa
just new in this area, from art background not science
what's ur name wwe
The Observable Universe Contains Between 200 Billion To 2 Trillion Galaxies.....
are u sure?
It actually hasn't been proven, people have just made estimations.
the thing is u can't count,maybe the our number system will collapse there...who knows?
What's the exact time when the laws of physics became active after the big bang ?
Actually we still don't know the exact number of galaxies in the universe, since the universe is bigger than we can imagine and it is still expanding even bigger today than yesterday
we didn't actually find the exact number but may be it is around 100 billion
why this universe is expanding?
Science Does Not Concern It Self With Why Questions, Science Is Geared For How And What Questions.....
Why Questions Form And Endless Loop.....
According to the universal law of cause and effect .any phenomenon that occur in this universe should have any reason , nothing happen without any reason.
Can you tell me the expanding speed of this universe.
yes that would be a great question to have answered what is the expanding speed of the universe?
Remember that most of what you can see is light years away. We're looking back in time, we can never know the current nature of our observations, only glimpse the galaxies and stars as they once were. Strange but true, and a little sad.
Correct, That's Why Telescopes Are Like Time Machines, You See The Past.....
really telescope can see the past?....i meant everything is in its place, though it rotates or revolve or whatever it is.......but how can this telescope can see the past.....can we see how our Universe was created,how it formed out of it? thats my question to all of you guys can you plz tell me.
Because We Dont See Any Object In Space As It Is, We See It As It Was..... Hope This Clears That Up For You.....
And One Day We Might Have A Telescope That Will Show Us The Big Bang.....
ok....... that will be nice one
ok....... that will be nice one
Travel at light speed in a spaceship at 186,000 miles every SECOND. At this speed it will take you 100,000 years just to cross our Milky Way galaxy, which is just a dot in the sky. Next stop, the Andromeda galaxy, after 2.5 million years, still in the Local Group, Virgo Supercluster, still a dot.
Pretty Much.....
there are nearly 200 million I suppose.
hi i need a telescope would you help mee
Nauman Reply
i can only advice you to go to some museum
wher is it?
but before that.......... i wanna know where are you from and from which state...and if you are nearer to that of the museum or will be able to go then.,.. i donot have any more words.
What Do Plan To Observe?
sorry . i am from asia
Check on amazon.Celeston telescopes are good for beginners.
can yoy introduce your self
can you
Asia...? from which country you belong into ...,,is it India or where?
Do You Want A Telescope With ALot Of Maintenance Or Zero Maintenance?
with alot offf
Any second hand online stores in your area? great for unused Christmas gifts etc..
So You Want A Newtonian Reflector?
hey guys I wanna ask you all that will it be safe or helpful to communicate with an intelligent civilization like are we safe ?
Is the Orion Sky Quest XT6 a good Dobsonian?
Look for a Sky Watcher 130BKS, it has an upgraded dual speed 2" focuser. You can use 1.25" or 2" eyepieces with it and it is quite easy to pollinate using a simple visual collimating eyepiece. This scope is great and ready for prime focus astrophotography if you choose to go that route.
Sorry, spell check error. Pollinate should be collimate. lol
Dobsonians Require A Lot Of Maintenance, If You Don't Mind Cleaning And Collimating On A Regular Basis, Then Go Ahead A Dobsonian Puts Up Excellent Views Especially For Deep Space Objects..... Now If You Don't Want Constant Maintenance Then A Schmidt Cassegrain Might Be A Better Fit For You.....
So what i would put for that answer ?
Abdulla Reply
The largest observatory complex in the world is on Mauna Kea, the tallest mountain on Earth. Whatare some factors astronomers consider when selecting an observatory site? Don’t forget practical ones.Should astronomers, for example, consider building an observatory on Denali (Mount McKinley) or MountEverest?
Abdulla Reply
i need help on the the second part
Should astronomers for example considered building an observatory on Denali (Mount Mckinley) or MountEverest ?
I would take into consideration. climate weather pattern.
also, could an establishment be built and be able to sustain years of being beaten by the weather?
so far Mount Everest to my understanding is a difficult mountain
it is because of light pollution in the cities that the observatories are situated in extremely remote areas.
Everest ist the tallest mountain
how far us milky way galaxy?
ShowsSpy Reply
oh man
We are part of the Milky Way Galaxy. We are within it, so it's not really away from us. Our galaxy is 100,000 light years in diameter. The nearest big galaxy is Andromeda, around 2 million lightyears away.
its 200,000 light years
Where is Rice University located?
Amanda Reply
it is in your vagina
that couldn't be any more wrong 😂😂
what even is this conversation😂😂!?
Show Some Class.....
Houston, Texas.....
what is the first step to take as far as getting into the field of astronomy?
john Reply
probably by being type 1or2 civilization...launching more telescopes into space...and stepping on mars. probably.
John, the first step is to want to focus a large part of your life studying, researching and investigating the universe... Including the planet we know the most about. Do well in math, physics, and chemistry, and attend a college that either owns astronomical equipment or facilities and has courses.
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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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