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How Many Scary Asteroids Really Go Past Earth Every Year?

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a425couple

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Aug 9, 2023, 5:23:33 PM8/9/23
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https://www.iflscience.com/city-killers-and-half-giraffes-how-many-scary-asteroids-really-go-past-earth-every-year-70149

(Some of the graphics are quite useful, view the citation
to see them.)

"City Killers" And Half-Giraffes: How Many Scary Asteroids Really Go
Past Earth Every Year?
There are far more small asteroids than large asteroids, and small
asteroids cause much less damage than large asteroids.
guest author image
STEVEN TINGAY
Guest Author


The distance between the Apophis asteroid and Earth at the time of the
asteroid's closest approach.

Image Credit: NASA/JPL-Caltech

The ConversationAsteroids are chunks of rock left over from the
formation of our Solar System. Approximately half a billion asteroids
with sizes greater than four metres in diameter orbit the Sun,
travelling through our Solar System at speeds up to about 30 kilometres
per second – about the same speed as Earth.

Asteroids are certainly good at capturing the public imagination. This
follows many Hollywood movies imagining the destruction they could cause
if a big one hits Earth.

Almost every week we see online headlines describing asteroids the size
of a “bus”, “truck”, “vending machine”, “half the size of a giraffe”, or
indeed a whole giraffe. We have also had headlines warning of “city
killer”, “planet killer” and “God of Chaos” asteroids.

Of course, the threats asteroids pose are real. Famously, about 65
million years ago, life on Earth was brought to its knees by what was
likely the impact of a big asteroid, killing off most dinosaurs. Even a
four-metre object (half a giraffe, say) travelling at a relative speed
of up to 60 kilometres per second is going to pack a punch.

But beyond the media labels, what are the risks, by the numbers? How
many asteroids hit Earth and how many can we expect to zip past us?

What is the threat of a direct hit?
In terms of asteroids hitting Earth, and their impact, the graphic below
from NASA summarises the general risks.

There are far more small asteroids than large asteroids, and small
asteroids cause much less damage than large asteroids.

Asteroid statistics and the threats posed by asteroids of different sizes.
Asteroid statistics and the threats posed by asteroids of different
sizes. NEOs are near-Earth objects, any small body in the Solar System
whose orbit brings it close to our planet. Image Credit: NASA

(A quite useful chart is here - click on the citation to view it.)

So, Earth experiences frequent but low-impact collisions with small
asteroids, and rare but high-impact collisions with big asteroids. In
most cases, the smallest asteroids largely break up when they hit
Earth’s atmosphere, and don’t even make it down to the surface.

When a small asteroid (or meteoroid, an object smaller than an asteroid)
hits Earth’s atmosphere, it produces a spectacular “fireball” – a very
long-lasting and bright version of a shooting star, or meteor. If any
surviving bits of the object hit the ground, they are called meteorites.
Most of the object burns up in the atmosphere.

How many asteroids fly right past Earth?
A very simplified calculation gives you a sense for how many asteroids
you might expect to come close to our planet.

The numbers in the graphic above estimate how many asteroids could hit
Earth every year. Now, let’s take the case of four-metre asteroids. Once
per year, on average, a four-metre asteroid will intersect the surface
of Earth.

If you doubled that surface area, you’d get two per year. Earth’s radius
is 6,400km. A sphere with twice the surface area has a radius of
9,000km. So, approximately once per year, a four-metre asteroid will
come within 2,600km of the surface of Earth – the difference between
9,000km and 6,400km.

Double the surface area again and you could expect two per year within
6,400km of Earth’s surface, and so on. This tallies pretty well with
recent records of close approaches.

A few thousand kilometres is a pretty big distance for objects a handful
of metres in size, but most of the asteroids covered in the media are
passing at much, much larger distances.

Astronomers consider anything passing closer than the Moon –
approximately 300,000km – to be a “close approach”. “Close” for an
astronomer is not generally what a member of the public would call “close”.

In 2022 there were 126 close approaches, and in 2023 we’ve had 50 so far.

Now, consider really big asteroids, bigger than one kilometre in
diameter. The same highly simplified logic as above can be applied. For
every such impact that could threaten civilisation, occurring once every
half a million years or so, we could expect thousands of near misses
(closer than the Moon) in the same period of time.

Such an event will occur in 2029, when asteroid 153814 (2001 WN5) will
pass 248,700km from Earth.

How do we assess threats and what can we do about it?
Approximately 95% of asteroids of size greater than one kilometre are
estimated to have already been discovered, and the skies are constantly
being searched for the remaining 5%. When a new one is found,
astronomers take extensive observations to assess any threat to Earth.
The Torino Scale categorises predicted threats up to 100 years into the
future, the scale being from 0 (no hazard) to 10 (certain collision with
big object).

Currently, all known objects have a rating of zero. No known object to
date has had a rating above 4 (a close encounter, meriting attention by
astronomers).


So, rather than hearing about giraffes, vending machines, or trucks,
what we really want to know from the media is the rating an asteroid has
on the Torino Scale.

Finally, technology has advanced to the point we have a chance to do
something if we ever do face a big number on the Torino Scale. Recently,
the DART mission collided a spacecraft into an asteroid, changing its
trajectory. In the future, it is plausible that such an action, with
enough lead time, could help to protect Earth from collision.
Steven Tingay, John Curtin Distinguished Professor (Radio Astronomy),
Curtin University

This article is republished from The Conversation under a Creative
Commons license. Read the original article.

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Jim Wilkins

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Aug 9, 2023, 5:57:01 PM8/9/23
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Telescopes can detect only asteroids that are illuminated by sunlight on the
side facing us, meaning that any that approach Earth from the sun's
direction, on their way back out, may be invisible until they pass or hit.

danny burstein

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Aug 9, 2023, 6:19:20 PM8/9/23
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It's a bit trickier, but since these asteroids will also be
blocking out stars they pass in front of...

Keping track of that a century ago would have been
just about impossible, but I'd completely wild assed
guess that there are computerized algorithms and
telescopes doing this... (plenty, make that PLENTY,
of defence/military reasons for tracking "black"
satellites...)


--
_____________________________________________________
Knowledge may be power, but communications is the key
dan...@panix.com
[to foil spammers, my address has been double rot-13 encoded]

Jim Wilkins

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Aug 9, 2023, 8:01:13 PM8/9/23
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"danny burstein" wrote in message news:ub13d5$493$1...@reader2.panix.com...

In <ub123b$326h$1...@dont-email.me> "Jim Wilkins" <murat...@gmail.com>
writes:

>Telescopes can detect only asteroids that are illuminated by sunlight on
>the
>side facing us, meaning that any that approach Earth from the sun's
>direction, on their way back out, may be invisible until they pass or hit.

It's a bit trickier, but since these asteroids will also be
blocking out stars they pass in front of...

Keping track of that a century ago would have been
just about impossible, but I'd completely wild assed
guess that there are computerized algorithms and
telescopes doing this... (plenty, make that PLENTY,
of defence/military reasons for tracking "black"
satellites...)
---------------------------------
https://www.livescience.com/space/asteroids/a-skyscraper-size-asteroid-flew-closer-to-earth-than-the-moon-and-scientists-didnt-notice-until-2-days-later

https://www.esa.int/Space_Safety/Planetary_Defence/NEOMIR_finding_risky_asteroids_outshone_by_Sun

Kym Horsell

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Aug 10, 2023, 1:11:39 PM8/10/23
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You should never say never or "only". :)

I've been using images from the TESS space telescope to look for objects.
TESS orbits in cislunar space so gets to see something of the volume between e & m
as well as various directions out into the solar
system and beyond.

The images have 24 bits of grey scale and if you
use ML software to twiddle the mappings you
can easily see things moving around out there.
I've made a series of movies of 100+ hrs of
looking in different directions.

The images are kinda unweildy for most image processing
s/w so I've only seriously looked at 8
"sectors" as the TESS people call them
(approx "positions of the telescope in the past 10 years").

Dialing up the contrast to 111 you can see "tons" of stuff moving out there.
Some sequences look like ant colonies in motion with stuff drifting around in all dirs.

The most spectacular are things that move in front of distant stars. Many sequences show objects apparently movine across 2.5 deg (approx width of a single sensor on the TESS) over
a period of 6-24 hrs.

The math suggests these can't be too distant or they would be solar escape vel.

Either they are rocks or -- more interestingly -- something else. :)

From the very prelim survey it seems like at any single time around 500-1000 of these things are moving around
out there and potentially visible in TESS images.

You can work up tables of size versus distance to explain the changes in brightness of stars the "rocks" move
in front of. They are mostly bigger than a breadbox.

I havent entirely cracked the code of how to translate coordinates inside frames with precise positions in the sky.
The images (FITS images) come with headers
that helpfully tell you what RA the telescope as a whole
is pointing at and what the "nominal rool"
of the camera is. But the slew of parameters
to translate individual pixels to RA and Decl dont seem to work out properyl.
So it's hard to tell whether these things are moving
with or against the general rotation of objects in the solar system.
But I can see at least a couple of these sequences
show diagonal movement as well as left to right and right to left movement.
So at least omse of the "rocks" are not doing
what you'd expect a dumb asteroid to be doing.
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