- 4327 - ASTEROIDS - how many come close? Last year, 2023, Earth had more than 100 close encounters with large asteroids. What are the odds of a direct hit in the near future?
-------------- 4327 - ASTEROIDS - how many come close?
- Asteroids are
chunks of rock left over from the formation of our Solar System. Approximately
half a billion asteroids with sizes greater than four meters in diameter orbit
the Sun, traveling through our Solar System at speeds up to about 30 kilometers
per second, about the same speed as the 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-meter object traveling at a relative
speed of up to 60 kilometres per second is going to pack a punch.
-
- 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. NEOs are
near-Earth objects, any small body in the Solar System whose orbit brings it
close to our planet.
-
- 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.
-
- 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-meter 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 big distance for objects a handful of meters 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,
300,000 km, 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.
-
- Consider really big
asteroids, bigger than one kilometer in diameter. The same highly simplified
logic as above can be applied. For every such impact that could threaten
civilization, 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.
-
- Approximately 95%
of asteroids of size greater than one kilometer 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
categorizes” 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).
-
- 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.
-
- “Centaurs” are
small planetary bodies that orbit between Jupiter and Neptune and have baffled
astronomers for sharing characteristics with both asteroids and comets.
-
- Centaurs got
their name after the mythical half-horse, half-human creatures called centaurs
due to their dual characteristics.
-
- James Webb Space
Telescope (JWST) has conducted a first-time detection of carbon dioxide in a
Centaur, this one designated “39P/Oterma”. A Centaur frequently crosses the
orbits of one or more of the gas giant planets within our solar system.
-
- Centaur has
been imaged up-close, they typically exhibit a combination of attributes
between comets and asteroids.
-
- While carbon
monoxide has been detected in two known centaurs, this recent discovery could
mark a turning point in how scientists understand the formation, evolution, and
composition of not only Centaurs, but of the early solar system, as well.
-
- Centaurs are
important to study since they are fairly well-preserved objects in space that
can provide insight on the chemical composition and physical processes of the
early solar system.
-
- “Centaur
39P/Oterma” was discovered on April 8, 1943.
While 39P/Oterma has long been classified as an inactive comet, it
currently exhibits a Centaur-like orbit between Jupiter and Saturn, meaning it
doesn’t approach the Sun, and has a radius of approximately 1.37 to 1.55 miles.
So, why was 39P/Oterma chosen for this specific study?
-
- “39P/Oterma is
what we call an active centaur, a centaur that develops a coma and a tail like
a normal comet. Since it is active, we
can use spectroscopy to observe molecules in their coma to gain insights into
their composition. 39P was chosen as one of our targets because it would be
active during the time of the proposed observations.
-
- JWST’s
Near-Infrared Spectrograph (NIRSpec) instrument was used and was supported by
ground-based observations from the Gemini North Observatory and Lowell
Discovery Telescope to investigate the characteristics of 39P/Oterma while it
was orbiting close to its perihelion (closest distance to the Sun) at 5.82
astronomical units (AU) from the Sun in July 2022.
-
- For context, the
perihelion of 39P/Oterma has gradually increased since its discovery: 3.39 AU
(1958), 5.47 AU (1983), and 5.71 AU (2023), and is projected to reach 5.91 AU
and 6.15 AU in 2042 and 2246.
-
- After analyzing
the JWST NIRSpec data, the researchers confirmed the first detection of carbon
dioxide in any Centaur, and the lowest amount of carbon dioxide ever detected
in any Centaur or comet. They also did not detect traces of water or carbon
monoxide, which are traditionally detected in Centaurs, specifically
29P/Schwassmann-Wachmann 1 (29P/SW1), another Centaur that shares approximately
the same AU distance as 39P/Oterma.
-
- And, though the
production rates are low, it shows a different chemical behavior from another
Centaur, 29P/SW1, seen at a similar distance (6 AU). This difference in
chemical behavior could be from the very different sizes of Centaurs 29P and
39P, or from having different orbital histories, or starting out with different
compositions, or possibly a combination of all of these.
-
- The finding of
carbon dioxide in a Centaur could be a game changer in terms of understanding
the compositions and characteristics of Centaurs, asteroids, and comets
throughout the solar system, along with potentially gaining better insight into
the formation and evolution of the solar system, overall.
-
- What new
discoveries about Centaurs will researchers make in the coming years and
decades and how many more Centaurs could possess carbon dioxide throughout the
solar system? Only time will tell, and this is why we science!
-
-
January 23, 2023
ASTEROIDS - how
many come close? 4327
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