Most of them are seen orbiting the
Sun staying within the main asteroid belt that lies between Mars and Jupiter. Their
size varies from 329 miles in diameter to 10 meters. Their combined total mass
is estimated to be less than Earth’s Moon.
They are mostly irregular in
shape. They revolve around Sun in elliptical orbits. They also rotate,
sometimes quite erratically, tumbling as they rotate. Around 150 asteroids are
known to have companion moons. There are also binary asteroids i.e., two rocky
bodies of roughly equal size orbit each other.
They are broadly grouped into
three categories:
- C-type (chondrite) asteroids – they are perhaps the oldest objects in the solar system. Known to mostly consist of clay and silicate rocks;
- S-types (Stony) – made up of silicate materials and nickel-iron; and
- M-types (metallic) –mostly contain nickel-iron.
The orbits of asteroids can be changed
by the massive gravity of Jupiter. Even their close encounter with Mars or
other objects can change their orbits. Such encounters can even knock asteroids
out of asteroid belt and hurl them into space in all directions across the
orbits of the other planets. Such stray asteroids or their fragments were known
in the past to slam into Earth and even other planets. Such hits played major
role in altering the geological history of the planets and the evolution of
life on Earth.
Scientists are continuously
monitoring the Earth-crossing asteroids whose paths intersect Earth’s orbit.
They also monitor the near-Earth asteroids that are within 28 million miles of
the Earth’s orbit, for they pose an impact danger.
The impact of asteroids on Earth
is dependent on its size, speed and angle at which it hits the Earth, and their
composition. If the asteroid hits on land, there would be a huge amount of dust
thrown up into the atmosphere. If it hits a water body, there would be an increase
in water vapour in the atmosphere resulting in rain and as a consequence, there
could be landslides and mudslides. There might even be hurricanes and tsunamis
due to increased kinetic energy. Hence, scientists constantly monitor them through
radars and trace the asteroid’s orbit, rotation, size, shape, and metal
concentration.
As a part of this monitoring
and defense system against an Earth-threatening asteroid, if any, in the
future, NASA has undertaken a mission to test check if intentionally crashing a
spacecraft into an asteroid is an effective way to change its course. This
planetary defense demonstration mission is named DART (Double Asteroid
Redirection Test) and is to be managed by NASA’s Planetary Defense Coordination
Office (PDCO).
To test check its capability,
DART has chosen the binary asteroid system, Didymos as its target. In Greek,
Didymos means ‘twin’, which explains the word, ‘double’ in the mission’s name. This
Didymos system consists of two asteroids: the larger asteroid called, Didymos
with a diameter of 780 meters and the smaller moonlet asteroid named Dimorphos
with a diameter of 160 meters. Dimorphos orbits the larger asteroid, Didymos.
These two are at a distance of 1.18 km from each other. It takes 11 hours and
55 minutes for Dimorphos to orbit around the Didymos.
Didymos is considered as the
most ideal candidate for the planetary defense experiment because as per the
current predictions it is not on a path to collide with the Earth and therefore
poses no actual threat to our planet. Secondly, it is considered easier to
detect the deflection, if any, caused by the ‘Kinetic impact’ caused by
crashing a spacecraft into it, in this local orbit of Dimorphos than a change
of orbit of an asteroid orbiting the Sun.
Accordingly, NASA launched DART spacecraft in November 2021. After traveling 11 mn km from Earth, the 600 kg DART spacecraft slammed into the smaller asteroid, Dimorphos in the early hours of Tuesday, 27 September, 2022. After the blast, Professor Alan Fitzsimmons, an astrophysicist at Queen’s University Belfast said: “What amazed and delighted me was that everything worked so well”.
The collision and its impact is
of course, yet to be assessed, for it takes longer time for the Licia Cube—the
small satellite with a camera that had accompanied DART but separated from it
15 days before the collision—to transmit the images to Earth for analysis.
Nevertheless, Lori Glaze, NASA planetary science director had this to say: “We
are now embarking on a new era for humankind—an era in which we potentially
have the ability to protect ourselves from a hazardous asteroid impact”.
On October 11, 2022, NASA
announced that the mission had successfully changed the orbit of the asteroid
Dimorphos. Using the data transmitted by the telescopes taken at the time of
impact as well as in the subsequent weeks, NASA scientists calculated that the
impact shortened Dimorphos’ orbit by 32 minutes.
However, to rightly understand
the effect of collision, further analysis of the ‘ejecta’—the many tons of
asteroidal rock displaced and launched into space by the impact—is needed.
Nevertheless, as Nancy Chabot, the DART coordination lead from the Johns
Hopkins Applied Physics Laboratory in Laurel, Maryland said, “DART has given us
some fascinating data about both asteroid properties and the effectiveness of a 'kinetic impactor' as a planetary defense technology”.
But the irony is: We could build-up
ability to protect ourselves from a hazardous impact from a celestial body but
could hardly protect ourselves from an incoming man-made missile …. ...
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