THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland, Maine 04103
(207) 780-4249      usm.maine.edu/planet
43.6667° N    70.2667° W  Altitude:  10 feet below sea level Founded
January 1970
2021-2022: XLVI
"The more we study the more we discover our ignorance."
-Percy Bysshe Shelley


THE DAILY ASTRONOMER Tuesday, November 23, 2021
Asteroid Crash
or
Dart on Dimorphos

________________________
We're off on an undeserved Thanksgiving holiday
after today. We'll post a quiz on Friday and then return for what should
be* a full week of DA's starting Monday.
A very happy Thanksgiving to you all!
_________________________

The scenario is quite a familiar one: a menacing, 100-mile wide asteroid
careens inexorably toward the third world and the forecast models range
from damn to dire. An impact will not only annihilate all structures and
life forms within a 1000 square miles of ground zero, but the explosion
-more powerful than the combined detonation of humanity's entire nuclear
arsenal- promises to set the world afire and cast multiple megatons of
displaced debris into the atmosphere. The protracted obscuration of
sunlight will precipitate a five-year long "nuclear winter," freezing most
of the globe and condemning 95% of all species, including those smug
humans, to extinction. Of course, some creatures will persist and
eventually evolve into a highly advanced, predominant race of beings who
will thrive, prosper and cheerfully exploit Earth's abundant resources
secure in knowledge that the planet was made solely for their convenience,
amusement and gratification.

A sleep-disturbing terror indeed.
Although Hollywood has capitalized richly on this doomsday scenario, the
prospect of an asteroid impact is all too real. After all, Earth has
sustained myriad impacts over its nearly five billion year history, some of
which were so powerful as to precipitate a mass extinction. While the
asteroid population has diminished somewhat since the solar system's
infancy, millions of those outer space rocks continue to tumble through the
gulfs of interplanetary space. Some of them, the *Aten and Apollo*
*asteroids*, even cross Earth's orbit! Eventually, something will hit us
hard and Earth life will change forever.

Then again, unlike the helpless dinosaurs, we humans have developed the
sinister science of astronomy. Many of its practitioners have cataloged and
tracked thousands upon thousands of these asteroids and they are paying
particular attention to the potentially hazardous ones, dubbed *PHA's. *
While some of the smaller rocks continue to elude detection,
asteroid-watching astronomers are confident that no major impact is
imminent. Consequently, we're all sleeping quite soundly at our desks,
thank you very much. However, what would we do if astronomers detected an
asteroid that was destined to hit our planet?

That problem is the cardinal concern of NASA's Planetary Defense Team (yes,
there is such an organization.) Their aim is to devise ways to prevent any
destructive impact, from city-withering strikes to those comparable to the
assault that propelled the dinosaurs into the ages. What ways have they
devised?
Well, while pulverizing the errant projectile with nuclear weapons works
brilliantly on the silver screen, that solution is quite problematic in the
real world for a whole host of reasons, not least of which relates to the
thousands of chunks produced by the explosion. A more reasonable solution
involves diverting any incoming asteroid with a bit of a "push." The theory
is that such a perturbation could alter an asteroid's trajectory enough to
cause it to miss our planet entirely. We don't need to "blow the asteroid
up." Instead, we just need to swat it away and leave us alone.

Yet, is that even possible?
NASA scientists certainly hope so and this week they are launching a
spacecraft named DART (Double Asteroid Redirection Test) which is destined
to crash on the smaller asteroid within a binary asteroid system. Weighing
500 kg (1,100 lb), the golf-cart sized vessel is scheduled to crash
onto Dimorphos,
a "moon" of the asteroid Didymos.   Approximately 520 feet in diameter,
Dimorphos is about the size of the Great Pyramid in Giza.         Planetary
scientists involved in this mission estimate that the impact of DART onto
Dimorphos at 4.1 miles per second will induce an initial velocity change on
Dimorphos equalling 0.4 millmeters per second.    That meager speed
alteration should result in a significant path change over a long period of
time.  If the calculations are correct, the DART impact will alter
Dimorphos speed by half a millimeter per second, thereby changing its
orbital period by ten minutes.       In a few years after the impact, the
Hera spacecraft, still under construction, will visit the Didymos system to
verify the orbital alteration.

The DART craft will launch tomorrow and, after a six million mile
journey,should  crash onto Dimorphos on October 2, 2022.    NASA scientists
hasten to assure us all that this binary asteroid is no threat to Earth.
Moreover, the orbital path change induced by DART will not re-direct it on
a collision course with Earth (as funny as that would be.)       The only
aim is to test the asteroid redirection impact theory: to determine if the
calculations are correct.

                                  [image: DART_satellite_draco_view.png]
                                       *       DART*


We know that our planet might one day be imperiled by a large asteroid
capable of wrecking widespread destruction.     We also know that such an
asteroid will be much larger than the minuscule Dimorphos.   However, the
DART mission represents the next stage in the complex process of developing
an effective planetary defense system, one that might literally save the
world someday.

Our hopes and thoughts are with you, DART!

*Yes, I know...

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