Again, we'll miss the myth today.
We promise not to make a habit of this...
THE SOUTHWORTH PLANETARIUM
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Altitude:  10 feet below sea level
Founded January 1970
Julian Date:  2459264.18
2020-2021:  XCIII


THE DAILY ASTRONOMER
Thursday, February 18, 2021
Exploratorium XXVI:  Touchdown Terror
______________________________________________
See the Perseverance Mars Rover landing live this afternoon:
https://mars.nasa.gov/mars2020/timeline/landing/watch-online/
______________________________________________________


*Location*
           Mars

*Time*
           This afternoon

Imagine the following scenario.   You -yes, you- drive to the store to buy
some milk.  Not that tricky an issue, is it?  Well, imagine that you had to
program the car to navigate its way to the store. Assume that you had to
ensure that it was able to  maneuver safely through the parking lot. The
car would then deploy a rover that would enter the store, locate the milk
and then tender the proper amount to the cashier prior to departing.    The
rover would then return to the car which would then drive back  to your
home and present you with the desired item.   During the time period
between the car's deployment and return, you would be powerless to
intervene, even to make any necessary last minute course corrections.
 You'd have to sit back, bite what remains of your nails and, even though
you're a scientific type, pray that all goes according to plan.       This
fanciful scenario -which  might become a common method of milk acquisition
in the near future- gives us an appreciation of what NASA scientists will
experience later today when the Mars Perseverance Rover sets down on the
Martian surface.

Launched on July 30, 2020, the Mars2020 mission intends to deliver the Mars
Perseverance Rover around Jezero crater, an ancient lakebed where water
might have freely flowed about 3.5 billion years ago.    One of the mission
aims is to detect traces of primordial life within the dessicated Martian
soil.    Before it can begin its work, however, the rover has to land
safely: no easy task.
[image: perseverance-edl-metric-980x578.png]

Our only focus today is what will hopefully be a safe landing: the seven
terrifying minutes when the craft has to go from a 12,500 mph flight to a
gentle touchdown on the planet surface.   The inability of engineers to
intervene during this descent engenders that terror in them all: if
anything goes awry, the vessel could be rendered inoperable and all the
efforts and resources invested in its design, construction and deployment
will be for naught.

But, bah humbug, it will all work out splendidly.
Here is what shall transpire this afternoon.


*3:38 p.m.  Eastern Time*
The spacecraft will encounter the tenuous Martian atmosphere at speeds
exceeding 12,500 miles per hour.     The vessel will be employing its
thrusters to help maneuver it towards its intended landing spot.   Also  it
will jettison the shell containing its fuel and sensors.

*3:40 p.m.*
After the vessel again employs its thrusters to halt its rotation, it will
deploy two 70-kg balancing weights to adjust its lift-to-drag ratio.    The
descent speed will approximately equal 10,500 miles per hour.

* 3:47 - 3:48 p.m.*
The vessel penetrates the lower part of the atmosphere, which will generate
tremendous amounts of heat energy, although less than that which a
descending probe would experience in Earth's atmosphere.   At this moment,
the craft is approximately 400 miles from its target.

*3:52 p.m.*
Speed reduced to approximately 1,068 miles per hour.   Just prior to this
time, the vessel will have used its thrusters to adjust its alignment.
The spacecraft also contains an entry system designed to make final
corrections to its alignment.  It will also eject six more balance weights
around this time.

*3:52 - 3:53 p.m.*
The parachute deployment phase.   As the Martian atmosphere is much more
tenuous than Earth's, the parachute will not impede the descent as much as
it would here.    The speed will still just slightly exceed 160 miles per
hour.

*3:53 p.m.*
The heat shield, which protected the craft, will now separate from the
vessel and will expose the Perseverance probe  that will record the
remaining descent phase.
Terrain navigation will also commence. The craft has been programmed to
seek out viable landing sites so as not to crash land on any boulders or
other protruding objects.

*3:54 p.m.*
The engine will engage that will ignite the retro-thrusters to reduce its
speed to 2 miles per hour.

*3:54 p.m.*
The rover's descent engines engage detach and start their flight over the
Martian terrain

*3:55 p.m.*
The rover drops from the descent stage and is lowered using cables along a
sky crane.

*3:55 p.m.*
The rover deploys its wheels and touches softly down on the terrain while
the sky crane crash lands away from the rover's landing site.


Assuming that all goes well, the probe will then begin to study the Martian
terrain and perhaps learn more about possible Martian life.   First things
first, though.    Let's just get through this afternoon.


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