"This royal throne of kings, this sceptred isle,
This earth of majesty, this seat of Mars,
This other Eden, demi-paradise,
This fortress built by Nature for herself
Against infection and the hand of war,
This this little world,
This precious stone set in the silver sea,
Which serves it in the office of a wall
Or as a moat defensive to a house,
Against the envy of less happier lands,--
This blessed plot, this earth, this realm, this
SOUTHWORTH PLANETARIUM"
207-780-4249       www.usm.maine.edu/planet
70 Falmouth Street  Portland, Maine 04103
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Founded January 1970


THE DAILY ASTRONOMER
Monday, May 23, 2016
Apollo Pandora

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We've added a new -and small- feature to the DA
Find it after the footnotes.
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When assailed by the tumults of this unquiet Earth, one often seeks to restore a sense of normalcy to one's life.    In this vein, we return today to Pandora's Jar: the vessel stuffed to the brim with astronomical questions inscribed on parchment paper.  What with all our adventures of late, we haven't had an opportunity to visit Pandora within the last week.        Sometimes, we select a question at random, as we prefer to let the Universe choose for us. (In other words, we're just feeling too darn lazy to examine the parchment pieces individually.)  Other times, such as now, we organize them by topic.      So, today, our two Pandora parchments pertain to what astronauts left on the moon, hence the title "Apollo Pandora."


"I have always been fascinated with the Apollo moon mission.  I've often wondered if can we see the Lunar rover with a telescope.  Can we?"
           -Seth P,  South Portland

Greetings!
Unfortunately, not even the most powerful telescopes will allow us to see such small objects on the moon as the Lunar Roving Vehicle.   The LRV is still parked on the surface, as shown by the LRO (Lunar Reconnaissance Orbiter) below.  However, the problem is in the resolution.  This vehicle cannot be distinguished against the moon's surface.      Remember that we are nearly a quarter of a million miles from the moon!   Simply,  we are too far away.

Now, to understand why distance matters, we will include some numbers.  First, a telescope's resolution depends on its effective diameter (or the diameter of its lens, a value known as "aperture.").   We can approximate a telescope's resolution power with the formula  R = 11.6/D, where D is the diameter in cm and R is the resolution in arc-seconds.*        The Hubble Space Telescope uses an objective mirrors with a diameter of 240 cm, so its resolution power is 0.05 arc-seconds.   By this calculation, we have determined that the Hubble Space Telescope can only resolve objects with resolutions greater than 0.05 arc-seconds.**   
We know that the lunar rover is about 4 meters across and its angular measure from Earth's distance is a vanishingly small  0.002 arc-seconds!  
Not even the Hubble Space Telescope can observe the lunar rover.

We know it is still there, however, even if it is out of our sight.


​This 2011 image of the Apollo 17 landing site captured by NASA's  Lunar Reconnaissance Orbiter.
The LRV, Lunar Roving Vehicle, is seen at the right hand edge of the
image.     As this orbiter was far closer to the moon than Earth-observers, it was
able to resolve the LRV against the lunar surface. (Image courtesy of NASA)



"Is it true that the American flag left on the moon is now white?"
-Miriam S,  Casco

Yes!
The red, white and blue is now, well, just white.
When the astronauts planted the stars and stripes onto the lunar surface, they did so with the knowledge that they were exposing old glory an onslaught of unfiltered solar radiation.  While Earth is shielded by an atmosphere, the moon is not.    The harsh star light strikes the moon without being softened by any intervening gases.   Consequently, it has completely bleached the American flag!  Were we to visit the old landing sites today, we would see still tracks from the lunar rover, astronaut foot prints that look as though the astronauts left five minutes ago, and stark white flags.       It looks as though the astronauts came to conquer the moon, thought better of it upon arrival and then left a flag of surrender behind.    And that explains, of course, why they've never returned.


Apollo 17 astronaut Eugene Cernan, the last human to walk on the moon,
poses with an American flag.  Even though the flag's colors are distinctive in this
photo, the flag is now most assuredly white, as the Sun's harsh radiation has
completely bleached it.   (Image courtesy of NASA.)
​


*There's always room for arc-seconds.   A circle contains 360 degrees.  A degree can be sub-divided into 60 arc-minutes.  An arc-minute can, itself, be divided into sixty equal parts called "arc-seconds."     To give one an intuitive notion about arc-seconds, an arc-second is equal to the width of a dime at a distance of 1.25 miles!   

**Life can be a mess.   This is a theoretical lower limit.  Generally, due to complicating factors, the Hubble's actual limiting resolution is about twice its lowest limit, or about 0.1 arc-second.


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FROM THE CATACOMBS OF INFINITE KNOWLEDGE:
Abe Silverstein, Director of Space Flight Development, proposed the name "Apollo"
for NASA's moon program.  NASA approved the name in 1960 and the rest is history.
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