Science Sunday 05-28-18

A Brief History of Flight – The Space Age – Part 2

Apollo 11

On July 20th of 1969, an event of epic proportions took place.  Two American astronauts, Neil Armstrong and Buzz Aldrin, landed their lunar excursion module (LEM) Eagle on the Moon, fulfilling the first part of President Kennedy’s challenge in 1961 that the United States “should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely.”

A third astronaut, Michael Collins stayed in orbit around the Moon in the command module Columbia, their return ticket to Earth.

It had taken them a little over 4 days to get to the Moon, leaving on July 16th.  Armstrong and Aldrin would stay on the Moon less than a day.  They would return to Earth in less than 3 days, successfully splashing down in the Pacific Ocean about 800 miles southwest of Hawaii, thus fulfilling the second part of President Kennedy’s challenge.

Even now, 48 years later, it is hard to conceive of the courage these three men would have to summon up, to sit on top of the (still) world’s largest rocket, the Saturn V, and blast off to the Moon.  They were still attached to the final stage of the rocket when it ignited, sending them into the translunar injection orbit (TLI) they needed to rendezvous with the Moon 4 days later.

When they first orbited the Moon, and were out of radio communication with Earth, the entire world held its collective breath.  Would the astronauts make it around the Moon and return to radio communication, or would they die on the far side of the Moon, which never presents itself visually to Earth?

When Armstrong and Aldrin landed on the Moon, in the Sea of Tranquility, between 500 and 600 million people on Earth watched from every corner of our planet.  It was a truly mesmerizing moment.

Although their stay was quite short, Armstrong and Aldrin planted the American flag there, set up a “Moon quake” seismograph, and installed a mirror to reflect laser beams from Earth so as to measure the Earth-Moon distance to within a quarter of a millimeter.

Both the landing and the blastoff (from the Moon) of the LEM were not without difficulty.  On landing, Armstrong could see that the computer driven LEM landing was going to miss its ideal landing spot, and place the craft in a rock strewn area.  He took semiautomatic control of the craft and landed it safely.

On departure, one of the switches needed for the LEM blastoff was broken, but the astronauts fixed the problem with a pen.  The blastoff knocked over the flag they have carefully positioned on the Moon.

Once docked with Columbia, the LEM was detached.  It would orbit the Moon on its own for a few additional months.

Columbia splashed down upside down, but inflatable balloons quickly righted the capsule.  The Apollo 11 astronauts had managed to bring back a little less than 50 pounds of lunar rocks.

After being quarantined for a good three weeks, the Apollo 11 astronauts returned to civilization with great acclaim, enjoying ticker tape parades in New York and Chicago.

As of this writing, both Aldrin and Collins are still alive, but Armstrong died in 2012.

There were seven Apollo missions, resulting in six successful landings.  Only Apollo 13 had to turn back because of damage to the command module Odyssey.

All in all, 12 men landed on the Moon, all returning successfully to Earth.  Four survive, with Apollo 12’s Alan Bean just recently passing.

Daffy About Pluto

_84270464_p_lorri_fullframe_colorWith the New Horizons Pluto Mission flying by Pluto this very day, there have been some questions in my class about the mission details, so I thought I would try to put them all here in one place.

The current speed of the mission probe is 31,000 mph, slightly slower than its speed as it left Earth, which was 36,000 mph, much greater than the escape speed of Earth, 25,000 mph.

The New Horizons probe’s speed makes it impossible to orbit the dwarf planet Pluto.  This was always the intention of the mission: to go slower would have meant a much longer mission time than was acceptable to the mission team.

The New Horizons probe will eventually leave the solar system (the heliosphere), but it will not “catch up” to Voyager 1, which, due to gravity assists during its mission lifetime, is now travelling at 38,500 mph.

Because of New Horizons speed leaving Earth, and because it never intended to be captured by the gravity of our Moon, New Horizons travel time to the Moon was a mere 9 hours, as opposed to Apollo 11’s time to the Moon of approximately 3 days (72 hours).

Yes, Apollo 11 achieved near escape velocity from Earth while in orbit around Earth, but it did not travel at that exact speed to the Moon, instead incrementally slowing down on its way to the Moon, due to the tug of Earth’s gravity;  When the Moon’s gravitational field became dominant for Apollo 11, the craft started to speed up again.

The slowest speed Apollo 11 had, relative to Earth was a “mere” 2,000 mph.  By the time Apollo 11 was in orbit around the Moon, it was travelling with a tangential (orbital) speed of 5,300 mph.

Using the average distance to the Moon of 239,000 miles, and a travel time of 72 hours, the average speed of Apollo 11’s journey to the Moon was 3,300 mph.  This looks to be about one tenth of the “average” speed of the New Horizons probe, something New Horizons’ principal scientist Alan Stern said on NPR’s “Science Friday” show this last Friday.

Pluto has a highly elliptical orbit, so its distance to the Sun varies from 3.67 billion miles to 4.58 billion miles.  Of greater interest here is the distance between New Horizons and the Earth at time of flyby.  That distance is 31.85 Astronomical Units.

An Astronomical Unit or AU is the average distance between the Earth and Sun, roughly 93 million miles, or 500 light seconds.  This means that the transmission time from the New Horizons probe to Earth (and vice versa) is 15,925 seconds, or roughly 4.5 hours.

Unfortunately, this does not mean that the first pictures from New Horizons will arrive on Earth 4.5 hours after flyby.  The probe will be very busy, as it only spends 3 minutes traversing the diameter of Pluto.  About 14 minutes later, it will pass by Pluto’s largest moon (of four), Charon, spending about the same time traversing its diameter.

Because of the distance between New Horizons and Earth, the data signal from New Horizons is very weak, and must be tracked by the Deep Space Network, consisting of three 70 meter dish antennas located in the US, Spain, and Australia.  Weak signals mean low data transmission rates.

The data transmission rate of the New Horizons probe is a mere 1 kilobit per second.  Compare this to a 1990’s modem which was 56 kilobits per second.  So the transmission of one picture from New Horizons will take 42 minutes to complete.

Since New Horizons will be busy during the flyby, we will not receive a first picture from it until Wednesday afternoon.  Because of the low data transmission rate, it will take 16 months to complete the transmission of all pictures and other data from New Horizons!

But don’t worry, the Pluto and Pluto moon pictures ultimately sent will be 100 times sharper than the image above.

Incidentally, the 46th anniversary of the Apollo 11 landing on the Moon is this Sunday.