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.



Author: Bob Mahoney

Physics teacher

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