Why it took 9.5 years to travel to Pluto
The Sunday Nation
Nairobi,
19 July 2015
This was an exciting
week for astronomers: the spacecraft “New Horizons” reached Pluto and
begun sending photos of the dwarf planet back to Earth. This project is
a test of endurance and patience.
At 4.7 billion
kilometers, Pluto is very, very far away. The journey there lasted nine
and a half years – from January 2006 to July 2015. You might be forgiven
to think that the spacecraft was moving too slowly but you would be
wrong.
Within half an hour
after launch, New Horizons accelerated to over 58,000km/h. This is the
highest launch speed ever recorded by a man-made object. At this rate,
it reached the moon’s orbit in just nine hours: compare that with the
three days that the Apollo missions took.
After one year of
travel (February 2007), New Horizons reached the vicinity of Jupiter,
the largest planet in the Solar System. There the spacecraft did a
“slingshot” maneuver using Jupiter’s immense gravity to boost the speed
to over 83,000km/h. From there, the spacecraft went into hibernation and
continued cruising at this high velocity for the remaining eight years.
Now, if it took 9.5
years to cover the 4.7 billion kilometers to Pluto, what was the average
speed of the spacecraft? 9.5 years is 3,468 days or 83,220 hours.
Therefore, the average speed is 4.7bn km divided by 83,220; that is
56,500km/h.
This is lower than
the cruising speeds of New Horizons (58,000km/h to Jupiter and 83,00km/h
afterward). The reason for the discrepancy is that the spacecraft did
not travel in a straight line trajectory.
Apart from the time
it took to travel to Pluto, the other test of patience comes from the
duration it takes to communicate with New Horizons. All signals are sent
by radio waves. These travel at the speed of light – 300,000km/second.
300,000km/s is
equivalent to about one billion kilometers per hour. So, it takes about
4.7hours for a signal sent from Earth to reach the spacecraft 4.7bn km
away. Upon receiving it, the spacecraft sends a confirmation reply that
also takes 4.7 hours to get here. In total, the signal round trip takes
almost 9.5 hours!
Cruising at
83,000km/h, New Horizons covers about 788,500km in 9.5 hours; that is,
over three-quarters of a million kilometers! So, in short, it is
impossible to know exactly where it is at any instant.
Now this was a fly-by
mission, meaning that the spacecraft did not land on or even get into
orbit around Pluto. The aim was to simply pass close enough to take
pictures and collect other scientific data without stopping. This lasted
just three days!
That sounds like
quite a waste: you go on a 9-year journey only to collect data for three
days? Well, that is how “expensive” scientific research is. By the way,
it will take another 16 months of continuous streaming to relay all the
data and pictures back to Earth!
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