A few billion kilometers from us is a small 478-kilogram probe hurtling through the Kuiper
Belt at over fourteen kilometers per second.
And on July 14th, 2015 it passed within 12 thousand 6 hundred kilometers of the surface
of everyone’s favorite dwarf planet, Pluto, just 9 years, five months, and 25 days after
launch.
Which means: It got there pretty darn fast.
Three, two, one, we have ignition, and lift off of NASA’s New Horizons spacecraft…
Riding on an Atlas V rocket, New Horizons’ January 2006 launch holds the record for the
highest speed launch from Earth, but that still wasn’t fast enough to get the probe
out to Pluto before Pluto’s atmosphere condenses and freezes, as the little guy moves further
and further away from the sun.
So in February of 2007, the probe whizzed by the planet Jupiter for a little bit of
a speed boost.
Now, I don’t know if you know this, but Jupiter is not a rocket.
So how did the planet give New Horizons the extra 3.8 gigajoules of energy the probe needed
to get up to its new speed?
Gravity & momentum.
Specifically, a maneuver called a gravity assist.
But Tech!
If you fly close to an object and then fly away from it, the additional speed gained
from gravitational attraction on the approach is lost whenever you leave.
Absolutely right!
At least, from Jupiter’s point of view.
Jupiter is big, about 318 times more massive than the Earth.
And like all things with mass, it exerts a gravitational pull on objects nearby.
As New Horizons approached Jupiter, it sped up because of the forward pull of Jupiter’s
gravity.
Then, as it left, it slowed back down at the same rate, because of the backwards pull from
Jupiter’s gravity.
This balance meant that the net change in speed from gravity alone was zero, though
there was a change in direction, and therefore a change velocity.
And that would have been all that happened, if Jupiter was the sun.
Everything inside of the solar system, including the Earth, Jupiter, Pluto, and
New Horizons, is being pulled along by the sun at a speed of 230 kilometers per second,
as the sun orbits the galactic center.
But from our limited point of view, the sun appears to generally be stationary, with the
planets and other objects moving around it.
We call this the sun’s inertial reference frame, and it’s common for everything in
the solar system.
Which means that if we were to try a gravity assist maneuver around the sun, we would get
exactly the results that we had previously predicted, which is, nothing would happen.
But, when we perform the maneuver around another object in the solar system, like say Jupiter,
we’re able to change our speed, and not just our velocity.
Jupiter is traveling around its orbit at a speed of 47,051 kilometers per hour, or about
13 kilometers per second.
And because of that, when we fly a probe like New Horizon’s nearby, we’re able to steal
some of Jupiter’s momentum.
The effect of gravity on its approach and retreat is going to be the same, adding up
to zero, but because Jupiter isn’t sitting still, it’s also going to pull the probe
in the direction of the planet’s motion.
And importantly, this direction is the same on both the approach and the retreat.
In general, the effect of a planet’s motion on a passing probe like New Horizons depends
on the relative angle of the probe’s approach.
New Horizons approached Jupiter perpendicular to the planet’s motion, at a speed of almost
19 kilometers per second.
In the fly by, Jupiter gave the probe an additional perpendicular velocity of one planet speed,
or about 13 kilometers per second.
When we add these two vectors together, we find a speed boost for New Horizons of about
4 kilometers per second, and a direction change of about 35 degrees.
And all of this was done with such precision that only two planned and minor course corrections
were needed to get the probe within 12,600 kilometers of Pluto, 8 years and four and
a half months later.
Which is the equivalent of firing a bullet from a spinning gun Las Angeles, and hitting
a 20-meter moving target in New York City.
Space is awesome.
For Tech Laboratories, I’m Tech Adams, saying Keep Thinking, and Thanks
for Watching.