the best summer intern ever

Recently I was chatting with some people about energy options and brought up the nuclear waste storage problem.  Someone suggested sending it to the Sun .. "after all" he reasoned "the Sun's gravity will just pull it in.."  That's a bad idea for a couple of reasons.  First there's a lot of waste and rockets have an unfortunate failure rate that is high enough to guarantee disasters - think massive dirty bombs.  But even if you had reliable launch vehicles there's this other problem --  it's very difficult to get something to the Sun.

 

While it's true that gravity is pulling us towards the Sun (and the Sun towards us), we're also traveling fast enough horizontally in an orbital path to constantly be falling around it.  Looking down from above the North Pole, the Earth is moving counter-clockwise around the Sun at about 30 kilometers per second - about 67,000 mph or 108,000 km/h.¹  To fall into the Sun you need to slow your horizontal orbital speed to zero...  your rocket has to be launched in a clockwise path at 30 kilometers per second.  That's fast.

 

 Really fast.

 

To go into Earth orbit you need about 7.8 kilometers per second.  Escaping Earth's gravity entirely requires about 11.2 kilometers per second.    Even making escape velocity is a tall order.  

 

Curiously it takes much less energy to escape the Earth's gravity and head out to the stars than it is to travel to the Sun (although making it to the stars takes longer).  Just as strange is what happens closer to the Sun.  The closer you are to the Sun, the faster you move.  Mercury orbits at about 48 kilometers per second.  So to reach the Sun from Mercury you need to burn off 48 kilometers per second.

 

This was recognized in the 50s when people started thinking about missions inside of Earth's orbit.  Initially it was thought that going outwards .. say to around Jupiter's orbit and then slowing to fall would be much easier than slowing from Earth's orbit.  But in the early 60s a Summer intern changed everything.

 

Michale Minovitch, UCLA grad student, had a Summer job at JPL.  He was working on orbital trajectories when it struck him that a spacecraft could steal the momentum from a planet and dramatically change its speed. Up until then people had been thinking of the motion of a spacecraft relative to the planet.  He did what physicists do to make problems easier... he chose a different reference frame looking at the motion with respect to the Sun.. effectively the rest of the solar system.

 

It's a bit difficult to explain without a bit of physics, but consider this analogy.  You're standing on a train platform and the express is about to shoot by at 100 kilometers per hour.  You're somewhat of a prankster and are holding a special bouncy ball that bounces off solid surfaces (almost) as fast as it strikes it.  The train is closing in.  The urge strikes.  You throw it at about 50 km/h directly at the front of the train and score a perfect hit. 

 

The train's engineer watches as you throw the ball.  From the cab she sees it coming at the train at 150 km/h.  It's a wonderful bouncy ball and bounces nearly elastically from the train.  Let's say it's a perfect elastic collision.  She watches it move away at 150 km/h.

 

You're watching the same collision.  From your point of view the ball is moving at the speed it leaves the train plus the speed of the train ... 250km/h. A bit of the train's momentum has been transferred to the ball.  The train slows down a tiny amount, the ball speeds up a lot.  

 

It's a bit more complex doing the orbital dynamics., but basically you approach a planet from behind going in the same direction and use gravity to transfer some of it's momentum to your spacecraft.   This is how spacecraft reach the outer planets and how they move to the inner solar system.  Some of the missions have multiple passes as they build up speed for long journeys.

 

Another dramatic bit was realized in the 60s - namely a lineup of planets would exist in the mid 70s that permitted a gravity assist trip that would take a spacecraft by all of them with very little fuel.  The opportunity, called the Grand Tour,  would only come once every 175 years or so.  NASA and JPL jumped on it and Voyager 1 and 2 set sail for the planets and beyond. They're still taking data and one is the most distant human-made object.   Pluto would have been possible, but was left off -- and that's another amazing story. But time to quit.

 

 

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¹ "about" is the right word.  Our orbit is elliptical and the speed varies.  It's slowest in early July when we're at our farthest distance.  All of the numbers here are approximations.. good to a few percent, but no better.

Comments

[Gregg Vesonder]

This is very cool. I did not realize it was discovered by an intern - should justify summer interns for the next millennium! Steve your explanation is wonderful. We should turn your blog entries into scientific cartoons!