The Science of Interstellar: Chapter II. Gargantua (Chapter 7. Gravitational Slingshots)

 

The Science of Interstellar: Chapter II. Gargantua

Chapter 7. Gravitational Slingshots   

 

Title: Interstellar

Director: Christopher Nolan

Cast: Matthew McConaughey, Anne Hathaway, Jessica Chastain, Matt Daymon

Genre: Science Fiction, Dystopia, Documentary, Action, Adventure

Language: English

Rated PG-13

Running time: 169 minutes

My Rating: 9.9/10 

"Look I can swing around that neutron star to decelerate"

    This is Cooper's line when the crews were deciding which planet they would head to (Miller's planet? Mann's planet? or Edmund's planet?). Wait, "swing?" What does Cooper mean by "swing"? - like a baseball bat? 

    In this line, Cooper is referring to gravitational slingshots. Have you ever heard of this? Don't worry at all if you haven't! I am going to introduce this concept to you very easily.

 

(click the image above if you want to review where the Endurance was parked and where Miller's planet was)

    Because they wanted to stay out of the time shift, the crews of the Endurance decided to park the Endurance just outside the Miller's planet. This is about at 5 Gargantua radii. While the Endurance moves at one-third the speed of light (c/3), Miller’s planet moves at 55% the speed of light (0.55c). This means that to reach Miller’s planet from the parking orbit, the Ranger must slow its forward motion from c/3 by about c/4 to reach the planet’s 0.55c speed and rendezvous with it. This deceleration would also enable sGargantua’s gravity to pull the Endurance downward. 

    But, is this possible with our current technological development? Well, the fastest that human spacecraft is likely to achieve in the 21st century is about 300 km per sec. However, the required changes of velocity from Endurance to Miller’s planet is about 3/c which is about 100000 km per sec. Then, did Christopher Nolan just make it to be possible because it is a sci-fi film? 

    The answer is No. Christopher Nolan never makes things work randomly in his sci-fi films. In fact, Nature provides a way to achieve the huge speed changes, c/3: by Gravitational slingshots around black holes smaller than Gargantua.

<Slingshot Navigation to Miller’s planet>

What exactly is Gravitational slingshot? 

In short, it is "a gravity assist."

In-depth, it is a type of spaceflight flyby which makes use of the relative moment and gravity of a planet/other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense.

Choosing what small black hole that is orbiting Gargantua to use for the slingshot, the crews choose a well-positioned one to gravitationally deflect the Ranger from its near-circular orbit and send it plunging downward toward Miller’s planet. And this is when Cooper says the line:

“Look I can swing around that neutron star to decelerate”

    Because the Ranger was moving c/4 faster than Miller's planet, it had to decelerate.  The neutron star will deflect and slow down the Ranger so it can rendezvous gently with the planet. However, if the deflector is a neutron star or a black hole with a radius of less than 10,000 km, the humans and the Ranger will be torn apart by something called: Tidal Forces. 

    Thus, to survive, the deflector must be a black hole at least 10,000 km in size. Black holes with this size are called Intermediate-mass black holes (IMBHs). The IMBH used for the gravitational slingshot in Interstellar was about earth-sized. It was 10,000 solar masses, but 2000 times heavier than typical black holes. Despite their big sizes, they are surprisingly 10 thousand times smaller than Gargantua. FYI, some IMBHs form in the cores of dense clusters of stars, called globular clusters, while some are found in the nuclei galaxies where gigantic black holes reside. 

    Wait, but didn't Cooper mention a "neutron star?" Yes, he did. But, this is because Christopher Nolan did not want to confuse the audience with 2 black holes. Instead, Christopher Nolan included one slingshot in Cooper's dialogue, at the price of using a scientifically implausible deflector: the neutron star. 

            

    When an IMBH passes through such a dense region, it gravitationally deflects the stars, creating a wake with enhanced density behind itself. The wake pulls on the IMBH gravitationally, slowing it down. This is a process called Dynamical friction. As the IMBH gradually slows, it sinks deeper into the vicinity of the gigantic black hole.

<NASA's Gravitational Slingshots in the Solar System>

Have you ever heard about the Cassini spacecraft? 

Cassini spacecraft was launched from Earth in 1997 with too little fuel to reach its destination: Saturn.  This problem, in fact, was dealt with by slingshots: once around Venus, second time around Venus again, third time around Earth, and lastly around Jupiter, arriving at Saturn in 2004. 

However, as you can see, the slingshots made Cassini deflect only mildly instead of strongly deflecting the spacecraft's direction of motion. Why is this?

This is because the deflector’s gravity was too weak to produce a strong deflection. Hence, it was an inevitably small deflection due to intrinsically weak gravity. In fact, Jupiter has much stronger gravity, but a large deflection would have sent Cassini in the wrong direction. Reaching Saturn required a small deflection. If you want to know more about gravitational slingshots used for Cassini, you can visit here!

https://science.nasa.gov/mission/cassini/gravity-assists/

    By contrast with these weak slingshots in the solar system, Gargantua’s intense gravity can grab even objects moving at ultrahigh speeds and throw them around on strongly bent slingshots. How interesting! Actually, this produced gravitational lensing, the key to seeing Gargantua, which will be introduced next week! 

     I trust your November was truly remarkable. How's the weather treating you in your corner of the world? Here in South Korea, the chill is starting to set in. Stay cosy, and get ready to delve into our next captivating topic: Gravitational Lensing. Until then, keep warm and I hope you enjoy the seasonal vibes!