In Interstellar, why does Cooper leave Brand?

Near the end of Christopher Nolan's space adventure film, Interstellar, Cooper and Brand are the only ones left to save the mission, and they're both flying around the black hole Gargantua. Cooper's plan is to detach the part of the ship containing TARS (Ranger 1) into the black hole in order to slingshot the rest of the Endurance towards Edmund's planet, and as an added bonus, TARS would transmit the quantum data to Cooper.

After Ranger 1 detaches, Ranger 2--containing Cooper--detaches as well. This is done so that the ship can be slingshot easier with less weight. My question is, why did Cooper have to leave with the ship as well? This seems to me to be an unnecessary sacrifice, as Cooper didn't actually know that he would be heading into a tesseract at the time. Why did he do this?

Cooper believes his actions (and death) will help complete Plan A which is of utmost important to him: i.e. saving Murphy.

In terms of the script backing this up:

Regarding consumable resources for them both to get home, Brand says:

You told me we had enough resources for both of us.

Apparently Cooper lied to her, as he responds:

We agreed, Amelia......90 percent.

(Meaning, he was only 90% truthful)

Why does TARS have to detach?

Cooper responds:

We have to shed the weight to escape the gravity. Newton's third law. The only way humans have ever figured out of getting somewhere......is to leave something behind.

Followed by:

It's what we intended, Dr. Brand. It's our only chance to save people on Earth. If I can find a way to transmit the quantum data I'll find in there......they might still make it.

In addition to the weight and resources issue - it appears as though Cooper believes he would be needed to assist in transmitting the data from the singularity to actually complete Plan A.

• I think the OP understood about the issue with the Ranger's weight, as evidenced by the sentence, "This is done so that the ship can be slingshot easier with less weight", but was asking why Cooper didn't leave the Ranger before dropping it into the black hole. Maybe the answer is just that there wasn't enough time. Mar 4, 2016 at 23:43
• Could the ship have been detached remotely? I've only seen the movie once, but I don't recall seeing any way to detach a ship without something, be it a person or robot, physically present to engage/push the proper levers/buttons. Mar 4, 2016 at 23:50
• TARS was attached to the ship so he could (presumably) have detached on his own. Whether Cooper really believes he needs to be on the detached ship as well for weight and consumable purposes or not is possibly open to debate - however, Cooper's last words to Brand indicate that he believes he will need to be present to transmit any data found. Ultimately - it seems Cooper believes his actions (and death) will help complete Plan A which is of utmost important to him: i.e. saving Murphy. Mar 5, 2016 at 0:08
• +1 to this. The novelisation concurs that his thoughts were that there weren't enough supplies for the both of them. Mar 6, 2016 at 10:30
• Your point on resources was very helpful! I hadn't thought of that. Mar 6, 2016 at 16:27

Cooper tells Brand that the lander's linkages have been destroyed. So the only way they can slingshot themselves out of the Black hole's orbit is by manually controlling the engines. Without Amelia knowing, the others(Cooper, TARS and CASE) decide that TARS and Cooper would jettison themselves from the Endurance to shed weight and make the slingshot easier. Although it is given as a hint by TARS, Amelia had no idea that Cooper would also follow the same as TARS. It seems as if Cooper could have done it without sacrificing himself. But, he says so himself that the gravitational slingshot can be done only if the engines of the Lander 1 and Ranger 2 are controlled manually. It means that someone had to be physically present in both the crafts for the plan to work.

It might also seem that CASE could have gone instead of Cooper. But then there is another issue. Just seconds before detaching Ranger 2, Cooper confesses that he lied that there was enough resources for both of them, meaning only one of them could survive. CASE could assist Amelia in Plan B once they reached Edmund's planet. Since CASE is a robot, he won't have need for the resources. Cooper decided to sacrifice himself for Amelia as well as trying to find a way to save the people on Earth. At that point, Cooper did not have much hope of saving the people on Earth. He might have thought that after TARS collected the quantum data, they could devise a plan to send that data to Murph.

Anyway, everything goes well. But when TARS and Cooper launch themselves into the sky to find Brand, do they have any idea how they are going to? I mean, the 'beings' closed the tesseract so the might have closed the wormhole too. Even if that didn't happen, how can the craft in which TARS and Cooper set off supply them with the resources they need for the journey?

After Dr. Mann severely damaged the Endurance it only had enough resources for one person to make the return trip to earth. If Cooper had stayed they would have both died.

It wasn't about the lander not being able to be detached remotely or Cooper wanting to dive face first into a black hole. The only reason they were even near the black hole was because they were so low on resources that the sling shot maneuver from its gravity was the only way they could generate enough speed to make it back to the wormhole and return to earth before oxygen ran out.

The fact that Cooper was able to find useful information beyond the event horizon and was actually able to transmit it back to his daughter was dumb luck. He went in fully expecting to die.