How did Dr. Yueh know Jessica and Paul would be sent to the desert?

Question

Lady Jessica was supposed to be "given" to Piter de Vries once she was captured. It is only once she is in front of the Baron and Piter that the latter renounces to have her and instead chooses to reign over Arrakis. It is then that it is decided that Jessica and Paul would be sent into the desert.

So how could Dr. Yueh know in advance of this, since he left a package and a note in the ornithopter beforehand, and told the Duke he would save them? Also, a deaf guard is used because of the Voice, so how could he possibly know they would survive?

Answer 1

It’s not that he somehow knew what their plans were, it's that he actually suggested taking that approach and helped to implement it.

Here’s the exchange between Piter and the two soldiers that ultimately carry them to the desert.

He looked at the first pair of troopers, turning so the deaf one could read his lips: ”Take them into the desert as the traitor suggested for the boy. His plan is a good one. The worms will destroy all evidence. Their bodies must never be found.“

And later on:

“This the ‘thopter we’re supposed to use?” he asked, and turned to watch his companion’s lips.
“It’s the one the traitor said was fixed for desert work,” the other said.

So Yueh planted the instructions and suggestions beforehand, both for taking them to the desert and about what means to use for the task. He could not have any certainties but after all, his whole plan was hanging a very thin thread anyway.

Answer 2

Yueh's knowledge was limited and not always accurate (he believed that Leto would kill the Baron as he died). But he made contingencies and his escape plan for Paul and Jessica may have been such a contingency.

He knew these things:

1. The Baron could not afford to keep Jessica alive and able to testify before a truthsayer.
2. On Arrakis, death by desert would be an easy form of execution and would satisfy the Baron's sense of cruelty (let the planet they have taken from me be their killer).
3. To get them into the desert, an ornithopter would be used.
4. Paul and Jessica were both highly trained and likely to be under-estimated by the Baron's men.

With those things in mind, he gave them tools that they could use if they applied all their skills, mental and physical, to their escape. Essentially, he gambled on Paul and Jessica.