Update 12/28/2018: Sky Sports released , and you can watch it in the US. It focuses mainly on Johnson getting acclimated to the McLaren. The original story appears as published on 11/28/2018 below.
It seems that every few years, a Formula One driver and a NASCAR driver swap seats to get a taste of what another form of racing is like. In 2003, Jeff Gordon and Juan Montoya, then driving for Williams, on the road course at Indy. Montoya must have liked it, since he ended up driving in NASCAR full-time just a few years later.
In 2011, in tough conditions. Both had a blast.
Now, Fernando Alonso and Jimmie Johnson have swapped, but instead of sending the F1 car to the US, Hendrick Motorsports sent Jimmie's car all the way to Bahrain. And then, the day after Alonso's last F1 race, the two swapped seats.
This was not done without a lack of preparation. Immediately after the NASCAR finale at Homestead, Johnson flew to England to spend time in McLaren's simulator to learn the foibles of the car he was driving, a 2013 MP4-28. That's not one of the current hybrid F1 cars, but rather one of the last V8-powered cars.
Alonso has been sampling all sorts of race cars lately, with a 2017 run in the Indy 500, a 2018 run at the Daytona 24 in an LMP2 car, and a drive–and win–at the 2018 24 Hours of Le Mans in Toyota's LMP1-H car.
What resulted from this car swap? Pretty much what you'd expect. Alonso had a blast pushing and sliding Johnson's car around while Jimmie was blown away by the F1 car's downforce and, particularly, the braking.
Take a look at some behind-the-scenes footage of what the day was like, as well as insights from engineers on both teams.