2010: The Year We Make Contact

After the mysterious failure of the Discovery One mission to Jupiter in 2001, which resulted in the deaths of four astronauts and the disappearance of David Bowman, the fiasco was blamed on Dr. Heywood Floyd, who resigned his position as head of the National Council for Astronautics. While an international dispute causes tensions between the United States and the Soviet Union, both nations prepare space missions to determine what happened to the Discovery. Although the Soviet ship, the Leonov, will be ready before the American spacecraft Discovery Two, the Soviets need American astronauts to help board the Discovery and investigate the malfunction of the ship's sentient computer, HAL 9000, which caused the disaster. The US government agrees to a joint mission when it is determined that Discovery will crash into Jupiter's moon Io before Discovery Two is ready. Floyd, along with Discovery designer Walter Curnow and HAL 9000's creator Dr. Chandra, joins the Soviet mission.
Upon arriving at Jupiter, the crew detect signs of life on Jupiter's seemingly barren moon Europa. They send an unmanned probe down to Europa to investigate the unusual readings, but just as it finds the source, a mysterious energy burst destroys the probe and its data. The "burst" then flies toward Jupiter. The Soviets believe the burst was simply electrostatic build-up, but Floyd suspects it was a warning to stay away from Europa.
After surviving a dangerous braking maneuver around Jupiter's upper atmosphere, the Leonov crew find the abandoned Discovery floating in space. Curnow reactivates the ship and Chandra restarts HAL, who had been deactivated by Dave Bowman before his disappearance nine years earlier. Also nearby is the giant alien Monolith that the Discovery was originally sent to investigate. Cosmonaut Max Brailovsky travels to the Monolith in an EVA pod, at which point the Monolith briefly opens with a burst of energy, sending Max's pod spinning off into space. On Earth, Dave Bowman, now an incorporeal being that exists inside the Monolith, appears on his wife's television screen and wishes her farewell. He also visits his terminally ill mother just before she dies.
On the Discovery, Chandra discovers the reason for HAL's malfunction: The National Security Council ordered HAL to conceal from Discovery's crew the fact that the mission was about the Monolith; this conflicted with HAL's basic programming of open, accurate processing of information, causing him to suffer the computer equivalent of a paranoid mental breakdown. Although the order bears his signature, Floyd is outraged that this was done without his knowledge.
On Earth, tensions between the United States and the Soviet Union escalate to a state of war. The Americans are instructed to leave the Leonov and move to the Discovery, with both crews ordered not to communicate with each other. Both crews plan to leave Jupiter separately when a launch window opens in several weeks; however, Bowman appears to Floyd and says it is imperative that everybody leave within two days. Stunned by Bowman's appearance, Floyd returns to the Leonov to confer with Captain Tanya Kirbuk, who remains skeptical. The Monolith then suddenly disappears, and a growing black spot appears on Jupiter itself. The spot is actually a vast group of Monoliths that are constantly multiplying. The Monoliths begin shrinking Jupiter's volume, increasing the planet's density, and modifying its chemical composition. This convinces the two crews that they must leave soon. Since neither ship can reach Earth with an early departure, they work together to use the Discovery as a booster rocket for the Leonov, though it means the Discovery's and HAL's destruction. Bowman's voice is heard once again as he speaks to HAL and tells him that they will soon be together after he transmits one final message to Earth:
ALL THESE WORLDS
ARE YOURS EXCEPT
EUROPA
ATTEMPT NO
LANDING THERE
USE THEM TOGETHER
USE THEM IN PEACE
The Monoliths engulf Jupiter, causing nuclear fusion that transforms the planet into a small star. Discovery is consumed in the blast after the Leonov breaks away to safety. The new star's miraculous appearance inspires American and Soviet leaders to seek peace. Europa gradually transforms from an icy wasteland to a humid jungle covered with plant life. A Monolith stands in the primeval Europan swamp, waiting for intelligent life forms to evolve.

Question: The U.S. government agrees to a joint mission with what nation?
Answer: Soviet Union