Apollo 13

On July 20, 1969, astronaut Jim Lovell hosts a house party where guests watch on television as Neil Armstrong takes his first steps on the Moon. After the party, Lovell, who had orbited the Moon on Apollo 8, tells his wife Marilyn that he intends to return to the Moon and walk on its surface.
On October 30, 1969, as Lovell conducts a VIP tour of NASA's Vehicle Assembly Building, his boss Deke Slayton informs him that he and his crew will fly the Apollo 13 mission instead of Apollo 14. Lovell, Ken Mattingly, and Fred Haise train for their new mission. Days before launch, Mattingly is discovered to have been exposed to rubella, and the flight surgeon demands his replacement with Mattingly's backup, Jack Swigert, as a safety precaution. Lovell initially resists breaking up his team, but relents when Slayton threatens to bump his crew to a later mission. As the launch date approaches, Marilyn has a nightmare about her husband's safety, but goes to the Kennedy Space Center the night before launch to see him off.
On April 11, 1970, Apollo 13 Flight Director Gene Kranz gives the go-ahead from Houston's Mission Control Center for launch. After the Saturn V rocket climbs through the atmosphere, an engine on the second stage cuts off prematurely, but the craft still reaches its Earth parking orbit. After the third stage re-fires, sending Apollo 13 to the Moon, Swigert docks the Command/Service Module Odyssey with the Lunar Module Aquarius and pulls it away from the spent stage.
Three days into the mission, the crew sends a live television transmission from Odyssey, which the networks decline to broadcast live. After Swigert performs a standard housekeeping procedure, one of two liquid oxygen tanks explodes, emptying its contents into space and sending the craft tumbling. The other tank is soon found to be leaking. Mission Control aborts the Moon landing, Lovell and Haise hurriedly power up Aquarius as a "lifeboat" for the return home, and Swigert shuts down Odyssey before its battery power runs out. In Houston, Kranz rallies his team to come up with a plan to bring the astronauts home safely, declaring "failure is not an option". Controller John Aaron recruits Mattingly to help restart Odyssey for the final return.
As Swigert and Haise watch the Moon pass beneath them, Lovell laments his lost chance of walking on its surface, then turns their attention to the business of getting home. With Aquarius running on minimum electrical power, the crew suffers freezing conditions. Swigert suspects Mission Control is unable to get them home and is withholding this from them. In a fit of rage, Haise blames Swigert's inexperience for the accident; Lovell quickly squelches the ensuing argument. When carbon dioxide approaches dangerous levels, an engineering team quickly invents a way to make the Command Module's square filters work in the Lunar Module's round receptacles. With the guidance systems on Aquarius shut down, and despite Haise's fever and the miserable living conditions, the crew succeeds in making a difficult but vital course correction by manually igniting the Lunar Module's engine.
Mattingly and Aaron struggle to find a way to power up the Command Module with its limited available power, but finally succeed and transmit the procedures to Swigert, who restarts Odyssey by transferring extra power from Aquarius. Jettisoning the Service Module, the crew finally sees the extent of the damage. They prepare for re-entry, unsure whether Odyssey's heat shield is intact (if it is not, they will be incinerated). They release Aquarius and re-enter the Earth's atmosphere in Odyssey. After a tense, longer-than-normal period of radio silence due to ionization blackout, the astronauts report all is well and splash down in the Pacific Ocean. Recovery helicopters bring the three men aboard the amphibious assault ship USS Iwo Jima.
As the astronauts receive a hero's welcome on deck, Lovell's voice-over describes the events that follow their return from spaceâincluding the investigation into the explosion, and the subsequent careers and lives of Haise, Swigert, Mattingly, and Kranzâand ends with him wondering when mankind will return to the Moon.

Question: WHO WAS MATTINGLY'S BACKUP?
Answer: JACK SWIGERT