Europe's Mars Express spacecraft has deployed the first of three booms that comprise its Marsis radar, which is to look for water within the Red Planet.
At 1530 BST on Wednesday, a command was sent to release the boom, which forms one half of Marsis' primary antenna.
The primary antenna's second boom is now due to be deployed on Sunday.
Fears that one or more of the antenna components could swing back and hit the spacecraft led deployment of the booms to be delayed for more than a year.
Spacecraft telemetry suggests the boom deployed successfully.
Two 20m-long (65ft) hollow fibreglass "dipole" booms make up Marsis' primary antenna, while a 7m-long (23ft) "monopole" boom acts as a receive-only antenna.
The second half of the primary antenna must be deployed for the radar to work. But the mood amongst instrument team members is now said to be positive ahead of Sunday's planned deployment.
Scientists are eagerly awaiting results from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (Marsis) instrument because it is thought the greatest reservoir of retained water on the Red Planet could be found beneath the planet's surface.
Some think such underground reservoirs may even provide a habitat for microbial life.
The radar altimeter can search for ice and liquid water up to 5km (3 miles) beneath Mars' surface.
An investigation carried last year out by engineers at the US space agency's Jet Propulsion Laboratory (JPL) concluded there was a possibility that one or all of Marsis' antenna components could whip back and strike the spacecraft.
Even though a European Space Agency (Esa) review board concluded that the energy of any impact between the booms and the spacecraft would be low, scientists voiced concerns that the booms could damage delicate instruments carried aboard the orbiter.