The critical event will involve a series of engine burns and maneuvers designed to slow the spacecraft from a velocity of 29,000 kilometers (18,560 miles) per hour relative to Venus just before the first burn to an entry velocity some 15 percent slower, allowing it to be captured into orbit around the planet.

During orbit insertion, the spacecraft will be about 125 million kilometers (80 million miles) from Earth, so round-trip signal time will be 13 minutes and 32 seconds.

Controllers will command the spacecraft to slew (rotate) and point the main engine nozzle in the direction of motion starting at 08:03 a.m. Central European Time. Venus Express then will burn the engine for approximately 51 minutes starting at 09:19 a.m.

Controllers also will order the spacecraft to position its solar arrays to reduce the possibility of excessive mechanical load during engine ignition.

Over the subsequent days, Venus Express will undertake a series of additional burns to lower its orbit apocenter, or farthest point from the planet, and control the pericenter, or closest point. The aim is to end up in a 24-hour orbit around Venus early in May.

Controllers must ensure that all steps take place in the correct sequence and the spacecraft is brought into the correct configuration in time for the main engine burn - which can only occur at a specific moment. The risk is that, if any problems develop, the spacecraft could miss its window for gravitational capture, "making any recovery extremely challenging," ESA said in a statement.

During the burn, Venus Express will enter an occultation as it travels behind the planet, so the line of site to Earth is blocked. During that time, it will lose radio contact with the ground for almost 10 minutes - in a situation similar to the recent insertion of NASA's Mars Reconnaissance Orbiter, which concluded successfully.

ESA controllers will expect reacquisition of radio contact at 09:56 a.m., when occultation ends.

"Venus orbit insertion is a complex step. The main challenge is that the maneuver must happen at the right time," said ESOC team member Jean-Baptiste Gratadour, an attitude and orbital-control-systems engineer.

"In the night between (Feb. 16 and 17), Venus Express passed its VOI readiness review after a successful burn of its main engine, providing a thrust of 400 Newtons," said Don McCoy, the VEX project manager.

He said that burn was followed by a minor mid-course correction by the spacecraft's four 10-Newton thrusters on Feb. 24. A 400-Newton engine generates 625 kilowatts of power, or considerably more than a standard automotive engine.

ESOC controllers have established a communications link with the 70-meter radio antenna at Madrid, part of NASA's Deep Space Network. The ground station will support the orbital insertion because of its geographic location. For routine operations, Venus Express communicates using X-band via ESA's new 35-meter deep-space antenna at Cebreros, Spain, near Madrid.

Controllers recently tested the link by broadcasting a live signal from the Venus Express low-gain antenna to Madrid and then to ESOC. During orbital insertion, the low-gain antenna will transmit data on the spacecraft's velocity. The Venus Express high-gain antenna, normally used to communicate with Earth, will be rotated away and out of line of sight of ground stations during the operation.

"We are also conducting continuous tracking of the spacecraft position through different techniques and using several ground stations, including ESA's Cebreros station and NASA Deep Space Network stations at Goldstone (USA) and Canberra (Australia) and Madrid," said Andrea Accomazzo, Venus Express spacecraft operations manager. "All our efforts are in fact now concentrated on the spacecraft navigation to prepare for the big day of arrival at Venus."

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