Venus received a visitor from its sister planet this morning when a European space probe completed a five-month interplanetary cruise and swooped into orbit to begin the first comprehensive scientific survey of its sultry atmosphere.

Venus Express used its main engine to safely enter orbit around the planet. Credit: ESA

Venus Express - a $266 million mission conducted by the European Space Agency - became the first spacecraft to enter orbit around the planet since the 1990 arrival of NASA's Magellan radar observatory.

Officials at ESA's Space Operations Center in Germany received confirmation of the successful completion of a make-or-break 50-minute engine burn at around 0807 GMT (4:07 a.m. EDT). The burn actually ended several minutes before, but it took almost seven minutes for radio signals to make the one-way trip through the 78-million mile void between Earth and Venus.

Venus arrival operations began in earnest at 0603 GMT (2:03 a.m. EDT) Earth received time, when the craft began to maneuver to the precise orientation required for the orbital insertion engine firing. The process aligned the probe's main engine with the direction of travel. Telemetry from the S-band low gain antenna showed Venus Express in the proper position for the burn about a half-hour later.

A NASA Deep Space Network ground station in Madrid relayed data verifying the ignition of the powerful main engine at about 0717 GMT (3:17 a.m. EDT). During the critical burn, the spacecraft passed behind Venus and controllers lost the carrier signal from the probe for around ten minutes. As expected, after the communications link was restored, the propulsion system was shut off at 0807 GMT (4:07 a.m. EDT), followed by an announcement a few moments later.

After the burn, commands ordered the solar arrays to begin tracking the Sun, and one of the X-band high gain antennas was pointed back toward Earth to begin beaming back heaps of information on the orbiter's health and status shortly after 0900 GMT (5:00 a.m. EDT).

During the engine firing, most of the 1,254 pounds of propellant stored aboard Venus Express was exhausted, also significantly reducing the mass of the spacecraft. The burn slowed the velocity of Venus Express by 15 percent from the initial approach of 18,000 miles per hour. The decrease in relative speed allowed Venus' gravity to capture the craft in what was planned to be an egg-shaped orbit stretching from a low point of less than 250 miles above the surface to a high point of over 215,000 miles away. Each trip around the planet in such an orbit takes nine days.

Venus Express is Europe's first mission to Earth's closest planetary neighbor. Credit: ESA

There is a unique opportunity begin early scientific observations later this week. Because of the distance the spacecraft will reach from Venus in its initial capture orbit, instruments will be able to view the entire planet at once. Investigations into the solar wind's interaction with the planet are on tap, as well as continuous long-range studies of the atmosphere for over four days. Images of the entire planet will also be taken.

It will take almost a month for Venus Express to gradually reduce its altitude to the planned operational science orbit. A total of seven burns - two with the main engine and five with less powerful thrusters - will be required to arrive in the final 24-hour polar orbit with a low point of approximately 150 miles and a high point of 41,000 miles. Plans currently call for this orbit to be reached by around May 7.

A simultaneous checkout of the probe's seven science instruments begins on April 22 and lasts through much of May. By June 4, officials hope to begin the scientific operations phase that should last two Venusian days, or about 486 Earth days. Venus Express carries enough propellant to double the planned mission duration if ESA elects to do so.

Venus Express seeks to answer key questions left from earlier exploration of the hostile planet. Its instrument package will largely focus on the thick atmosphere of Venus, which holds secrets that still elude scientists.

The atmosphere acts as a greenhouse on the planet, sending surface temperatures soaring to almost 900 degrees Fahrenheit. Thick cloud layers race around the planet in just four days, and violent winds are common at all levels of the atmosphere. Venus Express aims to reveal some of the elusive secrets of the Venusian atmosphere by observing the structure, circulation, and composition of the atmosphere from the surface to high altitudes to help determine why the atmosphere behaves as it does.

Venus Express will also study how charged particles in the solar wind interact with Venus, and how materials in the atmosphere escape into space due to the planet's lack of a strong magnetic field. Other instruments will also investigate surface geological processes and search for volcanic activity. A wide-angle visible, near infrared, and ultraviolet camera is also expected to take thousands of detailed pictures during the mission.

"With the arrival of Venus Express, ESA is the only space agency to have science operations underway around four planets: Venus, the Moon, Mars, and Saturn," said David Southwood, director of ESA's science programs. "We are really proud to deliver such a capability to the international science community."

"By observing Venus and its complex atmospheric system, we will be able to better understand the mechanisms that steers the evolution of a large atmosphere and the change of climates," said Jean-Jacques Dordain, ESA Director General. "In the end, it will help us to get better models of what is actually going on in our own atmosphere, for the benefit of all Earth citizens."

The Venus Express orbiter will examine the mysterious Venusian atmosphere and make new observations of the planet's surface. Credit: ESA

Tuesday's crucial milestone completed a 153-day, 250 million mile journey from the Earth toward the inner solar system after the craft's launch last November 9 atop a Russian Soyuz rocket. The booster's Fregat upper stage placed Venus Express on a course to Venus about 90 minutes after liftoff. See Spaceflight Now's coverage of the launch here.

Preparations for the arrival at Venus included a full main engine test firing in mid-February to evaluate its performance before being called upon again. Controllers transitioned into the orbital insertion phase of the mission on April 4 when the low gain antenna's radio transmitter was turned on. High bandwidth communications are not possible with Venus Express during the engine burn because those antennas are pointed away from Earth.

The set of commands for the critical hours surrounding the burn early Tuesday were uplinked to Venus Express last Friday. The timeline was stored in an on-board computer before being executed with apparent perfection.

Developed in just three years and on a relatively inexpensive budget, Venus Express borrows much more than just its design from other ESA missions. A nearly identical twin to Mars Express currently in orbit around the Red Planet, Venus Express carries five instruments first manufactured as flight spares for both the Rosetta comet chaser and Mars Express.

Industrial teams led by Astrium of France constructed the spacecraft after finishing their work on Rosetta and Mars Express. The decision to use components and personnel left over from these missions helped to significantly reduce costs and development time on Venus Express.