October 18, 2016

On September 30, a bright light of science was extinguished in the solar system. That day, the space probe Rosetta crash-landed on the comet it had been orbiting, marking the end of an ambitious mission that paid–and should continue to pay–huge dividends for astronomy.

The European Space Agency (ESA) launched Rosetta on March 2, 2004. Rosetta orbited comet 67P/Churyumov-Gerasimenko from August 2014 to September 2016. Scientists think that comets preserve dust, ice, and rock from the solar system’s formation. By gathering data on a comet, therefore, Rosetta helped scientists to learn more about the solar system’s composition and history. Rosetta was named for the Rosetta stone, an inscribed rock that enabled scholars to interpret ancient Egyptian hieroglyphs. Rosetta also carried a small craft named Philae to land on the surface of the comet’s nucleus (core). Philae was named for the Philae obelisk, which also bore inscriptions that helped decipher ancient Egyptian writing.

This artist’s impression shows the European Space Agency (ESA) lander Philae on the surface of comet 67P/Churyumov-Gerasimenko. Philae was released from the ESA probe Rosetta to gather detailed information about the comet’s structure and makeup. Credit: DLR German Aerospace Center

Rosetta overcame many difficulties to provide key insights into the history of the early universe. It was initially planned as a sample return mission to a different comet in collaboration with the United States National Aeronautics and Space Administration (NASA). But because of the tragic loss of the space shuttle Challenger in 1986, NASA pulled out of the Rosetta mission, and the ESA was forced to scale it back. Many years later, in 2002, an Ariane 5 rocket failed shortly after liftoff. Rosetta was scheduled to be carried into space on the next Ariane 5 later that year. The failure grounded the Ariane 5 for many months, and mission scientists changed their target comet to 67P/Churyumov-Gerasimenko.

In 2014, when the lander Philae touched down on 67P, its landing harpoons failed to trigger. The craft bounced high into space and came down on its side in a sunless area of the comet’s nucleus. Scientists worked feverishly to conduct experiments and gather data for 57 hours before the lander’s solar-powered batteries died. Despite Philae’s loss, Rosetta continued orbiting the comet, taking photographs and collecting data. On Sept. 2, 2016, with just weeks left in the mission, Rosetta discovered the wayward Philae in the shade of a small cliff on the comet’s surface.

In spite of all the bumps along the way, Rosetta was a fabulously successful mission. It became the first spacecraft to orbit a comet, and it released the first probe to land on (rather than crash into) a comet. It returned invaluable data about the evolution of comets as they approach the sun and the history of the early solar system. Scientists are only just beginning to draw conclusions from Rosetta’s data.

Rosetta’s collision with the comet was not accidental, but had been planned by mission scientists. The highly elliptical (elongated) orbit of 67P takes it as close as 115 million miles (185 million kilometers) and as far as 530 million miles (850 million kilometers) from the sun. This creates vast and lengthy temperature changes over the course of its six-and-a-half-year orbit. These temperature extremes ravage a spacecraft’s sensors and electronic equipment. As the comet tracked back away from the sun, scientists feared that Rosetta could not survive another hibernation in the icy depths of the outer solar system. Rather than risk it, they decided to send Rosetta out in style, crashing into the comet while collecting as much data as possible. On its final descent, Rosetta studied the comet’s gas, dust, and plasma environment very close to the surface. The probe also took some harrowing high-resolution images as it plunged toward the comet.

Despite the end of Rosetta, ESA has several other important missions in progress. LISA Pathfinder plans to study gravitational waves from space. The Gaia probe is in the process of creating the most detailed map of the galaxy ever. The ExoMars Trace Gas Orbiter is en route to Mars to study the planet’s atmosphere and release a lander in preparation for a future wheeled rover.

August 6, 2014

A photo of comet 67P/Churyumov-Gerasimenko taken by Rosetta on August 3, 2014, from a distance of around 180 miles (285 kilometers). (Copyright ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.)

A space probe launched in March 2004 has successfully met up with the comet it was sent to study. Ten years ago, the European Space Agency (ESA) launched the Rosetta probe, which has traveled 3.7 billion miles (6 billion kilometers) through the solar system to meet up with comet 67P/Churyumov-Gerasimenko (nicknamed Chury). The probe used a series of fly-bys past Earth and Mars to increase its speed during the chase—Chury travels at a speed of around 34,000 miles (55,000 kilometers) per hour. During the lengthy journey, the probe was put into “hibernation” for for some 2.5 years to save energy. Scientists at ESA’s flight center in Darmstadt, Germany, woke Rosetta in January for the last leg of its journey.

This morning, ESA scientists fired thrusters to stop Rosetta and maneuvered it into orbit around the comet; Rosetta will orbit Chury for the next 15 months. The probe will photograph and map the surface of the comet. Rosetta also carries a lander, Philae, which will be launched in November onto the comet’s surface. (Philae is an island in Egypt where an Egyptologist discovered an obelisk that was used in conjunction with the Rosetta Stone to decipher Egyptian hieroglyphics.) The lander carries a drill that will be used to take samples from below the surface.

Additional World Book articles:

• Comet (a research Guide)
• Space exploration

Feb. 16, 2012

To the surprise of scientists, data from the Venus Express space probe have indicated that Venus is rotating much more slowly than it was only 16 years ago. As a result, a day on Venus is now an average of 6.5 minutes longer. Why? For now, scientists are puzzled, though the reason may lie in the thick blanket of clouds that has long kept the planet shrouded in mystery.

The discovery of the slowdown was announced by scientists with the European Space Agency (ESA) who have been using an infrared device on the Venus Express to study surface features on the planet. A comparison of these images with radar maps of Venus made by NASA‘s Magellan spacecraft in the 1990′s at first caused the ESA scientists to think their calculations were seriously wrong.  During Magellan’s four-year mission, scientists had determined the length of a Venusian day by measuring the speed at which surface features passed beneath the orbiting probe. ESA scientists were confident that the Magellan measurements were accurate. But now, certain features were up to 12.4 miles (20 kilometers) away from where the ESA scientists thought they would be. Additional checks for possible errors led the scientists to conclude that the planet was, in fact, rotating more slowly. Since 1996, the length of a day on Venus has increased from an average of 243.0185 Earth days to an average of 249.5185 Earth days.

Thick clouds of sulfuric acid cover Venus, hiding its surface from most forms of light. The Pioneer Venus 1 orbiter took these pictures of the planet's atmosphere in ultraviolet light. Many kinds of light cannot penetrate the clouds. To map the surface, astronomers rely on spacecraft instruments that use radio waves, such as radar. NASA

Venus’s incredibly dense atmosphere may explain the mysterious slowdown. The atmospheric pressure on Venus is a crushing 90 times as great as the pressure at sea level on Earth. At the cloud tops, winds often blow at hurricane-force speeds of 200 miles (320 kilometers) per hour. Some scientists have speculated that friction between the surface and the turbulent atmosphere may be slowing the planet. They also suggested that the slower rotation rate may be part of a long-term weather cycle. The length of a day on Earth may also vary because of tides and winds. But on our home planet, a day may change by only a millisecond a year.

Additional World Book articles:

• Solar system
• Space exploration