This week, NASA released the first images that its Messenger probe sent back after it went into orbit around the solar system's innermost planet, Mercury. These weren't the first images taken of Mercury from a spacecraft, nor even the first images of Mercury taken by Messenger, which had passed by the planet several times as it maneuvered into orbit. So why is entering orbit a big deal?

In some ways, going into orbit means we're there to learn instead of simply discover. We picked up remarkable things as the Voyagers shot past the outer planets in our solar system, but the rapid flybys tended to provide a limited, static image of the planets they visited. When we returned with later orbiters, we got to track things like changing seasons on the planets and their moons, the evolution of their rings, and changes in their atmospheres. If one pass yields something interesting, it's possible to go back around and have another look.

So, in celebration of Messenger's achievement, we're going to take a look back at a series of firsts, with some of the initial photos taken from orbit (or a variation on that theme) for all of the planets. Dwarf planets need not apply.

Mercury/Messenger

On March 29, after several flybys, Messenger achieved orbit of the planet Mercury, the closest to the Sun. To protect its payload from the Sun, the probe features a sun shield and insulation, along with internal heat pipes and radiators—even the solar panels need mirrors to keep from overheating. In addition to imaging cameras, the probe carries three spectrometers and a variety of instruments to detect charged particles and magnetic fields. The goal is to get a sense of the planet's geological history, along with its current composition.

The probe's first image from orbit focused on the Debussy crater near the planet's South Pole.

Venus/Venera 9

The Venera 9 mission was part of an aggressive exploration of Venus performed in the mid-1970s by the Soviet Union. It was launched from the Baikonor Cosmodrome in 1975, and part of the mission became the first human hardware to orbit the planet. But on October 22, the other part actually landed on the planet, taking the picture shown below and transmitting data for nearly an hour. In addition to showing the planet's rocky surface, Venera 9 gave us the first direct measurement of the planet's high temperatures, thick clouds, and rather aggressive atmospheric chemicals.

Oddly, although the orbiter had a large array of instruments that included cameras, I've been unable to find photos taken from orbit. Based on our experience with Mars, however, the combination of orbital imaging and on-ground exploration provide a more complete picture of the rocky planets, so it seems reasonable to commemorate the landing on Venus.

Earth/Apollo 8

For our home planet, we're going to do the converse: commemorate the first picture of it taken from outside the Earth's orbit. In December of 1968, as part of the preparations for the first landing on the Moon, Apollo 8 left Earth's orbit, eventually taking the first photos of an earthrise while orbiting the Moon. So much has been written about how the images taken during that mission changed the way we perceive our planet that anything anything I write would be redundant. Besides, the image speaks for itself.

NASA has placed the entire Apollo 8 flight journal online, and has an entire page devoted to this image.