Now don't forget that we are trying to ascertain the age, but we are going to get there on a long and winding path, mostly because it's an opportunity to touch on some fun and interesting topics.

To begin, we go back in time, not quite 80 million years, to the Apollo program. Astronauts returned from the moon with many treasures, including 842 pounds (382 kilograms) of lunar material collected on six missions. In addition, three Soviet robotic Luna spacecraft came back with a total of 11 ounces (0.3 kilograms).

Earth's total inventory of lunar samples is larger. By comparing the chemical composition of that material with a great many meteorites, scientists have identified nearly 120 pounds (54 kilograms) of meteorites that were blasted from the moon by asteroid impacts and then landed on our planet.

Other meteorites are known to have originated on Mars. The principal method by which that connection was made was comparison of gases trapped in the meteorites with the known constituents of the Martian atmosphere as measured by the two Viking spacecraft that landed there 40 years ago. Scientists thus have 276 pounds (125 kilograms) of Martian material.

Of course, unlike the Apollo and Luna samples, the lunar and Martian meteorites were selected for us by nature's randomness from arbitrary locations that are not easy to determine.

The moon and Mars are two of only three (extant) extraterrestrial bodies that are clearly established as the source of specific meteorites. The third is Vesta, the fascinating protoplanet Dawn explored in 2011-2012. That world is farther away even than Mars, and yet we have 3,090 pounds (1,402 kilograms) from Vesta, or more than 11 times as much as from the red planet and more than three times as much as from the moon. We reflected on these meteorites during our travel from Vesta to Ceres.

It is thanks to Dawn's detailed measurements of the composition of Vesta that scientists were able to clinch the connection with the meteorites that were under study in terrestrial laboratories. The impact of an asteroid perhaps 20 to 30 miles (30 to 50 kilometers) in diameter more than one billion years ago excavated Vesta's Rheasilvia Crater. It left behind a yawning basin more than 300 miles (500 kilometers) across, a mountain more than twice the height of Mt. Everest, and a network of about 90 canyons with dimensions rivaling those of the Grand Canyon. And it launched a tremendous amount of material into space. Some of it settled back onto Vesta, resurfacing much of the southern hemisphere, but some of it departed with so much energy that it escaped Vesta's gravitational hold. Some of the biggest pieces liberated by that tremendous impact are now visible as small asteroids known as vestoids. And some of the small pieces eventually made their way to the part of the solar system where many of our readers (perhaps including you) reside. After Earth's gravity took hold of any of those wandering interplanetary rocks and pulled them in, they became meteors upon entering the atmosphere, meteorites upon hitting the ground, and keys to studying the second largest object in the main asteroid belt upon entering laboratories. One esteemed scientist on the Dawn team opined that with Dawn's detailed data and our Vestan samples, Vesta joined the ranks of the moon and Mars as the only extraterrestrial bodies that have been geologically explored in a rigorous way.

With so many meteorites from Vesta, why have we not linked any to Ceres? Is it because the rocks didn't get blasted away in the first place, or they didn't make it to the vicinity of Earth or to the ground, or we have not recognized that they are in our collections? While there are some ideas, the answer is not clear. For that matter, although Vesta and Ceres are the two largest residents of the main asteroid belt, why have we not tied meteorites to any of the smaller but still sizable bodies there? We will return to this question in a future Dawn Journal, but for now, let's get back to the question of how Dawn's pictures help with measuring the ages of features on Ceres.