In addition to the gamma ray spectra and neutron spectra, Dawn’s other top priority now is measuring Ceres’ gravity field. The results will help scientists infer the interior structure of the dwarf planet. The measurements made in the higher altitude orbits turned out to be even more accurate than the team had expected, but now that the probe is as close to Ceres as it will ever go, and so the gravitational pull is the strongest, they can obtain still better measurements.

Gravity is one of four fundamental forces in nature, and its extreme weakness is one of the fascinating mysteries of how the universe works. It feels strong to us (well, most of us) because we don’t so easily sense the two kinds of nuclear forces, both of which extend only over extremely short distances, and we generally don’t recognize the electromagnetic force. With both positive and negative electrical charges, attractive and repulsive electromagnetic forces often cancel. Not so with gravity. All matter exerts attractive gravity, and it can all add up. The reason gravity — by far the weakest of the four forces — is so salient for those of you on or near Earth is that there is such a vast amount of matter in the planet and it all pulls together to hold you down. Dawn overcame that pull with its powerful Delta rocket. Now the principal gravitational force acting on it is the cumulative effect of all the matter in Ceres, and that is what determines its orbital motion.

The spacecraft experiences a changing force both as the inhomogeneous dwarf planet beneath it rotates on its axis and as the craft circles that massive orb. When Dawn is closer to locations within Ceres with greater density (i.e., more matter), the ship feels a stronger tug, and when it is near regions with lower density, and hence less powerful gravity, the attraction is weaker. The spacecraft accelerates and decelerates very slightly as its orbit carries it closer to and farther from the volumes of different density. By carefully and systematically plotting the exquisitely small variations in the probe’s motion, navigators can calculate how the mass is distributed inside Ceres, essentially creating an interior map. This technique allowed scientists to establish that Vesta, the protoplanet Dawn explored in 2011-2012, has a dense core (composed principally of iron and nickel) surrounded by a less dense mantle and crust. (That is one of the reasons scientists now consider Vesta to be more closely related to Earth and the other terrestrial planets than to typical asteroids.)

Mapping the orbit requires systems both on Dawn and on Earth. Using the large and exquisitely sensitive antennas of NASA’s Deep Space Network (DSN), navigators measure tiny changes in the frequency, or pitch, of the spacecraft’s radio signal, and that reveals changes in the craft’s velocity. This technique relies on the Doppler effect, which is familiar to most terrestrial readers as they hear the pitch of a siren rise as it approaches and fall as it recedes. Other readers who more commonly travel at speeds closer to that of light recognize that the well-known blueshift and redshift are manifestations of the same principle, applied to light waves rather than sound waves. Even as Dawn orbits Ceres at 610 mph (980 kilometers per hour), engineers can detect changes in its speed of only one foot (0.3 meters) per hour, or one five-thousandth of a mph (one three-thousandth of a kilometer per hour). Another way to track the spacecraft is to measure the distance very accurately as it revolves around Ceres. The DSN times a radio signal that goes from Earth to Dawn and back. As you are reminded at the end of every Dawn Journal, those signals travel at the universal limit of the speed of light, which is known with exceptional accuracy. Combining the speed of light with the time allows the distance to be pinpointed. These measurements with Dawn’s radio, along with other data, enable scientists to peer deep into the dwarf planet.

Although it is not among the highest scientific priorities, the flight team is every bit as interested in the photography as you are. We are visual creatures, so photographs have a special appeal. They transport us to mysterious, faraway worlds more effectively than any propulsion system. Even as Dawn is bringing the alien surface into sharper focus now, the pictures taken in higher orbits have allowed scientists to gain new insights into this ancient world. Geologists have located more than 130 bright regions, none being more striking than the mesmerizing luster in Occator crater. The pictures taken in visible and infrared wavelengths have helped them determine that the highly reflective material is a type of salt.