Now it’s really getting interesting. Here are the two views of Ceres that the Dawn spacecraft acquired on February 12. The distance here is about 83,000 kilometers, the images taken ten hours apart and magnified. As has been true each time we’ve talked about Ceres in recent weeks, these views are the best ever attained, with arrival at the dwarf planet slated for March 6.
What I notice and really enjoy about watching Dawn in action is the pace of the encounter. Dawn is currently moving at a speed of 0.08 kilometers per second relative to Ceres, which works out to 288 kilometers per hour. The distance of 83,000 kilometers on the 12th of February has now closed (as of 1325 UTC today, the 18th) to 50,330 kilometers. Its quite a change of pace from the days when we used to watch Voyager homing in on a planetary encounter. Voyager 2 reached about 34 kilometers per second as it approached Saturn, for example, then slowed dramatically as it climbed out of the giant planet’s gravitational well. The same profile held with each encounter.
Have a look at this JPL graph of Voyager 2 assist velocity changes and you’ll see the profile for each planetary flyby. In each case, the spacecraft comes into the gravitational influence of the planet and falls towards it, increasing its speed to a maximum at the time of closest approach. The numbers at closest approach to Saturn are actually over twice what Voyager 2 is making right now — 15.4 kilometers per second — but it’s the climb out of the gravity well that slows the vehicle down, something that is clearly shown in the graph at each encounter. Voyager 2, as noted below, actually left the Earth moving at 36 kilometers per second relative to the Sun.
Image: Voyager 2 leaves Earth at about 36 km/s relative to the sun. Climbing out, it loses much of the initial velocity the launch vehicle provided. Nearing Jupiter, its speed is increased by the planet’s gravity, and the spacecraft’s velocity exceeds Solar System escape velocity. Voyager departs Jupiter with more sun-relative velocity than it had on arrival. The same is seen at Saturn and Uranus. The Neptune flyby was designed to put Voyager close by Neptune’s moon Triton rather than to attain more speed. Diagram courtesy Steve Matousek, JPL.
The result for those of us watching spellbound as the Voyager mission progressed was that the planetary flybys were over quickly, the newly seen planetary details and moons captured for later analysis. With the benefit of supple ion propulsion, Dawn approaches at a far more leisurely pace as it moves toward not flyby but orbit around its latest target. To judge from what we’re seeing in the latest imagery, Ceres is going to be a fascinating place to explore. Note the craters and bright spots that have people talking throughout the space community.
“As we slowly approach the stage, our eyes transfixed on Ceres and her planetary dance, we find she has beguiled us but left us none the wiser,” said Chris Russell, principal investigator of the Dawn mission, based at UCLA. “We expected to be surprised; we did not expect to be this puzzled.”
Given how much we learned about Vesta during Dawn’s fourteen month exploration of the asteroid, we can hope that a comparative analysis of Ceres will teach us much about the formation of these objects and the Solar System itself. The best resolution in the images above is 7.8 kilometers per pixel. But day by day a small world is opening up, slowly, majestically. We may be looking at a place that becomes a significant resource for an expanding interplanetary economy in some much to be hoped for future. I’m reminded of a work by the Irish poet Eavan Boland, drawn back to it this morning because of the title: ‘Ceres Looks at the Morning.’ Boland’s reference is to the classical goddess Ceres, but this excerpt seems apropos as we approach and reveal a new land:
look at me as a daughter would
look: with that love and that curiosity:
as to what she came from.
And what she will become.