The Dawn mission is going to command our attention this summer, with orbital capture around Vesta in mid-July and science data collection beginning a scant two weeks later as the spacecraft produces topographical maps from an altitude of approximately 2700 kilometers. Orbiting the main belt asteroid for one year, Dawn will close to within 200 kilometers to obtain closer images and perform other scientific measurements. And then it’s back into cruise mode for the trip to Ceres, another object of intense interest regarding Solar System formation.

This ambitious mission is the first ever mounted to orbit two Solar System targets. The image below is Dawn’s first look at Vesta, a mere five pixels across in these early approach images. This is the first image of the asteroid Dawn has taken, and images like it will be used to help scientists analyze Vesta’s precise location against background stars, helping to tune up the spacecraft’s trajectory on the approach. The image was taken when Dawn was some 1.21 million kilometers from the asteroid. Vesta is 530 kilometers in diameter and the second most massive object in the asteroid belt. We’ll be seeing it a lot better in coming months.

Image: This image, processed to show the true size of the giant asteroid Vesta, shows Vesta in front of a spectacular background of stars. It was obtained by the framing camera aboard NASA’s Dawn spacecraft on May 3, 2011, from a distance of about 1.2 million kilometers (750,000 miles). Since Vesta is so bright that it outshines its starry background, Dawn team members commanded a long exposure time to make the stars visible. They corrected the resulting exaggerated size of Vesta by superimposing a short exposure image of the target asteroid, showing its true size. Vesta is the small, bright pearl in the middle of the image. Credit: NASA/JPL.

Right now we’re still early in the approach phase, a time when Dawn is using its ion thrusters to match the asteroid’s orbit around the Sun, allowing it to enter into a spiraling orbit that begins with capture at about 16,000 kilometers. Also active are Dawn’s gamma ray and neutron detector instrument as well as its visible and infrared mapping spectrometer, taking early measurements and testing calibration to make sure they are ready for Vesta orbital operations.

“We feel a little like Columbus approaching the shores of the New World,” said Christopher Russell, Dawn principal investigator, based at the University of California in Los Angeles (UCLA). “The Dawn team can’t wait to start mapping this Terra Incognita.”

Yes, Vesta does have that feel about it, just as New Horizons will when it approaches Pluto/Charon. These are places we’ve only been able to examine from ground and space-based telescopes in the past, and as Dawn chief engineer Marc Rayman puts it, “Dawn will soon bring into focus an entire world that has been, for most of the two centuries scientists have been studying it, little more than a pinpoint of light.” Dawn’s thrusting will be interrupted once a week this month to take new images as Vesta goes from five pixels across to 12.

Rayman’s online journal of the Dawn mission is worth watching. In an entry from last year, he explains that Dawn’s ion thrusters allow it to gradually reshape its orbit around the Sun for orbital insertion at Vesta, a process far different from more conventional propulsion methods. Dawn’s thrusters are 10,000 times less powerful than the typical propulsion system on an interplanetary spacecraft, a gentle push that allows an entirely different flight profile, as Rayman explains by analogy:

To think about this facet of the difference between achieving this goal with the different technologies, imagine you want to drive your car along next to another traveling west at 100 kilometers per hour (60 miles per hour). The analogy with the conventional technology would be similar to heading north toward an intersection where you know the other car will be. You arrive there at the same time and execute a whiplash-inducing left turn at the last moment using the brakes, steering wheel, accelerator, and probably some adrenaline. When you drive an ion propelled car, operating with 10 times the fuel efficiency, you take a different path from the start, one more like a long, curving entrance ramp to a highway. When you enter the ramp, you slowly (perhaps even gently) build speed. You approach the highway gradually, and by the time you have reached the far end of the ramp, your car is traveling at the same speed and in the same direction as the other car. Of course, to ensure you are there when the other car is, the timing is entirely different from the first method, but the sophisticated techniques of orbital navigation are up to the task.

This July, the paths of Dawn and Vesta will have become so similar that, at a range of 16,000 kilometers, the spacecraft will be traveling at less than 50 meters per second relative to the asteroid. This is the point at which Vesta’s gravity will gradually take charge of Dawn. The ion thrusters will be shut down when the spacecraft finally reaches its ‘survey orbit,’ with each revolution taking about three days. Resumed thrusting after data acquisition will result in lower orbits as Dawn takes a closer look at its target. In 2012, the spacecraft will reverse its spirals and begin climbing away from Vesta as it moves toward its 2.5-year cruise enroute to Ceres.