Dawn Journal: The Final Countdown

Dear Vernal Dawnquinoxes,

A veteran explorer is leisurely orbiting the only dwarf planet in the inner solar system. Measuring space radiation high over Ceres, Dawn revolves once every 30 days in its gravitational master's firm grip. Dawn is well-known for its patience, and the pace of its activities has been decidedly relaxed in this orbit. That is about to change. There is now only one revolution to go before the spacecraft begins the final campaign of its long and rewarding deep-space adventure.

For eight months in 2015-2016, Dawn circled Ceres once every 5.4 hours at only 240 miles (385 kilometers). (The orbit has been variously designated as LAMO, then XMO1, and often as "the lowest orbit.") It then flew higher to pursue new objectives. The probe's orbit now takes it from slightly under 2,800 miles (4,400 kilometers) up to 24,300 miles (39,100 kilometers) and then back down again. (These values are a little different from what we presented in December, principally because the Sun's gravity gradually alters the orbit.) The orbit is known to people who call it extended mission orbit 5, or XMO5, as "extended mission orbit 5" or "XMO5" (following the nomenclature described here). XMO5 is illustrated in a figure below.

In contrast to the distant, serene probe, the operations team has been working quite intensively to prepare for a bold new phase of the mission. They have been assiduously working through all the tasks necessary to prepare for piloting this unique spaceship, late in its life and low on supplies, through maneuvers it was never designed for and to conduct observations never conceived of prior to late last year. Since the previous Dawn Journal, the team has generated more than 45,000 trajectories to study how to fly Dawn to two new orbits. Often there are more than 100 computers operating simultaneously to perform the necessary calculations. Many thousands more trajectories are yet to be computed and analyzed. If all goes well, by June, the probe will have followed an intricate flight plan that will allow it to glide a mere 22 miles (35 kilometers) above the alien landscapes almost every day in an orbit dramatically and poetically designated XMO7 (but occasionally summarized as "Whoa, that's low!").

Let's take a look at some of the plans the flight team is developing. As always, we will provide more details when Dawn is executing its complex assignments. In addition, as some parts of the plan are still being refined, there may be a few changes, and we will keep you updated on those as well. But plans are firm enough now that a preview is warranted.

On April 17, the spacecraft will fire up ion engine #2 and begin a downward spiral, gradually shrinking its elliptical orbit. Along the way to its final space destination, XMO7, the ship will moor at an intermediate orbit. On May 14, when it is in an orbit that ranges from about 235 miles (375 kilometers) to almost 3,000 miles (4,800 kilometers), it will shut down the engine. (This orbit is illustrated in the next two figures below.)

It is only coincidental that the lowest altitude of this intermediate orbit, XMO6, is so close to height of the lowest orbit so far. Indeed, the lowest point is not the most important point. The motivation for stopping in XMO6 is to collect infrared spectra and take pictures in the southern hemisphere in a range of about 900 miles (1,500 kilometers) to 1,600 miles (2,500 kilometers). It just so happens that when flying from XMO5 to XMO7, an orbit that provides that viewing opportunity dips down to the height of LAMO/XMO1 elsewhere in the orbit.

The XMO6 altitude in the south was chosen to be comparable to the altitude from which Dawn observed Ceres so extensively in its third and fifth mapping orbits (known as HAMO and XMO2, respectively). XMO6 will afford the probe views of the terrain with the illumination of southern summer that will make for the best comparison with what it has already observed farther north on the dwarf planet. Dawn photographed all of Ceres in full color in those earlier orbits, but it was not possible then to cover the vast surface with the infrared mapping spectrometer, which has a much smaller field of view than the camera. Therefore, scientists had focused their spectral mapping in the northern hemisphere, taking advantage of the lighting then. While some of the southern hemisphere was studied in infrared as well, the opportunity now to observe more of it will allow a more complete understanding of the distribution of minerals.

In XMO6 the spacecraft will fly over the south pole and then head north over the hemisphere of Ceres facing the Sun. It will go lower and lower as it does so. The lowest point in the orbit will occur between 50° and 60°N. Dawn already mapped that territory from LAMO/XMO1, but now it will take advantage of being low again to acquire some new color photography in the northern hemisphere.

As the spacecraft continues farther north, the altitude will increase again. It will sail higher as it travels over the night side before beginning its fall back down. It will take about 37 hours to complete one elliptical revolution.

Some readers may recall that for all of the mapping orbits at Vesta and Ceres, Dawn traveled south over the sunlit side and north over the hemisphere shrouded in the dark of night. (Readers who don't recall that are invited to trust that it's true.) Experts readily recognize that it is very, very difficult to reverse the orbital direction. Dawn did so, however, with the extensive maneuvering in February-April 2017 that allowed it to make the unique observation of opposition. Those who are interested can review the skilled piloting that reversed the direction.

The explorer will observe Ceres on 10 consecutive orbits in XMO6. To conserve precious hydrazine, Dawn will turn to point its main antenna to Earth and radio its findings after every other transit over the sunlit landscapes. In the other orbits, it will wait patiently, saving both data and hydrazine onboard for later.

On May 31, the spaceship will resume maneuvering. It will take about a week of ion thrusting to push down to the final orbit of the mission.

In XMO7 (shown in the two figures below), Dawn will range from as high as 2,500 miles (4,000 kilometers) to as low as about 22 miles (35 kilometers). (The minimum altitude will vary by a few miles, or kilometers, from revolution to revolution, for reasons we will explain in a future Dawn Journal.) It will take a little more than a day to complete one loop.

We have described before that photography will be very challenging, both because of the difficulty pointing the camera accurately enough to capture specific targets and the high speed so close to the ground. We will return to this problem in an upcoming Dawn Journal.

At the high point of XMO7, Dawn will move at only about 120 mph (190 kph). Then as gravity pulls it back down, the spacecraft will accelerate until it streaks northward at 1,050 mph (1,690 kph) above a relatively narrow strip of ground before starting to soar up again. Dawn was designed for mapping uncharted worlds, not making specialized observations under such conditions, and traveling so fast and so low means it cannot take pictures as sharp as you might expect. Nevertheless, even with a little bit of motion-induced blur at low altitude, any sights we photograph certainly will reveal finer details than we have seen before. This is going to be exciting!

The highest priority measurements will be the nuclear spectra, giving scientists the opportunity to take a sharper picture of the elemental composition of the faraway world, making a more accurate map of the concentration of atomic species that are important for Ceres' geology and chemistry. Dawn's gamma ray and neutron detector (GRaND) is not subject to the limitations of pointing accuracy and blur that can affect the photography. You can think of GRaND's gamma ray vision and its neutron vision as being broader but less acute than the camera's visible-light vision. Getting closer to the ground will help ensure the instrument sees a stronger nuclear signal than ever before and takes a clearer picture.

As the spacecraft races over the ground, GRaND will measure gamma rays and neutrons escaping into space from the atoms down to about a yard (meter) underground. It collected a large volume of such data from LAMO/XMO1, but being so much lower in XMO7 will allow scientists to identify and locate elements more accurately.

There are several GRaND (if not grand) objectives for XMO7. One is to see how the elemental composition differs at different latitudes. The instrument has already revealed that water is more plentiful near the surface at higher latitudes than near the equator, and now it may be able to refine this finding. One of the properties of XMO7 is that the low point will shift almost 2° of latitude south on each revolution. That is, each time Dawn swoops down to its lowest point, it will be south of the low point on the previous orbit. That will provide GRaND the opportunity to survey the concentration and distribution of underground ice at different latitudes. GRaND also may tell us more about other constituents, providing clues about the geological processes that shaped this exotic world. 

Of course, as Dawn orbits Ceres, Ceres turns on its axis, pirouetting beneath her admiring companion. So each time Dawn zooms down for a close look, it will not only be farther south than the time before but it will also be at a different longitude. The next Dawn Journal will focus on this and what it means for GRaND and for photography.

Controlling Dawn's orientation in the zero-gravity of spaceflight is harder at low altitude, where Ceres' gravitational pull is stronger. Dawn will use hydrazine much more quickly in XMO7 than at any other part of the mission, and the last of the propellant will be expended before the end of this year.

Dawn just celebrated the third anniversary of arriving at its permanent residence in the solar system. In the natural perspective of its current home, Dawn arrived about two-thirds of a Cerean year ago, or nearly 3,000 Cerean days ago. The explorer has now completed 1,600 orbits. Although hydrazine is dwindling, and the adventure is nearing its end, there is still plenty to look forward to. Stay onboard as Dawn prepares to delve further into the unknown. It's going to be a great ride!

Dawn is 10,800 miles (17,400 kilometers) from Ceres. It is also 1.87 AU (174 million miles, or 280 million kilometers) from Earth, or 740 times as far as the Moon and 1.88 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.

Dr. Marc Rayman
9:15 am PDT March 20, 2018