Dawn | In Final Mapping Orbit

SoccerBallImagine a soccer ball held 3.5″ (9 cm) from your eyes. That is the scale of how close the spacecraft Dawn is to the surface of Ceres in polar orbit of 5.4 hrs. Actual altitude is about 240 mi (385 km), as close as the International Space Station is to Earth. Dawn will remain in this low altitude mapping orbit (LAMO) forever. It does not have enough fuel to move to another orbit or location in space. The primary mission will end in mid-2016. The mission might be extended if science, resources, and money warrant.

The orbit diagram below shows the path of Dawn the final three months of 2015. In October, the spacecraft was 915 mi (1470 km) from Ceres traveling 400 mph (645 km/h) shown as an outer green ring. This orbit was called the high altitude mapping orbit (HAMO). Upon completion of scientific duties, the ion propulsion engine was turned on. That weak thrust caused Dawn to spiral inward toward Ceres. The blue lines in the diagram show the spiral path.

Image Mapping

After nearly 7 weeks of ion engine thrusting, Dawn reached the LAMO shown as the inner green ring. Because it fell deeper into the gravity field of Ceres, it gained speed. Dawn is now going 610 mph (980 km/h). Images are returned daily and are available to browse at this fine link. Bookmark and return often.


NASA | JPL-Caltech

This image from LAMO gives a sense of the detail and perspective possible on the surface of Ceres.

NASA | JPL-Caltech | 12-22-2015 | Click to embiggen


GRaND View Mapping

Because Dawn is in polar orbit the entire surface is visible to the instruments on board. There are three primary types of instrumentation. As noted above, images will allow a detailed visual map to be made of the surface features. In addition, scientists want to know the elements and chemical makeup of the surface. Mapping those from orbit is accomplished using the gamma ray and neutron detector (GRaND). The 21 sensors in GRaND measure the energy of gamma photons and neutrons emitted by matter at the surface. The signals are very weak. The LAMO allows for the strongest signals and most detailed mapping possible.

Cosmic rays from space strike the surface of Ceres. The nucleii of elements are excited by the absorption of the cosmic ray energies. The nucleii in turn emit gamma photons and neutrons to rid themselves of the extra energy. By analyzing the spectra of the gamma and neutron energies, GRaND will allow scientists to learn the composition of the various substances on the surface.

Gravity Field Mapping

Scientists are eager to probe the interior of Ceres over the next months. As Dawn passes over the surface at close range, it will feel slightly speeded up and slowed down by the presence of mass concentrations beneath the surface. The variations in the speed of Dawn will be measured from Earth using the communications antennae. The speed variations will show up as doppler shifts in the frequency of the telemetry signal. A global map of gravity strength, and therefore density of underlying material, will emerge. An example of such a map of Earth is from the GRACE mission. Red bumps indicate high density and stronger than normal gravitational pull. Blue is less density and weaker pull. Images credit: NASA | JPL

Mission updates are journaled periodically by Marc Rayman, Chief Engineer, Mission Director JPL and can be found at this link. I will also include summaries here. Stay tuned.


15 thoughts on “Dawn | In Final Mapping Orbit

  1. The gravity field mapping was most interesting to me. It’s been a while since I thought about Doppler shifts, but it’s clearly one of the phenomena that stuck from science classes. When I went to double-check, my basic understanding was right, but this is far beyond those fire engine examples.

    My hunch was that the low-density return from the Gulf of Mexico is Sigsbee Deep. It sure looks like it from other maps I compared to the graphic. That would make sense, I suppose, given that the Atlantic, Pacific and Great Lakes — other deepwater spots — show low density.

    It seemed counterintuitive to see a low-density return in the middle of Africa, but in fact, that spot is the Congo basin, which not only is threaded with rivers, and geologically depressed, but which also has a huge area of sand and sandstone beneath it.

    What a great and interesting post. It will be fun to see what they come up with in the coming months.

    Liked by 2 people

  2. You have good insights into the geology of the different areas. I also think the gravity field mapping is quite interesting. It is impressive how the slight variations in speed can be analyzed from Earth via Doppler shifts. And to think the first demonstrations of it used musical instruments on board moving rail cars.

    Thank you for your interesting comments.


  3. I like the global map of gravity strength, that I can visually those areas more easily, and NASA has always been so good with imaging. Have you not been doing more photos like you used to?

    Liked by 1 person

    • The European Space Agency landed a probe on a comet last year. Gravity on the surface is very weak. You could jump up and not come down for hours. When you did, the crash would be slow and gentle. Careful not to go too fast. You could escape and never return.

      Liked by 1 person

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