Tag Archives: NASA

The Bright Spots on Ceres Come into Focus

Peter Lobner

In my 20 March 2015 post, I discussed the Dawn spacecraft mission to the large asteroid Vesta and the dwarf planet Ceres, both of which are in the main asteroid belt between Mars and Jupiter. Dawn arrived in orbit around Ceres on 6 March 2015, at an initial altitude of 8,400 miles (13,518 kilometers). On approach and from this high altitude orbit, Dawn photographed two very bright spots on the surface of Ceres.

Ceres seen from Dawn  Source: NASA

After spending six months mapping the surface of Ceres and gradually descending to lower altitude orbits, Dawn currently is in a much lower “high-altitude mapping orbit” (HAMO) at 915 miles (1,470 kilometers) above the surface. Ceres’ diameter is about 587 miles (946 kilometers). Due to the low mass of this dwarf planet, Dawn’s orbital speed in the HAMO is only 400 mph (645 kph). The spacecraft completes one orbit in about 19 hours.

From its current vantage point in HAMO, Dawn has provided a much better view of the bright spots on Ceres. The following composite photo shows the bright spots at a resolution of 450 feet (140 meters) per pixel.

ceres-bright-spots-Sep2015,jpg  Source: NASA

The source of the bright spots has not yet been determined. We’ll get a more detailed view later in 2015, when the spacecraft descends to the “low altitude mapping orbit” (LAMO) at an altitude of 230 miles (370 kilometers).

You can keep up with the work of the Dawn project team at the following NASA / Jet Propulsion Lab website:


 9 December 2015 Update:

NASA’s Jet Propulsion Laboratory (JPL) released closeup photos of the bright spots, which appear to be globally distributed on Ceres. JPL scientists reported that Ceres has more than 130 bright areas, and most of them appear to be associated with impact craters.   There is evidence that the bright spots may be salt deposits left behind after a subterranean briny water-ice deposit was exposed by an impact and the  ice-water sublimated into space.  Here is a closeup, false-color photo of the Occator Crater, emphasizing the deposits of bright material on the crater floor.

Occator Crater - Ceres_JPL

You can read more on this subject on the JPL website at the following link:


1 November 2018 Update:

On 1 November 2018, NASA reported the end of the Dawn mission:

“Dawn missed scheduled communications sessions with NASA’s Deep Space Network on Wednesday, Oct. 31, and Thursday, Nov. 1. After the flight team eliminated other possible causes for the missed communications, mission managers concluded that the spacecraft finally ran out of hydrazine, the fuel that enables the spacecraft to control its pointing. Dawn can no longer keep its antennae trained on Earth to communicate with mission control or turn its solar panels to the Sun to recharge.”

You’ll find more information about the Dawn mission and its many accomplishments on the NASA / JPL website at the following link:


New Horizons Spacecraft Rapidly Approaching Encounter with Pluto

Peter Lobner

New Horizons is rapidly approaching Pluto for a fast fly-by encounter with closest approach at 7:49 am on Tuesday, 14 July 2015. You’ll find basic information about the New Horizons mission in my 14 March 2015 post on this subject. Detailed information is available at the NASA New Horizons mission website at the following link:


The spacecraft will fly past Pluto at 30,800 mph (49,600 kph), and is expected to fly within 7,750 miles (11,265 km) of Pluto’s surface. The close-encounter segment of the flyby is quite brief, as shown in the following diagram of New Horizon’s trajectory through the Pluto system.

New Horizons trajectorySource: NASA/Applied Physics Laboratory/Southwest Research Institute

On 9 July, New Horizon’s Long Range Reconnaissance Imager (Lorri) took the following photo from a range of 3.3 million miles. Some basic surface features have been noted by the NASA project team, along with a diagram indicating Pluto’s north pole, equator, and central meridian.

Pluto pic 1

Source: NASA/Applied Physics Laboratory/Southwest Research Institute

On 11 July, the project team released the following slightly more detailed photo that reveals linear features that may be cliffs, as well as a circular feature that could be an impact crater.

Pluto pic 2

Source: NASA/Applied Physics Laboratory/Southwest Research Institute

Below is a photo released on 9 July showing both Pluto and it’s largest moon, Charon, which orbit each other around their common center of gravity. You’ll find more information on the unusual orbital interactions among Pluto and it’s five known moons in my 6 June 2015 post on that subject.

Pluto pic 3

Source: NASA/Applied Physics Laboratory/Southwest Research Institute

Hubble Space Telescope 25th Anniversary Didn’t Come Easily

Peter Lobner

The Hubble Space Telescope was launched on 24 April 1990 by the space shuttle Discovery on mission STS-31, and was deployed into orbit on 25 April. You can find details on the design of Hubble at the following link:


During system checkout, it was determined that a design error had been made and Hubble’s primary optics suffered from spherical aberration. This optical problem was corrected in 1993 on Servicing Mission 1 (SM1), which also resolved several other issues. Over Hubble’s 25 year operating life, five servicing missions were conducted by space shuttle astronauts.

SM-1 – launched 2 Dec 1993, shuttle Endeavour
SM-2 – launched 11 Feb 1997, shuttle Discovery
SM-3A – launched 19 Dec 1999, shuttle Discovery
SM-3B – launched 1 Mar 2002, shuttle Columbia
SM-4 – launched 11 May 2009, shuttle Atlantis

The Hubble today is quite a different machine than the one launched in 1990. You can see details of each servicing mission at the following NASA website:


NASA’s Hubble mission website is at the following link:


Here you have access to details about Hubble’s 25-year mission, including an extensive photo gallery. NASA’s official photo to commemorate the 25th anniversary is the following photo of the Westerlund 2 star cluster taken by Hubble’s near-infrared Wide-Field Camera 3, which was installed during SM-4.

image Source:  NASA

Messenger Spacecraft Mission at Mercury About to End

Peter Lobner

Updated 12 January 2016

The 1,069 pound Messenger (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft is only the second spacecraft sent to Mercury. Mariner 10 flew past Mercury three times in 1974 and 1975. Messenger was launched on 3 August 2004 and flew for 6-1/2 years on a circuitous trajectory that included 15 orbits of the sun, one flyby of Earth, two flybys of Venus, and three flybys of Mercury before entering orbit around Mercury in 18 March 2011. The series of planetary flybys allowed Messenger to decelerate relative to Mercury and achieve orbit with minimal use of fuel.

The NASA Messenger mission website is at the following link:


Messenger is solar-powered, with its science payload and propulsion system located behind a sunshade to protect against the intense solar radiation encountered at Mercury’s close orbit of the Sun.

image  Source: Johns Hopkins University/APL

Messenger has instrumentation for mapping and characterizing Mercury using imaging cameras, laser altimeter, various spectrometers, magnetometer, and a radio science package to measure slight velocity changes in orbit. You can read details on the spacecraft instrumentation systems at the following link:


After four years in orbit, fuel needed to maintain orbit is expected to be depleted in April. Messenger’s orbit will decay and the spacecraft eventually will crash at perigee into Mercury’s surface at its orbital speed of 8,750 mph.

12 January 2016 update:

On 30 April 2015, Messenger crashed into the surface of Mercury on the side facing away from Earth.  Before crashing, Messenger orbited Mercury 4,105 times and collected more than 277,000 images.  A composite photograph of Mercury created from thousands of Messenger images is shown below:

Mercury composite imaages from MessengerSource: NASA /  Johns Hopkins University/APL

Radioisotope Thermoelectric Generators (RTG) for Spacecraft: History and Current U.S. Pu-238 Production Status

Updated 5 March 2021

Peter Lobner

Radioisotope Thermoelectric Generators (RTG), also called Radioisotope Power Systems (RTS), commonly use non-weapons grade Plutonium 238 (Pu-238) to generate electric power and heat for National Aeronautics and Space Administration (NASA) spacecraft when solar energy and batteries are not adequate for the intended mission. In comparison to other RTG heat sources (Strontium-90, Cesium-137), Pu-238 has a relatively long half-life of 87.75 years, which is a desirable property for a long-life RTG.

Approximately 300 kg (661 lb) of Pu-238 was produced by the Department of Energy (DOE) at the Savannah River Site between 1959 – 1988. After U.S production stopped, the U.S. purchased Pu-238 from Russia until that source of supply ended in 2010.

Limited production of new Pu-238 in the U.S re-started in 2013 using the process shown below. This effort is partially funded by NASA.  Eventually, production capacity will be about 1.5 kg (3.3 lb) Pu-238 per year. The roles of the DOE national laboratories involved in this production process are as follows:

  • Idaho National Engineering Lab (INEL):
    • Store the Neptunium dioxide (NpO2) feed stock
    • Deliver feed stock as needed to ORNL
    • Irradiate targets provided by ORNL in the Advanced Test Reactor (ATR)
    • Return irradiated targets to ORNL for processing
  • Oak Ridge National Lab (ORNL):
    • Manufacture targets
    • Ship some targets to INEL for irradiation
    • Irradiate the remaining targets in the High Flux Isotope Reactor (HFIR)
    • Process all irradiated targets to recover and purify Pu-238
    • Convert Pu-238 to oxide and deliver as needed to LANL
  • Los Alamos National Lab (LANL):
    • Manufacture the Pu-238 fuel pellets for use in RTGs

Pu-238 production process

Source: Ralph L McNutt, Jr, Johns Hopkins University APL, 2014

In 2015, the U.S. had an existing inventory of about 35 kg (77 lb) of Pu-238 of various ages.  About half was young enough to meet the power specifications of planned NASA spacecraft. The remaining stock was more than 20 years old, has decayed significantly since it was produced, and did not meet specifications.  The existing inventory will be blended with newly produced Pu-238 to extend the usable inventory. To get the energy density needed for space missions while extending the supply of Pu-238, DOE and NASA plan to blend “old” Pu-238 with newly produced Pu-238 in 2:1 proportions.

Since 2010, NASA’s RTG for spacecraft missions has been the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which It is based on the SNAP-19 RTG flown on the two Viking Mars landers (circa 1975) and the Pioneer 10 and 11 deep space probes (circa 1972). At beginning of life, the current MMRTG can provide about 2,000 watts of thermal power and 110 watts of electrical power from eight General Purpose Heat Source (GPHS) modules that contain a total of 10.6 pounds (4.8 kilograms) of plutonium dioxide fuel. Electric conversion efficiency is about 6%.

Assembled MMRTG on a transport dolly.  Source: NASA

MMRTG cut-away diagram.  Source: NASA

You’ll find a NASA MMRTG Fact Sheet here:  https://rps.nasa.gov/resources/86/multi-mission-radioisotope-thermoelectric-generator-mmrtg/?category=fact_sheets

NASA had a program to develop an Advanced Stirling Radioisotope Generator (ASRG), which was designed to produce about four times the power of the MMRTG per unit of Pu-238. Electric conversion efficiency was about 26%. The ASRG required a total of 2.7 pounds (1.2 kilograms) of plutonium dioxide in two GPHS modules. However, the ASRG would produce less waste heat, which can be used productively to warm electronics in the interior of a spacecraft, such as the Mars rover Curiosity.  In November 2013, NASA announced that ASRG development had been discontinued because of budget cuts. You’ll find a NASA ASRG Fact Sheet at the following link:  https://rps.nasa.gov/resources/65/advanced-stirling-radioisotope-generator-asrg/

You can read a history of RTGs and more information on current U.S. Pu-238 production status in a 2014 presentation by Ralph L McNutt, Jr, at the following link: https://www.lpi.usra.edu/sbag/meetings/jan2014/presentations/08_1545_McNutt_Pu238_SBAG.pdf

9 February 2016 Update:

On 22 December 2015, DOE reported the first U.S. production in nearly 30 years of Pu-238.   This production demonstration, which was partially funded by the NASA, was performed at ORNL and yielded 50 grams of Pu-238.  The last U.S. production of Pu-238 occurred in the late 1980s at the Savannah River Plant in South Carolina.

DOE reported that it plans to set an initial production target of 300 – 400 grams (about 12 ounces) of Pu-238 per year. After implementing greater automation and scaling up the process, ORNL expects to reach the the production target of 1.5 kg (3.3 lb) Pu-238 per year.

The next NASA mission that will use an RTG is the Mars 2020 rover, which will use an MMRTG,  as used on NASA’s Mars rover Curiosity. 

You can read the ORNL announcement of initial Pu-238 production at the following link: https://www.ornl.gov/news/ornl-achieves-milestone-plutonium-238-sample

3 January 2019 Update:

In the past three years, ORNL has made scant progress in producing Pu-238.  In a 13 December 2018 article, “NASA Doesn’t Have Enough Nuclear Fuel For Its Deep Space Missions,”author Ethan Siegel reports:  “Although current production (at ORNL) yields only a few hundred grams per year (less than a pound), the laboratory has the eventual goal of ramping up to 1.5 kilograms (3.3 pounds) per year by 2023, at the earliest.  Ontario Power Generation in Canada has also begun producing Pu-238, with the goal of using it as a supplemental source for NASA.”  You can read the complete article on the Forbes website at the following link: https://www.forbes.com/sites/startswithabang/2018/12/13/nasa-doesnt-have-enough-nuclear-fuel-for-its-deep-space-missions/#1a73d47e1c18

The Canadian plans for becoming a source of Pu-238 was announced on 1 March 2017:  “Ontario Power Generation (OPG) and its venture arm, Canadian Nuclear Partners, are participating in a project to produce isotopes in support of deep space exploration. Under the agreement, OPG would help create isotopes at the Darlington nuclear station east of Toronto that will help power space probes.” You can read the complete OPG press release here: https://www.opg.com/news-and-media/news-releases/Documents/20170301_DeepSpace.pdf

Also see the OPG public relations piece, “OPG looks to the stars,”  here: https://www.opg.com/news-and-media/our-stories/Documents/20170802_OPG_Deep_Space.pdf

4 August 2020 Update:

The NASA Mars rover, Perseverance, was launched from Cape Canaveral on 30 July 2020, with an expected landing date of 18 February 2021 in the Jezero crater on Mars. Once on the surface, Perseverance will be powered by an MMRTG.

The Pu-238 and some other special materials for the Perseverence MMRTG were produced in the U.S. at ORNL, as described in the following short (2:03 minutes) video, “ORNL-produced tech fuels NASA’s Perseverance mission to Mars”:

In a 20 July 2020 news release, ORNL provided more information on the U.S. production process for Pu-238 and reported that, “the lab has been consistently increasing its Pu-238 production capabilities, aiming to produce 1.5 kilograms per year by 2026.”  You can read this ORNL press release here: https://www.ornl.gov/news/ornl-produced-plutonium-238-help-power-perseverance-mars

At the planned U.S. production rate for Pu-238, NASA should be able to conduct an MMRTG mission at about four-year intervals. If NASA MMRTG missions will be more frequent than this, the U.S. will need to purchase additional Pu-238 from another source, perhaps Canada.

5 March 2021 Update:

The Perseverance rover landed on Mars on 18 February 2021, in the planned target area in Jezero Crater.  Power from the MMRTG was nominal after landing.  Perseverance will spend at least one Mars year (two Earth years) exploring the landing site region.

The next NASA mission with an MMRTG-powered spacecraft is the Dragonfly mission to Saturn’s moon Titan, which will launch in 2026 and arrive on Titan in 2034.

The Voyager 1 and 2 spacecraft were launched in 1977, each with three RTGs delivering a maximum of 470 watts of electrical power at the beginning of the mission.  Both spacecraft have left the solar system (Voyager 1 in 2013 and Voyager 2 in 2018) and continue to transmit from interstellar space in 2021 with their RTGs operating at a reduced power level of about 331 watts after 44 years of Pu-238 decay during the mission.  NASA plans to continue the Voyager missions until at least 2025.

For more information:

Where on Earth Does Lightning Flash Most?

Peter Lobner

According to satellite observations summarized in the following map, lightning occurs more often over land than over the oceans and more often closer to the equator.

image   Source: NASA

The map above shows the average yearly counts of lightning flashes per square kilometer from 1995 to 2013. The map was created using data collected from 1998–2013 by the Lightning Imaging Sensor (LIS) on NASA’s Tropical Rainfall Measuring Mission satellite, and from 1995–2000 by the Optical Transient Detector (OTD) on the OrbView 1/Microlab satellite. Flashes above 38 degrees North were observed by OTD only, as the satellite flew to higher latitudes.

Areas with the fewest number of flashes each year are gray or purple; areas with the largest number of lightning flashes—as many as 150 per year per square kilometer—are bright pink.  Be careful where you pitch your tent if you go on safari in central Africa.

Check out the story at the following link:


Spitzer Space Telescope “Warm Mission” Continued into 2020

Peter Lobner

Updated 19 February 2020

The Spitzer Space Telescope, an infrared space observatory, was launched on 8 August 2003 into an “earth-trailing” orbit around the Sun. It is one of four “Great Observatories” launched by NASA; the others being the Hubble Space Telescope, the Compton Gamma-ray Observatory; and the Chandra X-ray Observatory.

Spitzer_Telescope_Handbook013   Diagram source: NASA

The primary mirror is 85 cm in diameter, made of beryllium, and until May 2009, was cooled by liquid helium to 5.5 degrees K. With the on-board liquid helium supply exhausted, most of the instruments were no longer usable. However, the two shortest wavelength modules of the Infrared Science Archive (IRAC) camera remained operable at their original sensitivities. This allowed the mission team to continue with the “Spitzer Warm Mission”.

You can read about the design of the Spitzer Space Telescope at the following link:


An example of an image from the Spitzer Space Telescope is this view of Eta Carinae:

The tortured clouds of Eta Carinae  Photo source: NASA

You can see this and many other images from the Spitzer telescope, and related image data, at the following NASA / JPL / Caltech website:


Update 19 February 2020

On 30 January 2020, NASA reported,

“After more than 16 years studying the universe in infrared light, revealing new wonders in our solar system, our galaxy and beyond, NASA’s Spitzer Space Telescope’s mission has come to an end…..the spacecraft was placed in a safe mode, ceasing all scientific operations.”

You can read the NASA announcement and a summary of the accomplishments of the Spitzer mission here:


NASA’s Opportunity Rover Just Completed an 11-year Marathon on Mars

Peter Lobner

The Opportunity rover landed on Mars on January 25, 2004. It originally was designed for a 90 Sol mission (a Sol, one Martian day, is slightly longer than an Earth day at 24 hours and 37 minutes). The rover has proven to be far more durable than expected.  A marathon is a long-distance running event with an official distance of 42.195 kilometers (26 miles and 385 yards).  The rover Opportunity is the first extraterrestrial rover to complete this distance. The record time for this event was 11 years and 2 months.

Opportunity rover Rover Opportunity; source:  NASA

Opportunity path om Mars 26+ miles    Map of the marathon trek; source: NASA

8 July 2015 Update:

On 2 July 2015, NASA/JPL uploaded an 8-minute time-lapse video of rover Opportunity’s marathon road trip on Mars. This compilation of images from the low-resolution, wide-field-of-view hazard-avoidance cameras on rover Opportunity between January 2004 and April 2015 shows the rover’s-eye-view of the Martian marathon from its landing location to the rim of Endeavour Crater. The sound in the video is synthesized from accelerometers on rover Opportunity.  You can view this video using the link below.  The view from the rover is in the left-hand panel, and a map of the rover’s path is on the right-hand panel of the video.

The URL for this embedded video is:


Kepler Observatory has Built an Impressive Record for Detecting Extrasolar Planets

Peter Lobner

Updated 18 May 2016 and 30 October 2018

Kepler observatory Source: NASA

NASA’s Kepler extrasolar planet (exoplanet) hunting observatory was launched on 7 March 2009 into an Earth-trailing orbit around the Sun. From this vantage point, Kepler has a continuous, unobstructed view of the selected target areas in the “northern” sky, above the ecliptic plane (plane of the solar system).  Kepler has a very narrow field of view, which is about twice the size of the scoop of the Big Dipper as we see it from Earth (about 1/400th of the sky).

NASA’s Kepler mission website is at the following link:


The Kepler mission team primarily uses the “transit” method to detect exoplanets. NASA explains the transit method as follows:

 “When a planet passes in front of a star as viewed from Earth, the event is called a ‘transit’. On Earth, we can observe an occasional Venus or Mercury transit. These events are seen as a small black dot creeping across the Sun — Venus or Mercury blocks sunlight as the planet moves between the Sun and us. Kepler finds planets by looking for tiny dips in the brightness of a star when a planet crosses in front of it — we say the planet transits the star.

Once detected, the planet’s orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler’s Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet’s characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered”.

The following figure illustrates the effect on brightness of a planet transiting across a star.

Kepler transit methodSource: NASA Ames

Kepler originally used four reaction wheels to orient the spacecraft to observe specific regions of the sky.  After failure of a second reaction wheel in 2013, Kepler no longer was able to maintain a desired orientation with sufficient accuracy.  The mission was stopped and the spacecraft was put in “safe mode” until a modified mission plan, dubbed “K2” or “Second Light” was tested and then implemented in May 2014.  This K2 mission plan defined new target areas that could be observed using the two remaining reaction wheels and thrusters to orient and stabilize the spacecraft.  Measured pointing accuracy in the K2 configuration has been shown to be almost equal to the pointing accuracy with three reaction wheels.  The K2 mission is expected to continue through the end of 2016.

The Kepler mission team announced discovery of the first Earth-sized planet in the habitable zone of a star in April 2014.  This planet, named Kepler-186f, orbits a red dwarf, the most common type of star in the Milky Way, about 500 light years from Earth.

You will find current information on the confirmed and candidate exoplanets discovered by Kepler at the following link to the NASA Kepler website:


In March 2015, NASA’s Kepler team won the National Air and Space Museum 2015 Trophy for Current Achievement.  Read more about this award at the following link:


 Update – 18 May 2016:

The 10 May 2016 issue of The Astrophysical Journal includes an article entitled, “False Positive Probabilities for all Kepler Objects of Interest: 1284 Newly Validated Planets and 428 Likely False Positives,” by T.D. Morton et al. This paper reports the results of the first large-scale demonstration of a fully automated transiting planet validation process that calculates the probability of an observation being a false positive. A remarkable aspect of this work is that the automated validation process was applied to every Kepler Object of Interest (KOI). The key results reported in this paper are as follows:

  • Out of 7,056 KOIs, the team determine that 1,935 have probabilities <1% of being astrophysical false positives, and thus may be considered validated planets.
  • Of these, 1,284 have not yet been validated or confirmed by other methods.
  • 428 KOIs are likely to be false positives, but have not yet been identified as such by other means

You can download the complete paper at the following link:


Update – 30 October 2018:

NASA announced that the Kepler Space Telescope ran out of fuel today and no longer could orient itself to study cosmic objects or transmit data to Earth.

“As NASA’s first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington. “Not only did it show us how many planets could be out there, it sparked an entirely new and robust field of research that has taken the science community by storm. Its discoveries have shed a new light on our place in the universe, and illuminated the tantalizing mysteries and possibilities among the stars.”

You can read the NASA press release here:


The complete Kepler and K2 missions are described in detail on the NASA website:


The Dawn Mission to Vesta and Ceres

Peter Lobner

The Dawn spacecraft is the only spaceship built to orbit two extraterrestrial bodies; the large asteroid Vesta and the dwarf planet Ceres, both of which are in the main asteroid belt between Mars and Jupiter. You can see that Vesta and Ceres are quite small in comparison to Earth’s moon.

Small planet comparison Source: NASA

Dawn spacecraft showing ion propulsion engine firing:

Dawn_Flight_Configuration Source: NASA

Dawn’s mission was enabled by an advanced solar-electric xenon ion propulsion system. You can read more about the propulsion system and Dawn’s instrumentation at the following NASA link:


Launched in September 2007, Dawn made a gravity-assist flyby of Mars before reaching Vesta in July 2011 and remaining in orbit for about 14 months.

View of Vesta from Dawn

Vesta_from_Dawn Vesta. Source: NASA

From the collected data, NASA created a 360 degree view of Vesta, which you can see at the following link:


Using its ion propulsion system, Dawn departed Vesta in September 2012 and headed for it’s rendezvous with Ceres, arriving in orbit on 6 March 2015. You can read about the orbital insertion and see a 360 degree view of Ceres at the following link:


View of Ceres from Dawn, showing two bright spots of unknown origin.

Ceres seen from Dawn Ceres.  Source: NASA

Dawn’s initial orbit was at an altitude of 8,400 miles above Ceres. Over the next month, Dawn is using it’s ion propulsion system to spiral down to a “survey orbit” 2,700 miles above Ceres. After the survey is complete, the ion propulsion system will be used again to spiral down gradually to a 910 mile “high-altitude mapping orbit”. Late in 2015, the spacecraft will descend again until it reaches a “low altitude mapping orbit” at an altitude of 230 miles. Because of the very low thrust of the ion propulsion engines, the transitions between orbits takes a long time (several weeks). You can read the details about the in-orbit management of the spacecraft at the following link: