Marc Rayman
Marc Rayman
Chief Engineer/ Mission Director, JPL
Dawn Journal | March 31

by Marc Rayman


Dear Correspondawnts,

Powering its way through deep space, Dawn draws ever closer to dwarf planet Ceres. To reach its destination, the interplanetary spaceship gently reshapes its path around the sun with its extraordinary ion propulsion system. In about a year, the spacecraft will gracefully slip into orbit so it can begin to unveil the nature of the mysterious world of rock and ice, an intriguing protoplanetary remnant from the dawn of the solar system.

Even as Dawn ascends the solar system hill, climbing farther and farther from the sun, penetrating deeper into the main asteroid belt between Mars and Jupiter, its distance to Earth is shrinking. This behavior may be perplexing for readers with a geocentric bias, but to understand it, we can take a broader perspective.

The sun is the conductor of the solar system symphony. Its gravity dictates the movements of everything that orbits it: Earth as well as the other planets, Vesta, Ceres, and myriad smaller objects, including asteroids and Dawn. (Actually, the gravity of every single body affects how all of the others move, but with more than 99 percent of the entire solar system’s mass concentrated in the gargantuan sun, it dominates the gravitational landscape.)

Vesta and Ceres in Virgo

Artist’s concept depicting Dawn thrusting with its ion propulsion system as it travels from Vesta (lower right) to Ceres (upper left). The galaxies in the background are part of the Virgo supercluster. Dawn, Vesta, and Ceres are currently in the constellation Virgo from the perspective of viewers on Earth. Image credit: NASA/JPL

Whether it is for a planet or Dawn orbiting the sun, a spacecraft or moon orbiting a planet, the sun or other stars orbiting the Milky Way (the Milky Way galaxy, that is, not your correspondent’s cat Milky Way), or the Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to an appreciable fraction of our readership), any orbit is the perfect balance between the inward tug of gravity and the inexorable tendency of objects to travel in a straight line. If you attach a weight to a string and swing it around in a circle, the force you use to pull on the string mimics the gravitational force the sun exerts on the bodies that orbit it. The effort you expend in keeping the weight circling serves constantly to redirect its course, forcing it to curve; if you release the string, the weight’s natural motion would take it away in a straight line (we are ignoring here the effect of Earth’s gravity on the weight).

The force of gravity dwindles as the distance increases, so the sun pulls harder on a nearby body than on a farther one. Therefore, to remain in orbit, to balance the relentless gravitational lure, the closer object must travel at higher speed, resisting the stronger attraction. The same effect applies at Earth. Satellites that orbit very close (including, for example, the International Space Station, 250 miles, or 400 kilometers, above the surface) must streak around the planet at about 17,000 mph (7.6 kilometers per second) to avoid being drawn down. The moon, orbiting almost a thousand times farther above, needs only to travel at less than 2300 mph (about 1.0 kilometers per second) to balance Earth’s weaker hold at its remote location.

For that reason, Mercury zips around the sun faster than any of the other planets. Mars travels more slowly than Earth, and the still more distant residents of the asteroid belt, whether natural (all of them but one) or a product of human ingenuity (one: Dawn), proceed at an even more leisurely pace. As Earth makes its relatively rapid annual trip around the sun, the distance to the spacecraft that left it behind in 2007 alternately shrinks and grows.

We can visualize this with one of the popular models of clocks available in the Dawn gift shop on your planet, in which the hour hand is longer than the minute hand. Imagine the sun as being at the center of the clock. The tip of the short minute hand represents Earth, and the end of the hour hand represents Dawn. Some of the time (such as between noon and shortly after 12:30), the distance between the ends of the hands increases. Then the situation reverses as the faster minute hand begins moving closer and closer to the hour hand as the time approaches about 1:05.

Earth and Dawn are exhibiting the same repetitive behavior. Of course, their relative motion is more complicated than that of the clock hands, because Dawn’s ion thrusting is constantly changing its solar orbit (and so the distance and speed at which it loops around the sun), but the principle is the same. They have been drawing closer since August 2013. Earth, coming from behind, is now about to pass Dawn and move ahead. The stalwart probe will not even take note however, as its sights remain firmly set on an unexplored alien world.

Dawn's interplanetary trajectory

Dawn’s interplanetary trajectory from launch through its arrival at Ceres next year. The positions of the spacecraft and Earth are shown on April 10, 2014, when their independent orbits bring them relatively close together. Credit: JPL/NASA

On April 10, the separation will be 1.56 AU (1.56 times the average distance between Earth and the sun, which means 145 million miles, or 233 million kilometers), an almost inconceivably large distance (well in excess of half a million times farther than the International Space Station, which orbits Earth, not the sun) but less than it has been since September 2011. (The skeptical reader may verify this by reviewing the concluding paragraph of each log in the intervening months.) Enjoy the upcoming propinquity while you can! As the ship sails outward from the sun toward Ceres, it will never again be this close to its planet of origin. The next time Earth, taking an inside track, overtakes it, in July 2015 (by which time Dawn will be orbiting Ceres), they will only come within 1.94 AU (180 million miles, or 290 million kilometers) of each other.

By the way, Vesta, the endlessly fascinating protoplanet Dawn unveiled in 2011-2012, will be at its smallest separation from Earth of 1.23 AU (114 million miles, or 183 million km) on April 18. Ceres, still awaiting a visitor from Earth, despite having first been glimpsed from there in 1801, will attain its minimum distance on April 15, when it will be 1.64 AU (153 million miles, or 246 million km) away. It should not be a surprise that Dawn’s distance is intermediate; it is between them as it journeys from one to the other.

A  printable map of Ceres and Vesta's paths through Virgo in 2014--copyright permissions required by Sky & Telescope.

This finder chart can help you locate Vesta and Ceres (and even Dawn, although it is too small to see) in the constellation Virgo. Click it for a larger version (© Sky & Telescope). Credit: Sky & Telescope Magazine

Not only is each one nearly at its shortest geocentric range, but from Earth’s point of view, they all appear to be near each other in the constellation Virgo. In fact, they also look close to Mars, so you can locate these exotic worlds (and even the undetectably small spacecraft) in the evening sky by using the salient red planet as a signpost. In July, the coincidental celestial alignment will make Vesta and Ceres appear to be separated by only one third the diameter of the full Moon, although these behemoths of the asteroid belt will be 0.57 AU (52 million miles, or 85 million kilometers) from each other.

We mentioned above that by constantly modifying its orbit under the persistent pressure of its ion engine, Dawn complicates the simple clock-like behavior of its motion relative to Earth. On Halloween 2012, we were treated to the startling fact that to rendezvous with Ceres, at a greater distance from the sun, Dawn had to come in toward the sun for a portion of its journey — quite a trick! In that memorable log (which is here, for those readers who didn’t find every detail to be so memorable), we observed that it would not be until May 2014 that Dawn would be as far from the sun as it was on Nov. 1, 2012. Sure enough, having faithfully performed all of the complex and intricate choreography since then, it will fly to more than 2.57 AU from the solar system’s star in May, and it will continue heading outward.

With the sun behind it and without regard to where Earth or most other residents of the solar system are in their orbits, Dawn rises to ever greater heights on its extraordinary expedition. Distant though it is, the celestial ambassador is propelled by the burning passion for knowledge, the powerful yearning to reach beyond the horizon, and the noble spirit of adventure of the inhabitants of faraway Earth. The journey ahead presents many unknowns, promising both great challenges and great rewards. That, after all, is the reason for undertaking it, for such voyages enrich the lives of all who share in the grand quest to understand more about the cosmos and our humble place in it.

Dawn is 11 million miles (18 million kilometers) from Ceres. It is also 1.57 AU (146 million miles, or 235 million kilometers) from Earth, or 625 times as far as the moon and 1.57 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 26 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT March 31, 2014

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7 Responses to “Dawn Journal | March 31”

  1. Hi Marc,

    for a 2015 astronomy calendar I’m trying to figure out in which week of the year to best place the story on Dawn’s “arrival” at Ceres. But at this moment not even the proper month is clear to me, as the diagram here says “April ’15” while the one in said “Mar/Apr ’15” – and the one in says “Feb ’15”. Apparently the planning is a bit fluid, but it would be nice to have a target day and be it a NET (no earlier than) one.



    • Marc Rayman says:

      Hi Daniel,

      You are quite correct that the date is a bit fluid. This is a natural consequence of the use of ion propulsion, which provides a degree of flexibility largely unavailable to missions using conventional propulsion. (I have written about this a few times, including on May 27, 2011 and July 29, 2013.) We could pin the date down more precisely, but we can squeeze even more out of the remarkable mission this way (and our operations processes can easily accommodate the uncertainty).

      As you might expect, the actual date gets more firm as we get closer, so some of the variation you refer to is simply a result of the publication date. It is difficult to keep all the old material up to date. (Note that the fact sheet you cite was produced even before Vesta, and Dawn arrived earlier and departed later than what is shown there.)

      Right now we are using March 23 as a convenient “target date” for Ceres arrival, but it easily could be later by a few weeks. It is difficult to specify a “no earlier than” date, but I would suggest March 11. It is important to note that it is quite unlikely to be that early.

      As a reminder, I described in December that well before it is captured into orbit (which is how we define “arrival”), Dawn will send us views of Ceres much better than we currently have. Cool and important as arrival will be, the mission will provide us exciting results even before then.

      I hope this is helpful.


  2. Scott says:

    Oh, you KNOW I’ll be with this one ’till the end, just like I was with DS1!

    And sorry for the typos I did not see them until right as I hit submit.


  3. Scott says:

    Hi Marc,

    I hate to skip ahead, I know there is a LOT oeft to d, but I was wondering if there are plans already for Dawn after Ceres?

    • Marc Rayman says:

      Hi Scott,

      Thank you for your question.

      Dawn will end its remarkable journey of discovery in orbit at Ceres. In brief, because two of its four reaction wheels have stopped working, the mission is critically dependent on the remaining supply of hydrazine propellant, which the spacecraft uses to point its solar arrays at the sun, its sensors at Ceres, its antenna at Earth, or one of its ion engines in the direction needed to travel elsewhere. When it exhausts that precious resource, the mission will conclude. I wrote a little more about the end of the mission in answer to a similar question submitted to my February log.

      That fate is still more than two years away, and you are certainly correct that there is a LOT left to do. After all, we have a mysterious alien world to explore, one that has never been visited. Indeed, there is no other solar system body this large that we have known about for this long (more than two centuries) that we have not yet sent a spacecraft to. So, fantastically successful as Dawn has already been at Vesta, we still have an exciting and ambitious mission ahead.

      I hope you stay with the mission to the end and share in the thrill of the discoveries, the wonder, the challenges, the triumphs, the inspiration of a grand extraterrestrial adventure, and the gratification in what collectively humankind can accomplish as we reach into the cosmos.

      Thanks again for your question.


  4. Ben says:

    Hi Marc,

    Thank you very much for writing this blog.
    If you would be kind enough to please let us know why Hubble is able to clearly resolve images of objects that are light years away, but can’t image Ceres clearly, it would be appreciated. Thank you for your answer and best wishes to you and to the Dawn team.


    • Marc Rayman says:

      Hi Ben,

      Thank you for reading my blog! And I appreciate your submitting a question.

      The key to how clearly Hubble (or, for that matter, you) can see something is a combination of two values: size and distance. You might consider a view of a tree from a few hundred feet away to be very clear, and yet you could hardly appreciate the sight of a leaf if it were only 20 feet away. The distant objects Hubble sees are truly enormous, so they seem to show plenty of detail.

      Dwarf planet Ceres, more than 600 miles (970 kilometers) in diameter, is the giant of the main asteroid belt. But when Hubble imaged it, the alien world was more than 150 million miles (246 million kilometers) away, so Ceres presented a very small target even for the great telescope’s powerful vision. For comparison, this would be like looking at a soccer ball 35 miles (56 kilometers) away. It’s remarkable that Hubble could make out any detail at all!

      Let’s compare that with some other objects Hubble has imaged. One of its many famous photos is of the lovely columns of interstellar gas and dust where stars are forming in the Eagle Nebula. The nebula is 6,500 light-years away. Of course, if Ceres were at such a vast distance, it would be completely undetectable. But the reason the picture is so clear is that we are looking at something huge. The tallest pillar in the photo is four light-years high, nearly the distance from the Sun to Alpha Centauri! For this view, we bring our soccer ball to 862 feet (263 meters).

      While the Eagle Nebula is in the Milky Way galaxy, Hubble takes us even to other galaxies. Another of its photos shows the gorgeous spiral galaxy M83, sometimes called the Southern Pinwheel. At a distance of 31 million light-years, the galaxy appears so sharp because it is 50,000 light-years across. Now it is like seeing a soccer ball less than 220 feet (67 meters) away.

      Instead of using a soccer ball as a reference, we can measure the appearance of Ceres, the Eagle Nebula, and the Southern Pinwheel galaxy in terms of an astronomical object you see regularly: the moon. The moon is so close that you can see it easily with your unaided eye, although you certainly discern more with binoculars or a telescope. (And, by the way, the moon looks the same size as a soccer ball at 81 feet, or 25 meters.) The Southern Pinwheel, despite being millions of light-years away, is so large that it appears to be well over a third the size of the moon. (The light coming from that great distance is far too faint for you to see, but telescopes overcome that by collecting more light with very large optics and long exposures.) The smaller and closer Eagle Nebula is still about one tenth of the size of the moon. The next time you gaze at the moon, think of the relative size of these objects, and you can imagine a powerful telescope like Hubble being able to show a great deal of detail. Ceres, on the other hand, looks to be only 0.00044 times the size of the moon — that’s 2,300 times smaller than the moon. Peer again at the moon, and you can understand that even with the great power of a large telescope above Earth’s atmosphere, it would be very difficult indeed to get a clear view.

      Now we should be careful here. For all of these examples, I have only been considering the angular size of the target. When we look at the fabulous picture of the Southern Pinwheel, it looks crisp because we see many fine details, but those “fine” details are light-years across. The galaxy is so large, however, that there is room for a great deal of variety across the picture, so we deem the picture sharp. The finest details in the Hubble views of Ceres are much, much smaller, only a few tens of miles (kilometers) across. But Ceres itself just isn’t large enough for that to appear to our brains as showing much detail. The entire object only fills a tiny fraction of a Hubble picture. Similarly, when you see a tree, it looks clear if you can see not only its overall shape but also the trunk and many branches and perhaps even distinguish individual leaves. But for a leaf to be clear, you might say you need to be able to make out its shape and see its petiole, midrib and veins. Most people’s criterion for assessing the clarity of a picture is not the absolute amount of detail measured in fractions of an inch, miles, or light-years, but rather how many individual elements (pixels) we can divide the subject of the picture into.

      Hubble’s unique vision transports us to many exciting destinations in the universe, but it cannot take us everywhere. To acquire the sharp views of Ceres that will thrill us and help us unlock many of its intriguing mysteries, we have to use different means to get closer. Thanks to the extraordinary work of the Dawn team, soon we will!

      I hope this answers your question.