- #386
mfb
Mentor
- 37,097
- 13,934
From January last year.
Is "region A" the one with the two bright spots? That would be very interesting.
Is "region A" the one with the two bright spots? That would be very interesting.
What distinguishes water ice from CO2 ice? Color of visible light?Dotini said:A crater with water ice at the north pole of Mars.
http://www.esa.int/spaceinimages/Im..._view_of_crater_with_water_ice_-_looking_east
orbit dates altitude(km) pixel(m) XHubble period analogy
RC3 April 23–May 9 (13,500) (1,300) 24 15 days (3.0 meters)
Survey June 6-30 (4,400) (410) 72 3.1 days (1.0 meters)
HAMO Aug 4–Oct 15 (1,450) (140) 215 19 hours (33 cm)
LAMO Dec 8–end of mission (375) (35) 850 5.5 hours (8.5 cm)
Thank you for mentioning this, Marcus. I grabbed this morning's shots and noticed that the Earth, Sun, and Mars were just out of the "Ceres-Dawn" graphic.marcus said:The simulated view of Ceres from Dawn's perspective now includes the Sun and Mars. A couple of bright Gemini stars are in the background.
http://neo.jpl.nasa.gov/orbits/fullview2.jpg [Broken]
this is the first time i can recall seeing the views merged like that
usually the sun, Mars, Earth only show in http://neo.jpl.nasa.gov/orbits/fullview1.jpg [Broken]
and fullview1 does not include Ceres.
marcus said:Dotini, depending on the time of day I'll be plumping either for salt flats or "some combination of the the above". I've read that the rest of the surface is very low albedo (10%, didn't Emily Lakdawala say something like that?) So the bright spots might not actually need to be very bright by ordinary standards--I expect they could still show up very bright by contrast.
"Dawn was scheduled to carry a magnetometer to measure any magnetic fields around Vesta and Ceres, but cost overruns forced Dawn investigators to drop the instrument from their final design."Jimster41 said:Unlikely it has an EM field right?
It truly boggles the mind, what these people were able to figure out, before calculators, electric lights, and modern dentistry even existed... But then I guess, the night sky was still pretty dark, and it's not like you could watch TV.marcus said:GAUSS AND CERES
Ceres was observed only briefly in early 1801 and then disappeared in the glare of sunlight. Gauss took the observations (spanning only 9 degrees of sky angle, during the first 40 days of that year) and calculated from them where to look at a later time, around the end of the year, when it would be out of the glare. Here are excerpts from a nice student term paper about this (at a Rutgers History of Science website)
Thanks to Leorah Weiss and http://www.math.rutgers.edu/~cherlin/History/Papers1999/weiss.html
===excerpts===
Then,on January 1, 1801, the Italian astronomer Joseph Piazzi discovered a planetoid, working from an observatory in Palermo, Italy. This object, which he christened Ceres, was moving in the constellation Taurus. Astronomers were only able to observe the planetoid for 41 days, during which its orbit swept out an angle of only 9 degrees. Ceres was then lost to sight when its light vanished in the rays of the sun, and the astronomers could no longer find it. There was now a challenge of calculating Ceres' orbit using only the observations Piazzi made, so that astronomers would be able to sight Ceres when it reemerged. [1,5,6,8,10]
...
...
The technical execution of Gauss's method is very involved, and required over 100 hours of calculation for him. His first tactic was to determine a rough approximation to the unknown orbit, and then refine it to a high degree of precision. Gauss initially used only 3 of Piazzi's 22 observations, those from January 1, January 21, and February 11. The observations showed an apparent retrograde motion from January 1 to January 11, around which time Ceres reversed to a forward motion. Gauss chose one of the unknown distances, the one corresponding to the intermediate position of the 3 observations, as the target of his efforts. After obtaining that important value, he determined the distances of the first and third observations, and from those the corresponding spatial positions of Ceres. From the spatial positions Gauss calculated a first approximation of the elements of the orbit. Using this approximate orbital calculation, he could then revise the initial calculation of the distances to obtain a more precise orbit, and so on, until all the values in the calculation became coherent with each other and with the three selected observations. Subsequent refinements in his calculation adjusted the initial parameters to fit all of Piazzi's observations more smoothly [11].
In September of 1801, Zach published several forecasts of the prospective orbit, his own and Gauss's among them; Gauss's prediction was quite different from the others and expanded the area of the sky to be searched [1]. Using Gauss's ephemeris for Ceres (astronomical almanac showing its predicted location at various times), astronomers found Ceres again between November 25 and December 31. Zach, on December 7, and then Olbers, on December 31, located Ceres very close to the positions predicted by Gauss. Between the discovery of Ceres in 1801 and the present day, over 1,500 planetoids have been identified, with Ceres remaining the largest [5,10]. While continually improving and simplifying his methods, Gauss calculated ephemerides for the new planetoids as they were discovered. When Olbers found Vesta in 1807, Gauss calculated the elements of its orbit in only 10 hours. His calculations of parabolic orbits were even faster, as is natural. He could calculate the orbit of a comet in a single hour, where it had taken Euler 3 days using the previous methods [5,6].
Gauss published his methods in 1809 as "Theoria motus corporum coelestium in sectionibus conicus solem ambientium," or, "Theory of the motion of heavenly bodies moving about the sun in conic sections." [1,2,3,5,6,11]. Gauss first wrote this work in German, but his well-known publisher, Perthes, requested he change it to Latin to make it more widely accessible (sic). In fact, the astronomical methods described in Theoria Motus Corporum Coelestium are still in use today, and only a few modifications have been necessary to adapt them for computers [11]. Gauss's determination of Ceres's orbit made him famous in academic circles worldwide, established his reputation in the scientific and mathematical communities, and won him a position as director at the Gottingen Observatory. [5,10]
==endquote==
Gauss was born in 1777, so in 1801 when Piazzi first observed Ceres and Gauss made the first successful calculation of its orbit, he was 24 years old....
OmCheeto says:
Your comment is awaiting moderation.
April 29, 2015 at 8:13 am
Hi Dr Rayman!
I read on Dawn’s Twitter page the other day, that Dawn’s total change in solar powered velocity was around 24,000 mph or 39,000 kph. Does this make Dawn the fastest man made solar powered vehicle in the known universe?
Thanks!
OmHi Om,
Your comment came in while our web team was transitioning the website to a new system that does not allow blog comments (in compliance with the JPL Blog). It wasn’t my idea, but it is what it is. I would have been happy to answer this one for everyone, because this kind of question comes up so often, but now I’ll reply only to you.
The information on Twitter now comes from me but it may be reworded by the person who actually does the tweeting. So sometimes it may be a little misleading. Nevertheless, it is quite correct that the total delta-v now exceeds 24,000 mph. As of today, it is 10.8 km/s, and since you are technical, I’ll stick with metric.
Your question shows a common misconception. Missions that travel to higher orbits (in this case, higher solar orbits) generally go slower. Dawn’s heliocentric velocity today is 17.1 km/s. You and I are traveling around the sun today at 29.5 km/s. So we have not used the ion propulsion system to accelerate but rather to decelerate (as well as to accomplish other changes in the orbit, including inclination).
I have written about this quite extensively in my Dawn Journals. See http://dawn.jpl.nasa.gov/mission/journal_02_28_13.asp for one discussion. Also, every September 27, my Dawn Journal described how much faster Earth travels than Dawn. Of course, all objects on Earth travel at essentially the same heliocentric velocity, and spacecraft sent to the inner solar system travel still faster. MESSENGER traveled at Mercury’s heliocentric velocity, so it was much faster than Dawn. Most (but not all) spacecraft sent to the outer solar system, such as Cassini, Voyager 2, Juno, and others, travel even more slowly than Dawn. But to answer your question more directly, Dawn is one of the slowest man made solar powered vehicles. (As all man made objects are in the known universe, that qualification isn’t important.)
Of course, in orbit around Ceres (and Vesta), Dawn uses its ion propulsion system to accelerate (relative to the central body) so that it can go to lower orbits. Then at Vesta it decelerated relative to Vesta to climb to higher orbits and escape before resuming its heliocentric travels.
I hope that answers your question.
I also hope that my previous email answered your questions, but I have no idea. You don’t need to acknowledge my responses, but if you do, you might find me to be even more helpful (or, at least, happy to help) in the future. Everyone (or, at least, I) likes to know their efforts are worthwhile. Once again, however, it is not necessary.
By the way, I took a quick peek again at the physicsforums again not too long ago, as you had invited me to some time ago. I didn’t have time for more than a minute or two there. There are far too many points to comment on (and virtually every question had already been answered in my Dawn Journals, although I know they are impractical to scour or even search). I don’t have time now to look it up again, but I will reiterate a comment I believe I made to you once before, although I may have posted it on the blog. The Mystic simulator, as one of the forum participants observed, is consistent. Consistency does not equate to accuracy, however. It is never based on actual flight data. The numbers in my Dawn Journals, now the information in tweets (because it comes from me), and the mission status reports are more accurate. I love the Mystic simulator, but one needs to be careful about using it to conclude what the spacecraft is really doing. It is based on a design reference trajectory.
Regards,
Marc
mfb said:Oh, the joys of orbital mechanics, where you thrust forwards to slow down.
We just need the right reference frame, then Dawn is fast again. Dawn is the fastest man-made object with ion drives in the reference frame of Earth.
Dawn is between Ceres and the Sun again, which means we will reach the end of RC3 soon (scheduled: Saturday May 9). 3 times better resolution in one month (also faster orbits, 3 days instead of 15).
Orbit dates altitude(km) pixelsize(m) res/HST period soccerball at
RC3 April 23–May 9 (13,500) (1,300) 24 15 days (3.0 meters)
Survey June 6-30 (4,400) (410) 72 3.1 days (1.0 meters)
HAMO Aug 4–Oct 15 (1,450) (140) 215 19 hours (33 cm)
LAMO Dec 8–end of mission (375) (35) 850 5.5 hours (8.5 cm)
Last time I tried to figure out something impossible, I popped a blood vessel.marcus said:Mfb suggested this orbital mechanics paradox: In the opposite sense from his example, to spiral in the craft points its ion thrust beam ahead in the direction it is going, thrusting in reverse as if to slow down. But as it spirals in it actually speeds up. So braking speeds you up.
As it moves in closer, the planet's gravity is stronger. The craft moves faster and in tighter spiral loops. It takes more work to cancel the gravitational energy difference between orbit levels.
...
The Dawn spacecraft was launched in 2007 with the purpose of studying two of the largest bodies in the asteroid belt, Vesta and Ceres. It was the first spacecraft to orbit two extraterrestrial bodies and provided valuable insights into the formation and evolution of our solar system. Running out of fuel in Ceres orbit marked the end of its mission and the completion of its objectives.
Dawn's fuel, hydrazine, was used to power its ion thrusters, which helped it maneuver and maintain its orbit around Ceres. After more than 11 years of operation, Dawn's fuel supply was depleted, causing it to lose control and crash into the surface of Ceres.
Dawn's mission to Vesta and Ceres provided valuable insights into the formation and evolution of our solar system. Some of the key discoveries made by Dawn include the presence of water on Ceres, evidence of past geological activity on both Vesta and Ceres, and the confirmation of Vesta as a protoplanet, or a large asteroid that is a precursor to a planet.
Dawn's mission to the asteroid belt provided scientists with a wealth of data and images that helped to improve our understanding of this region of our solar system. It helped to confirm the theory that the asteroid belt was once a much larger and more active region, and provided evidence for the formation of some of the largest asteroids in the belt.
Dawn's mission has paved the way for future missions to explore and study other small bodies in our solar system. The data and images collected by Dawn will continue to be analyzed and studied by scientists, providing new insights and discoveries for years to come. The end of Dawn's mission also marks the end of an era, as it was one of the first missions to use ion propulsion, a technology that is now being used in other spacecraft and is revolutionizing space travel.