Mercury mission to take 7 years?

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nopatience

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Why is the Mercury MESSENGER mission going to take 7 years? <br /><br />didn't it take Cassini 7 years to reach Saturn? and I thought saturn was much further away?
 
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heyscottie

Guest
The distance isn't the problem with either Saturn or Mercury. The real problem is different orbital speeds. We don't have the ability, using current rockets, to either economically slow down enough to match Mercury's orbital speed, or to economically speed up enough to match Saturn's orbital speed.<br /><br />Instead we sling probes near other planets, using gravity assist maneuvers to give us greater delta velocities than we can with our chemical rockets alone. This is a time-consuming process, and we have to plan trajectories very carefully so that we make the desired passes near the other planets.<br /><br />Both Cassini and Messenger made (will make) multiple passes of Earth, as well of other planets, before they reach their destinations.
 
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thalion

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^<br />Ditto.<br /><br />The fact that it's an orbiter mission instead of a fly-by like Mariner 10 also lengthens the trip, as it must travel slower, while Mariner 10 had a favorable planetary alignment, and the option to hot-foot it. Then there is the issue of the money a heavier booster would have cost.
 
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spacester

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I always question authority in matters of orbital mechanics. IMO the "experts" are typically more concerned with what you <b>cannot</b> do in space than in what you <b>can</b> do.<br /><br />So I wrote some software to figure out things for myself. It was a big project, but when things like this come up, I can quickly determine for myself the difficulties.<br /><br />It turns out the experts were right in this case <img src="/images/icons/laugh.gif" /> Mercury is very expensive in deltaV. The most fuel efficient transfer, the Hohmann transfer, takes about 110 days. It varies quite a bit because Mercury's orbit is quite eccentric. For one particular alignment, the trip takes 105.82 days and requires a departure dV of 7.4 km/s and an arrival dV of 14.4 km/s. These are large numbers.<br /><br />For that example, even if you were able to drop the stage which gives you the departure velocity, you find it is nearly impossible to build a vehicle to do the job.<br /><br />The mass fraction of the vehicle you need to build to arrive at mercury can be found by:<br />mo / mf = e ^ (dV / Ve) = e ^ (dV / (g * Isp))<br />mo= mass of vehicle fully fueled<br />mf = mass of vehicle after burning the fuel to achieve orbit<br />g = 9.81 m/s^2<br />Isp = 450 s for a really kick-ass rocket engine<br />mo / mf = e ^ (14400 / (9.81 * 450)) = e ^ (3.262) = 26.1<br /><br />Which means that the mass of your propellant needs to be 25.1 times as much mass as the rest of the vehicle. This is technically possible, but rather ridiculous in practice. Especially when you consider that the specific impulse is for LH2/LOX engines, any other propellant combo would be much worse, and LH2 needs really big tanks.<br /> <div class="Discussion_UserSignature"> </div>
 
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CalliArcale

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spacester's always got the math for this. <img src="/images/icons/wink.gif" /><br /><br />The MESSENGER website has a great diagram: MESSENGER Trajectory. It will be making <i>five</i> gravity assists -- Earth once (next year; wave when it goes by!), Venus twice, and then Mercury itself three times. Carefully follow the colored lines as shown in the key to see what happens with each gravity assist.<br /><br />Simplified a bit, this is most of what happens: the Earth flyby will lower perihelion so MESSENGER can encounter Venus in 2006. The first Venus flyby will lower aphelion. The second Venus flyby will lower perihelion so MESSENGER can encounter Mercury in 2008. Then there will be three Mercury flybys to lower perihelion until MESSENGER is much closer to Mercury's orbital parameters, making the actual orbit insertion relatively easy in terms of delta-vee (change in velocity) because they'll be most of the way there already. Additionally, there are five major trajectory correction maneuvers (engine burns) planned besides the actual orbit insertion. These will mostly just fine-tune the orbit, though. Most of the energy will be coming from gravity assists. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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remcook

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orbital mechanics is fun <img src="/images/icons/smile.gif" /><br /><br />a lot of results in orbital mechanics are counter-intuitive, which makes it very hard to judge things on forhand.<br /><br />Example: when you fire your rocket in space, your speed decreases (you get into a higher orbit).
 
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nopatience

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WOW! that is interesting. <br /><br />so let me get this streight, the only reason that it is going to take 7 years is because we cannot carry the fuel that would slow down probe fast enough to enter mercury orbit?
 
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spacechump

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Correct. The best way, without carrying an exuberant amount of fuel, is to shed velocity off of planet flybys, in the same way that probes also gain velocity from planet flybys.
 
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jcdenton

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I'm not sure using bigger rockets would work. As you know, the probe is orbitting Earth and Venus numerous times in order to build up momentum. If the JPL could think of better ways to build up momentum than the "sling-shot" method, than the Messenger would probably reach Mercury in shorter time. <div class="Discussion_UserSignature"> </div>
 
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spacester

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<font color="yellow">Could the probe get there sooner if we simply used a bigger, more powerful rocket?</font><br /><br />Nope. All that would get you is a shorter burn time at arrival. I am assuming 'bigger and more powerful' means more thrust. The total impulse (braking force) is the thrust multiplied by the burn time. No matter how you slice it, you need a given amount of total impulse to insert into orbit. The example I gave is basically the lowest possible total impulse.<br /><br />The specific impulse can be thought of as the fuel economy. We need better fuel economy to get there faster (weird, huh?), but the Isp = 450 seconds is pretty much the maximum possible with chemical propulsion.<br /><br />What would be interesting is to play with some scenarios where you combine staging strategies with fly-bys. But that would take actual work on my part . . . <img src="/images/icons/wink.gif" /><br /> <div class="Discussion_UserSignature"> </div>
 
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nopatience

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Okay, I will ask a stupid question, and wait for what ever answer I get. <br /><br />If infact you had the thrust to intercept Mercury without gravity assists, what are the choices in regards to braking. (asside from chemical burns)<br /><br />here's the stupid part- is there any chance of air braking by means of a "anchor"? or someting similar?<br />I'm lol at this idea! But the curiosity is real.
 
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spacechump

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<i>here's the stupid part- is there any chance of air braking by means of a "anchor"? or someting similar?<br />I'm lol at this idea! But the curiosity is real.</i><br /><br />Do you mean aerobraking? Because in this case no. Mercury's atmosphere is pretty much useless in this case given how thin it is. Besides aerobraking mostly only works when you've achieved orbit anyhow and want to decrease your orbital speed to change your orbit.
 
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tom_hobbes

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Wouldn't a bigger rocket simply mean there's more mass to slow down for 'parking' in mercury orbit. I was under the impression that it's not getting there quickly that is the problem, it's getting there and slowing down enough. Messenger also needs to carry enough fuel for constant adjustments once in orbit. <div class="Discussion_UserSignature"> <p><font size="2" color="#339966"> I wish I could remember<br /> But my selective memory<br /> Won't let me</font><font size="2" color="#99cc00"> </font><font size="3" color="#339966"><font size="2">- </font></font><font size="1" color="#339966">Mark Oliver Everett</font></p><p> </p> </div>
 
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spacechump

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Exactly Tom. Messenger already has 55% of its mass as fuel. There's no point in trying to fight the sun's influence with larger rockets that just pack on more mass to slow down anyhow.
 
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Leovinus

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If we had one of those low-weight low-thrust ion drives, could we get there any faster? <div class="Discussion_UserSignature"> </div>
 
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spacechump

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You'd get there faster but you'd still have to slow down once you got there or be prepared to either plow into the sun or be caught in its orbit and not Mercury's.<br /><br />The best use of low-thrust ion drives will be to send the probe off onto a close solar orbit and then use the ion drives to slowly slow your velocity to match that of Mercury's orbit. Once you get them approximately relative to each other you can drop it into orbit. This concept wouldn't work with chemical rockets and would probably still take quite a while with an ion engine.
 
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remcook

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These low-thrust engines take a while to start up and so they are relatively good in missions that require long distance to cover.<br /><br />remember...SMART-1 takes a long time to get to the moon!<br /><br />But once they have speed, they can go on..and on..and on
 
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spacechump

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Well it could make it there much quicker if they really wanted it to. But the point of the mission is to test the endurance and reliability of the ion engine...thus the constant orbits around the earth and moon...to give it enough time for a thorough breakin and test period.
 
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yurkin

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spacehump<br />Smart is trying to go for the most energy efficient route to the moon. It’s not going slowly to give engineers more time to study the engines endurance. In fact they would have less time if this were the case. <br />Smart is going at full throttle whenever its engines are on. At its current orbit it’s on a third the time. If the engines were on all the time Smart would get to the moon faster. But this would be a less energy efficient route. To make up for it the engines would have to be on longer. Therefore the elapsed time of engine operation would actually be longer if Smart tried to get there sooner.<br /><br />Even if Smart had enough fuel to burn full time, and didn’t take a month off to test all its systems, your still looking at several months travel time.<br />
 
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spacechump

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And I quote from the web page:<br /><br />" The main mission objective of SMART-1 is to demonstrate innova-tive and key technologies for scientific deep-space missions. One of the objectives is the flight demonstration of Electric Primary Propulsion for a scientific lunar orbi-ting spacecraft delivered into a standard geostationary transfer orbit. SMART-1, however contains other technology elements both in the spacecraft bus and in the instruments carried onboard.<br /><br />The Spacecraft will also carry a scienti-fically relevant payload. The space-craft is designed with regard to the power needed for the electric propulsion, the severe radiation environment that is a consequence of the slow earth escape trajectory and the need for on-board autonomy.<br /><br />During this 18-month transfer phase, the solar-electric primary propulsion’s performance, and its interactions with the spacecraft and its environment, will be closely monitored by the Spacecraft Potential, Electron & Dust Experiment (SPEDE) and the Electric Propulsion Diagnostic Package (EPDP) to detect possible side-effects or interactions with natural electric and magnetic phenomena in nearby space.<br /><br />A promising technology, Solar Electric Primary Propulsion could be applied to numerous interplanetary missions in the Solar System, reducing the size and cost of propulsion systems while increasing manoeuvring flexibility and the mass available for scientific instrumentation.<br /><br />In addition to Solar Electric Primary Propulsion, SMART-1 will demonstrate a wide range of new technologies like a Li-Ion modular battery package; new-generation high-data-rate deep space communications in X and Ka bands with the X/Ka-band Telemetry and Telecommand Experiment (KaTE); a computer technique enabling spacecraft to determine their position autonomously in space, which is the first step towards fully autonomous spacecraft navigation. "<br /><br />It's just a test platform that is conveniently being sent to the m
 
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yurkin

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Spacehump<br />Just because the primary objective of the mission is to demonstrate ion drive operation is beside the point. Where do you see it say that the craft could get there faster then it is? Where do you see it say that Smart has to go slower then it can in order to perform its objectives? It just lists its goals, which are totally independent of travel time. Nothing in that quote has anything to do with your point.<br /><br /><br />Force of Smart-1 Hall Effect Thruster = 0.068 N = 0.015287 pounds = 6.934 grams<br />You think you can go from GTO to lunar orbit with a 7 gram thruster, on the first orbit? Keep in mind Smart will weigh in at 437 kilograms.<br />http://smart.esa.int/science-e/www/object/index.cfm?fobjectid=34201
 
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vogon13

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The final orbits of Messenger around the sun are entirely within Venus' orbit. Messenger does not experience a Mercury fly-by during every orbit around the sun. It's period around the sun does change after each close Mercury flyby. (there are also "tweaking" orbital correction burns of Messenger's propulsion system that change the orbital period slightly). I don't have the video handy, but I seem to recall that Messenger goes around the sun twice while Mercury goes around three times to set up another fly-by, then after this fly-by, Mercury goes around the sun 5 or 6 times while Messenger goes around one less time. Each fly-by nudges Messenger's orbit closer to that of Mercury, but as you get closer, it takes more revolutions to get the next shot lined up. Most of Messenger's time is actually spent waiting . This trajectory is actually quite an amazing thing and if someone knows who came up with it, please let us know! We need more smart people to revere! Although suggested by Clarke and a British astronomy club in the 1940's, the mathematics for these gravity assist trajectories wasn't really worked out until the 1960's at JPL by Flandro and Minovich. The subtlety and elegance of the Messenger path to Mercury is an unsung 'modern marvel'.<br /><br />If the US congress would fund more planetary missions I would find the waiting times more bearable, as there would be more going on around the solar system to keep us occupied while the probes are getting to their destinations. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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