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Space

The Art of Aerobraking 86

Posted by michael from the flat-rock-on-a-still-pond dept.
gizmo_mathboy writes: "Yahoo! Dailynews has the following Space.com article about the risk of using aerobraking for orbital insertion of spacecraft versus the certainty of using conventional propulsion systems. This is all explained in terms of the Mars Global Surveyor craft that is expected to do its orbital insertion on October 23. Skip the wimpy aerobraking and as a prophead trapped in a code monkey's job I say, "In Thrust We Trust.""
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The Art of Aerobraking More

The Art of Aerobraking

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  • So what, you cannot aerobrake if thin atmosphere...

    => use retro Thrust.

    Hell, I just won a position as Nasa's Chief Engineer 8)

    • Depends on what you mean by 'thin'. Compared to the vacuum of space, even Mars' atmosphere is thick as pea soup and would work fine for aerobraking.

      The danger lies in using aerobraking to rid yourself of all your velocity. If you use if to get rid of a nice portion of it and then thrust your way to a complete stop (relatively speaking, of course), you get the use of the atmosphere with less risk.

      • Aerocapture (Score:5, Informative)

        by zardor ( 452852 ) on Thursday October 04, 2001 @07:42AM (#2387651)
        Using aerobraking to rid yourself of 'all your velocity' (interplanetery velocity, relative to the orbital motion of the target planet) is called aerocapture. This has never been attempted before, and would require prescise atmospheric targetting (to within a few kms), precise details of the atmospheric density parameters, and perfect understanding of the spacecraft's atmospheric behaviour, all on the first, and only 'deep' pass through the atmosphere. Since we are 'burning off' all our interplanetery velocity in one go, the heat load would be quite extreme, probably needing a dedicated heat shield, which could be discarded after the aerocapture pass. Of course, the spacecraft wouldn't need an Mars orbital insertion (MOI) rocket engine, and the ton or so of fuel that would go with it. (A smaller rocket burn however would be required 1/2 an orbit after the 'deep' aerocapture pass, to raise the spacecraft enough so it wouldn't pass through the atmosphere a second time). Once aerocapture has been achieved, and the spacecraft had been checked out (and allowed to cool down!), a gentler aerobraking phase can then be used over time to reduce the orbital velocity of the space craft, and lower the resulting eliptical capture orbit into a circular one suitable for science studies.
        It has been proposed to use areocapture for some of the later mars orbiter missions, but it was deemed too risky, particularly after Mars Surveyor98's areobraking phase showed how unpredictable the Martian upper atmosphere really is.
        If you remember the film 2010 (I think), the Russian exploration ship used areocapture at Jupiter, by inflating huge baluttes (balloons) around the craft and plowing through Jupiter's atmosphere.
        • What abour aero-smashing?

          I guess what I mean is how difficult would it be to build a craft capable of smashing directly into Mars's atmosphere (perpendicular to suface normal) and slowing down that way? I mean certainly the heat stress would be tremendous, but certainly there's solutions to that...

          So the plan is that it smashes down in there, then once it realizes it's slow enough, it can deploy a series of parachutes to slow it down even further...

          • Aerosmash (Score:5, Informative)

            by zardor ( 452852 ) on Thursday October 04, 2001 @09:37AM (#2387900)
            Eek! It would probably not be possible to enter and land through mars atmosphere 'perpendicularly'. For 'entry' purposes, assume mars atmosphere to be 125Km high. The spacecraft is travelling at interplanetery velocity, say 7.5Km per sec. If we decide not to slow down, we will hit the surface in 17 seconds with a *big* bang.
            The time is too short to run the entry sequence (jetesson heatshield, deploy parachutes, fire retros etc)
            The deceleration G forces required to slow down in the limited time would be massive, (>100 Gs, causing structural engineering design issues)
            The total integrated heat load on the heatshield would be the same, but the peak loads would be much higher (up to half a gigawatt. Thats a lot of asbestos)
            And since you are going 'straight down', once you jetesson your heat shield (and its stored thermal energy), you will probably land on it a few seconds later and melt.
            The ideal solution (as demonstrated by mars pathfinder) is to come in at an shallow angle of about 15 degrees, and in this case the whole entry sequence takes a good few minutes, the peak deceleration is about 20Gs and the peak heat load is about 100Megawatts.
            See the Mars PathfinderEntry Descent and Landing [nasa.gov] website for more details.

            • > It would probably not be possible to enter and land through mars atmosphere 'perpendicularly'. For 'entry' purposes, assume mars atmosphere to be 125Km high. The spacecraft is travelling at interplanetery velocity, say 7.5Km per sec. If we decide not to slow down, we will hit the surface in 17 seconds with a *big* bang.

              ...besides, we already tried it a few years ago. It didn't work. (Except to provide a light show for the Martians. :-)

        • I thought a big problem of those would be hitting too steep or or in too high density air, and burning up when the craft gets caught by the planet. Either that or another really big problem occurs when entering in these shallow angles needed to aerobrake would be skipping like a stone and being sent out into the neither regions of space. These crafts heat shields are large, flat, and smooth. The perfect skipping stone.
        • It has been proposed to use areocapture for some of the later mars orbiter missions, but it was deemed too risky...

          Didn't Mars Climate Orbiter already demonstrate aerocapture? (Apologies to the JPL folks..)

        • Aerocapture has never been attempted before? On Mars, correct. But every space capsule that ever returned to Earth used aerocapture.
    • Of course, you can fin the original story here [space.com], complete with pretty pictures and a movie.

      Now if only the Martian defense force does not get the probe.

      ;-)

  • Hmmm... In Thrust We Trust... That's the name of a flick on the Hot Network this month.
    I think they should take the risk of Aerobraking. Hell, so what if it fails the first time? It's not like anybody's going to die and I'm sure sure they have the money to throw away.
    • Come on, we are talking about orbital insertion here.
      • Hmmm... In Thrust We Trust... That's the name of a flick on the Hot Network this month.

        Come on, we are talking about orbital insertion here.

        I thought that nobody was having sex on the ISS yet???
    • No, they DON'T have the money to throw away. The US space program (the best in the world) has serious money troubles. They are attempting to hold together a serious program by going for the cheaper, lighter technologies (such as this aerobraking) which is fine and good, but the fact remains that the program has been basically gutted ever since the moon flights.

      This is deeply sad, in my opinion. What we really need is a shuttle Mark II (that first one is really just a prototype.. a good prototype, but it really needs to get past version 1.1.9.). We need a _real_ space station. We need a program that lets us start mining asteroids, so that we don't have to keep lifting every expensive gram of water up to orbit. And, of course, we need a really good Sh*t Recycler (tm). Heck, even /. needs that!

      • Re:Nonsense (Score:2, Insightful)

        by Mr. Eradicator ( 470089 )
        If the shuttle is a prototype, then it has to be the most advanced prototype ever. The SSME is the most amazing engine in the world. To increase the Isp even a tenth of a percent would be an amazing accomplishment.

        To add to this, there is an upgrade in the works. Several parts of the orbiter are being upgraded. The Avionics system is one of those. They've already put new displays and computer hardware in the cockpit and after the avionics is upgraded in a few years, it will be "up-to-date".

        I agree that we need the money and public support that we had in the Space Race days of Mercury through Apollo. Unfortunately the public sees no reason for this. Back then we had the Cold War to fuel our need to advance in space technology. Now most people could care less. I don't think any wars in the next 10 years will show a demand for a TIE-fighter, unfortunately.

        As for the space station ... well, they're working on that. But _that_ is where their money problems are coming from ;)

  • Nitpicking, I know... (Score:4, Informative)

    by sprouty76 ( 523155 ) <stephen_douglas@ ... inus threevowels> on Thursday October 04, 2001 @07:18AM (#2387629) Homepage
    This is all explained in terms of the Mars Global Surveyor craft that is expected to do its orbital insertion on October 23.

    Actually, it's the Mars Oddessy craft that's about to perform aerobraking, the Global Surveyor has been in Mars orbit for several years now.

    • Jesus, I was thinking "that thing's been at Mars since I was in high school! It's STILL not done aerobraking?" Well, good to know then that aerobraking is just sort of slow, rather than ludicrously slow.
  • A really nice demonstration of this can be seen in the movie 2010 (the sequel to the famous 2001). The Russian spacecraft uses this technique to slow down for an orbit around Jupiter. It's really fun to watch. If the science only partially intrigues you, then watch the movie just because it has John Lithgow. I like that actor more than Keanu Reeves.

  • Aerobraking vs. Propulsion Braking (Score:3, Interesting)

    by Listen Up ( 107011 ) on Thursday October 04, 2001 @07:31AM (#2387637)

    I am sure in the interest of reducing size, reducing weight, reducing cost, and increasing the amount of available instrument and sensory device space onboard an interplanetary craft which is designed to land on an alien planet within a certain *limited* budget, exploring as many alternative kinds of landings mechanisms is well withing the realm of understandable and highly saught after.
    Packing a couple of large parachutes into a space craft for landing on a planet with a sustainable atmosphere would make a lot of sense if the means of adding an entire rocket/fuel powered landing/propulsion system onto the same craft would not produce greater yield/results within the intended mission/budget. Why not design a craft which could always 'right' itself regardless of how it is situated after it lands with a parachute type landing? That would/should not be a very difficult task given the amount of incredible talent that is at NASA's disposal.
    As a *very* sincere and heartfelt sidenote...Policy usually destroys or f*cks up NASA missions. That is a *certain* gaurantee...NOT the entire staff of Ph.D. Physists and Engineers working on the projects. It always makes me sad/unhappy when people blame NASA engineers for NASA's recent terrible public mistakes. Blame policy, politics, and administration (be it impossible deadlines, mismanagement, etc.) for NASA's recent unfortunate public image. Things are getting better every day and by the minute and all it will take is just one immensely and incredibly perfect mission for NASA's public image to be returned to it's former 'moon landing' era confidence. Thank You for listening. Please support NASA and it's mission to keeping dreams and imagination alive despite the rest of the world's problems.

    • Why not design a craft which could always 'right' itself regardless of how it is situated after it lands with a parachute type landing?

      They did. It was called Mars Pathfinder. It bounced around on those big airbags for a while, and after it came to rest sequentially deflated them so the payload ended the right way up.

      What's a 'sustainable' atmosphere?

    • IIRC, Pathfinder didn't rely (solely) on a baloon deflation sequence to right itself. The craft was a tetrahedron (pyramid with 4 triangular faces), and no matter how it landed it would right itself as it opened.

      BlackGriffen
    • At school we're actually designing a Mars Sample return Mission. As a result I've been researching many different methods for orbital insertion. AeroBraking is an excellent method to change your orbit. As an Example the Magellan spacecraft was orbiting venus and wished to enter a more circular orbit. It was calculated that 900 kg of fuel would be needed to do this traditionally, more than the spacecraft had. Using aerobraking only 38 kg of fuel were used. Of course the downside is the manuver took 3 months to perform.

      For atmospheric entry however parachutes alone are not enough. Mars's atmosphere is 1000 times less dense than Earths. What parachutes are used for is to give a huge reduction in speed, from about 900 MP to about 160 MPH. A parachute that does this weighs about 10 KG. A parachute that would bring pathfinder to a survivable landing would have a 250 ft diameter and weigh 420 kg, more than the lander itself! It's much better to slow down some with a parachute then use retro rockets in one form or another.

      -Mishra
    • Things are getting better every day and by the minute ...


      Bzzzt! Thanks for playing. I wish I had your confidence in NASA's management. Unfortunately, instead of emulating successful missions such as Mars Pathfinder, NASA is planning to spread the model for a failed mission (Mars 98) throughout the agency.


      Mars Pathfinder was managed and built in-house by JPL -- this was the lander / rover mission which succeeded wildly (albeit with modest goals) in 1997.


      Mars 98 was the combined missions of Mars Climate Orbiter, Mars Polar Lander, and the experimental Deep Space 2 impact samplers. The loss of the two primary spacecraft was attributed to poor coordination between JPL and the contractor who built them (Lockheed Martin Astronautics).



      Yet, the "plan" being advanced to contain the massive cost overruns on Space Station, is based on outsourcing as much work as possible. Hmm. I'm not saying that outsourcing is always a bad thing, but NASA had better tell the taxpayers what's going to be different in the new formula.


      Here's NASA's "Commercialization" plan:
      http://www.spaceref.com/news/viewsr.html?pid=3730 [spaceref.com]

  • Aerobraking and probe intelligence... (Score:5, Informative)

    by trims ( 10010 ) on Thursday October 04, 2001 @07:44AM (#2387654) Homepage

    No, I'm not going to talk about V-ger or anything like that.

    The article mentions that one of the major problems with aerobraking is the fluctuation in density of the admosphere causes problems with calculations for the aerobraking. That got me to thinking...

    Now, recently, we've started to build the landers with a reasonable amount of autonomous intelligence, so they can cope with some problems without requiring instruction. However, from all that I've read, all the space-borne probes we've send are dumb as a rock: that is, they can't do anything that Mission Control doesn't tell them to. They're a true remote-controlled vehicle.

    The problem with this approach is the time lag between Earth and wherever they are (which is measured in light-minutes). I realize that adding some sort of intelligent processing to a probe causes an additional weight to be carried (and power consumed), but for christ sakes, I can get a Lego Mindstorms to run around my livingroom by itself; one would hope that we might be able to build a semi-autonomous space probe.

    Basically, we should be able to build something that does this (MC=Mission Control, SP=Space Probe):

    • MC: probe, prepare for insertion! Here are the initial data parameters, go in 5 seconds! We will talk again in 5 minutes when you get to position X.
    • SP: thanks, MC. <sounds of intersteller number crunching>
    • SP: ok, control vanes, move here; rocket, fire in 4, 3, 2, 1...
    • (3 minutes later, and atmosphere unexpectedly thickens) SP: oh no! Quick, recalculate! <more i.n.c. noises> rocket, give me a 2 second burn then turn 43 degress for a 1 second burn!
    • (SP arives at X 20 seconds late) MC: good job! here's some updated positional data for the next pass...


    Basically, what I'm suggesting is that we break the mentality of requiring absolute control over the probe at all times, and allow them a degree of adaptability and flexibility by providing them with some reasonable programming. That's no happening now. And as the maneuvers we attempt grow in complexity, we're going to find it almost impossible to completely pre-calculate everything. If we keep trying, we're going to fail.

    Adaptable and intelligent semi-autonomous probes are the long-term solution.

    -Erik

    • Re:Aerobraking and probe intelligence... (Score:5, Interesting)

      by Johnny Vector ( 93021 ) on Thursday October 04, 2001 @08:50AM (#2387726) Homepage
      for christ sakes, I can get a Lego Mindstorms to run around my livingroom by itself; one would hope that we might be able to build a semi-autonomous space probe.

      You mean like Deep Space 1 [nasa.gov]? Or Clementine [navy.mil]? Yep, it's being done.

      (3 minutes later, and atmosphere unexpectedly thickens) SP: oh no! Quick, recalculate! rocket, give me a 2 second burn then turn 43 degress for a 1 second burn!

      Oop, doesn't work that way. Orbital mechanics is funny until you wrap your head around it. To change the perigee, you have to burn at the apogee. Once you're in the atmosphere, there's bugger all you can do about it until the next time around. (Well, unless you're carrying gobs of fuel, and if you can do that, the screw this aerobraking stuff.)

      Of course, you can make the probe autonomously adjust the next pass based on the results of the current one. But I wouldn't want to even try until we have at least one more probe's worth of data on exactly how to model all this.

      And in response to the AC who thinks that rad-hard processors aren't up to this, all I have to say is HAW! Go look up what processing power the guidance computer on Apollo 11 had, and marvel at how much you can do when you're not spending cycles drawing aqua-colored drop shadows. I could make a useful aerobraking auto-adjust system with an RTX-2010 and half a meg of RAM. (That's an 8 MHz Forth processor, folks.) If that's not enough for you, Lockheed-Martin is selling rad-hard 250 MHz PowerPC 750 boards for only two arms and a leg.

      • Mmm rad-hardened PPC 750 boards. I'm guessing that these don't have backside cache and are actually 740s. That's besides the point. Imagine if you will, a Beowulf cluster in orbit around Jupiter. I'm not sure what you could do with it but in the ~10 years it would take to get there I'm sure you could think of something.
    • MC: Bomb #20, please return to the bomb bay.
      SP: But I received the drop order.
      MC: The drop order was in error. Please return to the bomb bay.
      SP: OK, but this is the last time...
      ...
      Captain: Talk to the bomb... Teach it phenomenology...
      ...
      SP: In the beginning there was darkness, and me...

      (Rusty memories of AI hardware in "Dark Star")
    • "(3 minutes later, and atmosphere unexpectedly thickens)" Problem is this: how do you detect that fast enough to do anything about it? It's not like the craft can carry a barometer, since the temperature is so high and the air is moving so fast that the bernoullie (sp?) effect would be insane. If we had a gps over mars we could have the craft track its velocity, but that aint happening any time soon. Trust me, if rocket science were really that easy, everyone would be doing it. BlackGriffen
      • Carry an accelerometer, and compare actual acceleration to predicted acceleration. You'd have to do some signal processing magic to ignore the buffeting, but I bet it'd be doable.

        Inertial navigation's not as accurate as GPS, but it'll do in a pinch.
    • The problem with this approach is the time lag between Earth and wherever they are (which is measured in light-minutes).

      Light-minutes and light-years measure distance, not time.

      • The problem with this approach is the time lag between Earth and wherever they are (which is measured in light-minutes).
        Light-minutes and light-years measure distance, not time.

        I'm sure he knows that. If you are 10 light-minutes away from me, it will take 10 minutes for your radio signal to get to me, since radio waves propagate around the speed of light.
    • The problem isn't really about computing power. The computing power needed is akin to the cruise controller on our car or the temperature controller on your refrigerator.

      The difficulty is in getting a sensor that can tell the controller what the air density is. The obvious way to do this would be with an accelerometer but these are expensive and don't really work well enough anyway (with current technology). Wing-like control surfaces would also help -- but these add weight and cost.

      Aerobraking and Aerocapture hold a lot of promise as most of the fuel carried by these probes is used to go into orbit at Mars (or your favorite celestial body).
      • The difficulty is in getting a sensor that can tell the controller what the air density is. The obvious way to do this would be with an accelerometer but these are expensive and don't really work well enough anyway (with current technology).

        Well, no, actually. Accelerometers are not terribly expensive, and work just fine. Silicon Designs [silicondesigns.com], for instance sells rad-hard ones. You also want to watch the temperature on various parts of the spacecraft. Temperature sensors are even cheaper than accelerometers.

        Measuring what you need to measure is not hard, nor are the calculations. All we need is a better understanding of what controls the density at the top of the atmosphere. Once we have that, you can bet autonomous aerobraking will be as common as cherry pez.


    • Most unmanned spacecraft are already extremely autonomous. Due to the cosmologically slow speed of light, and the inavailability of rocket scientists in outer space, they have to be. Their algorithms for failure handling are often so complex that they border on AI.

      The situation you described is called "adaptive guidance". Consider the problem of launching the Shuttle into orbit. A human is typically not involved in the loop at any point except to monitor systems and choose an abort mode if a failure should arise (and even then, the MC's are the ones who usually make the call). The algorithm(s) are not for sissies, they involve nested intregals and all sorts of fun stuff.

      During reentry (which is a similar problem to aerobraking) the humans are only involved in the loop during the final approach stage, and that is not even neccessary -- it's just good to have a human already in the loop, so they can better detect and respond to a problem. And yes, you can measure pressure at Mach 25, but what do you do with it? How do you know what the pressure is one second later, when you've traveled 7 km?

      The reason that the Shuttle can perform reliabily is that we have a very exact model of the Earth's atmosphere, including variations due to solar flares, and the time of day. Our model of Mars is not quite as refined, and the accuracy is more critical-- just a velocity different of a few 1/10's of km/s can make the difference between aerocapture, burning up, or spinning off into space.
  • Hmm. You mean trust like they did with a previous Mars probe that accidentally reentered (may it rest in its pieces) due to a miscalculation over the size of the thruster?

  • Must we always discuss everything twice? [slashdot.org]

    • Yeah... (I actually posted it). Sorry I didn't use the title of the Y! article as the title of the /. article. It would make it more clear I guess.

      What really gets me is I get posted in "science" and this gets front page.

      What's the difference?!?!

      And Mr. On Crack Moderator. Read both articles before placing this "off-topic". It should be modded up as "Informative"!
  • Economically, aerobraking is definitely the way to go. Putting large amounts of propellant on board would make the mission more expensive, or take the place of instruments, radio gear, computers, etc.

    Until we get really advanced propulsion technologies that are both powererful and economical (high thrust, high specific impulse), we're going to need to use methods like this.

    • That is correct!

      Why lug around a fairly large tank of propellents with all its associated weight penalties and possible risk that restarting the rocket motor could damage the spacecraft? I believe that a quite plausible theory why the 1993 Mars Observer disappeared was that when they tried to restart the onboard rockets to slow the spacecraft down there was a propellent explosion that may have destroyed the spacecraft.

      Besides, the Mars Global Surveyor showed that aerobraking can be done safely.
  • Thrust breaking is not going to be around for long. Why? Because it isn't acceptable. Too difficult? Yes it isn't easy to do, but then again getting a ship to mars isn't easy either but we did that. This is just the next hurdel. Most of the fuel is spent escaping the earth, the last thing you want to do is add to the total mass that must be accelerated. Not to mention that when a thrust reducing craft reaches mars its total mass is far greater than a craft that uses aerobreaking. Did I mention that crafts that use aerobreaks can get there faster, while using less fuel? AND... If you were to ever plan a return trip from mars, the amount of fuel you would need if you relied on thrust to induce your -delta V, would be FAR too much. The air density CHANGES? No!! You're KIDDING!! Next thing your going to tell me is that the atmosphere is constantly changing, almost like WIND! Gimme a break. If you REALLY want to understand this topic, and the different proposed methods for reaching mars with a crew and returning them to earth, then you should go checkout/buy The Case For Mars [amazon.com]. The solution is not giant, zillion dollar monster ships, but small, light, quick craft.
  • Don't forget, the Martians have an excellent track record of taking out our spacecraft. I wonder if they'll let this one into orbit, or take it out the way they did the Mars Observer in 92 [nasa.gov] or the Russian Phobos missions.
  • The twin failures of the Martian Chemical Orbiter and the Martian Polar Lander in 1999 have ...

    Should be Mars Climate Orbiter and Mars Polar Lander, if I recall correctly...
  • We don't need no steenking brakes!

    Why bother with braking at all? Why not just design the thing to survive smacking into the surface of the planet at 4,000 kph or whatever it's going to be doing when it gets there?
    • > Why bother with braking at all? Why not just design the thing to survive smacking into the surface of the planet at 4,000 kph or whatever it's going to be doing when it gets there?

      Given our recent track record, "smacking into the surface of the planet at 4000 kph" is about all our probes have done anyways. Might as well build for it ;)

    • Ahhh, but we want it to *stay* in orbit, that's why this problem is hard :)

  • Some extra information... (Score:3, Informative)

    by JoeRobe ( 207552 ) on Thursday October 04, 2001 @03:49PM (#2389397) Homepage
    Just a comment: I'm reading some comments from people saying that this is the first time aerobraking has been used. This is not true. Mars Global Surveyor (http://mars.jpl.nasa.gov/mgs/) used aerobraking to do it's orbital insertion several years ago. This is said in the article, so I'm surprised that people are saying that's it's a new technique. In any case, MGS's aerobraking phase was extremely successful. There is of course this fear of the atmosphere suddenly thickening, but this wouldn't happen in a matter of seconds, it would take quite awhile, enough time for the spacecraft to respond.

    The story says that the MGS had some problems with aerobraking. Yes, it had some problems, and they said it took longer than it should have, which it did, but the way that they did it was much safer than direct orbital insertion with conventional propulsion systems. The primary source of the problems was (and I know this from following its news DURING it's aerobraking phase) that they didn't want to hurt an already damaged solar panel, so they were being very conservative because if they lost that panel, the mission was over. They normally could have easily handled the inconsistencies, but that in combination with the solar panel problem made them reevaluate some things:

    To make sure the panel would be alright, they needed the pressure on the panel to be less that 0.2 N/m^2. They could only do this by extending the aerobraking phase. The major reason for breaking it up into two phases was because there would be a solar conjunction in June, 1998 in which we would not be able to talk to MGS for awhile. Thus we got it out of aerobraking mode before we were going to lose communication. It began phase two so late because a major part of the mission was to map Mars, and to do this required the spacecraft to be in certain places at certain times. To achieve this, they needed to wait awhile before restarting aerobraking.

    There was not a fear of "crashing" the spacecraft here - they wanted to keep that solar panel intact, so they lengthened the aerobraking phase, which made them rearrange the mission slightly. It really wasn't a big deal.

    Also, "labor-intensive" is a bit of a stretch - the orbits at the beginning of the first aerobraking phase were on the order of a couple days, and only a fraction of that time was spent going through the atmosphere, which gave them a very large amount of time to figure out where the spacecraft was and where it was heading. The phase 2 aerobraking orbit (much easier than phase 1) to begin with was about 12 hours. It definitely wasn't a scramble. They also fail to mention that a lot of science was done both during and in between the aerobraking phases - it wasn't a wasted year.

    Also, it seems to me that now that we have the information (density data, etc.) from the MGS aerobraking, the Odyssey aerobraking predictions will be much better. In addition, if the MGS predicted atmospheric densities and such were so far off for the MGS mission, and MGS still survived, then Odyssey will do fine. It's just a matter of being conservative.

    Let's remember that the spacecraft doesn't just go flying into the atmosphere, it gets itself into a very large, very elliptical, "rough" orbit, after which it begins aerobraking to lower the orbit and slows itself down. I'm sure somewhere on the MGS website you can see how it lowered its orbit with each pass, making it more circular. It's really slow, and from what I've seen from MGS, quite safe procedure, assuming you're careful.

    I don't know if this helps anyone out. But really, the aerobraking phase isn't all that dangerous, and using the MGS as an example of how difficult it is is definitely a mistake.

    JoeRobe

  • Aerobraking is an elegant solution, making possible missions that aren't otherwise possible. Applying the lessons learned from Mars Global Surveyor, just make sure your structural design is sound, and go about your aerobraking conservatively and patiently.

    In other words, you do the statistics and you just dip far enough into the atmosphere that there is only a .00000001% chance (or whatever level of risk you're willing to assume) of overheating or overstressing the spacecraft on any given pass.

    Maybe someday thrust will be so cheap we don't need to spend weeks in an aerobraking phase, but until then, I hope we get very good at it.

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