The Art of Aerobraking

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.""

Re:"Uncertain atmosphere"

[Dyolf Knip]

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.

For those who want to see it in action.

[Dragon218]

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

[Listen Up]

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.

Re:Aerobraking and probe intelligence...

[Johnny Vector]

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.

Atmosphere Cheap - Thrust Expensive

[Manhigh]

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.

Re:Aerobraking and probe intelligence...

[EccentricAnomaly]

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).

Aerobraking better than "Trust in Thrust"

[GPS Pilot]

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.