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Rosetta is en route to rendezvous with Chury. Briefly, the mission comprises an orbiter, and a lander. The latter named Philae.

Wikipedia writes to say

... The lander is designed to touch down on the comet’s surface after detaching itself from the main spacecraft body and “falling” towards the comet along a ballistic trajectory. ...

  • Why is Philae not provided with a propulsion system?
  • Could a random event (say, a tumble even outgassing), potentially, deflect Philae's planned trajectory?
  • Is it's mass of 100kg on Earth, and Chury's infinitesimal gravity adequate to ensure touch-down on the comet along the planned trajectory?
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Let me just expand a bit on @Tildalwave's answer. Most landers on airless bodies need a propulsion system, because they will be going too fast otherwise to land. But that's only because most landings have been done on objects with a lot of gravitational mass. Let's just try and figure out what the escape velocity would be. Wikipedia gives us the following clues, including some comet info:

  • Size: 4 km diameter
  • Shape: Irregular, but roughly oval shaped.
  • Average comet density: 0.6 g/cm^3

Okay, that's not a lot to go on, but what can we gleam from that? Well, not a lot, but let's just try and figure out a mass, and then escape velocity from that. Here's the numbers leading up to that:

  • Volume: (4 km diameter sphere) $3.4 \times 10^{16}$ cm$^3$
  • Mass: $2.0 \times 10^{13}$ kg
  • Escape Velocity (2 km from center of mass): 1.2 m/s

Okay, that's a pretty low escape velocity, walking would more than do it, and you could easily jump off of the comet! So, what would you need to do if you were going to actively try to land on it?

  1. Some sort of a radar system, to know how close you were to the ground
  2. Propulsion system

Those take power and mass, adding much complexity. The alternative is to have Rosetta put you in to a ballistic trajectory, landing at exactly the escape velocity. It's not hard to absorb an impact of 1 m/s. In fact, the Mars Phoenix Lander landed at 2.4 m/s on Mars, making this even more realistic.

The thruster system makes the whole thing more complicated. The only real advantage would be some sort of an abort capability, and a slightly improved landing from Rosetta, but that's fairly insignificant compared to the added complexity of the mission. The delta v required to change from orbiting to ballistic is negligible for Rosetta, and it already requires such capabilities anyways. Why go through the bother of adding on another several complex systems?

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Actually, Philae does have a propulsion system. As explained in this related question, its Active Descent System uses a cold-gas thruster to propel the lander towards the comet if needed.

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  • $\begingroup$ Didn't that break or leak or something? (Oh, heh, they only discovered the problem yesterday, and this post is 7 months old. oops) $\endgroup$ – Mooing Duck Nov 13 '14 at 23:17
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It doesn't really need it and it would needlessly add to its mass. Rosetta will assume a relatively slow and probably highly elliptical orbit around 67P/Churyumov-Gera...aaagh! Chury!, with the perigee at only roughly a kilometer away from it. The orbit has not yet been determined though, see my related question and the answer there. It likely won't be until Rosetta transmits its own close proximity observations of the comet, and might even change in time as the comet's coma, tails and in general its surface activity increases during its flyby closer to the Sun. Rosetta of course also has its own semi-autonomous collision avoidance system onboard, that will enable it to react to any debris in its path and adjust its trajectory.

Anyway, such elliptical orbits leave mission control plenty of leeway to later (when the Rosetta will be in orbit) decide at what point to release Philae on its ballistic trajectory. My guess would be this trajectory will be attempted from the point of Rosetta's flying past the comet in the opposite direction of its movement, to reduce the chance of incoming debris to near zero. The lander also has a harpoon (see the spike in the middle of the legs frame) that will fire towards the comet immediately after the touchdown to hook itself on it. Additionally, each of the three legs have battery powered ice screws in between the two padded gears, to additionally grip to the comet and pads to cushion the collision / landing:

   Philae lander

   Training sessions for the Philae comet lander (Source: DLR, National Aeronautics and Space Research Centre of Germany)

To answer your questions more directly tho, adding propulsion system to it would be needless weight. The Philae lander wouldn't be able to react fast enough with its Attitude Control System (ACS), if it had any onboard, and to avoid any possible obstacles. Instead, the lander will be put on a collision course (or ballistic trajectory, your pick) with the comet with an approximate velocity of 1 m/s. That is, I would say, higher relative velocity than lowest achievable with its release system, and the resulting kinetic potential should suffice to negate effects of any collisions with smaller debris or pressure in the opposite vector to its movement from outgassing, as you mention. The lander's mass is negligible in gravitational sense, but it still adds to its inertial mass. With a bit of smart decision making from the mission control, they ought to have plenty of opportunities to assure its touchdown. The harpoon, soft pads and powered drills should do the rest in assuring the lander stays put where it landed.

Of course, there are chances all of this will go sour. One of the main concerns I've seen mentioned online is, that the lander itself creates with the harpoon and the screws deep enough cracks for a part of the comet on which it landed to simply chip away from the comet's main body. That doesn't sound too serious as the lander could still do all of its experiments (some might become less conclusive tho, like for example the CONSERT comet nucleus radio wave sounding experiment), just on a smaller piece of the icy rock, but it could cause the lander to completely unhook from any piece of the comet's surface, or the chipped away piece starts spinning, squashes the lander between the chipped away piece and the comet's main body and causes it to lose its grip to it.

I guess we'll have to wait and see, but adding propulsion system to it wouldn't have mitigated main concerns I've seen mentioned over its chance of success either. In fact, it would only add to the problem, with more parts that could malfunction and larger mass of the lander increasing its physical influence on the comet.

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  • $\begingroup$ Wouldn't go ballistic/no-go ballistic be impaired by the lag between Mission Control, and Rosetta? How much mass would a minimal propulsion system add? Albeit that last may qualify as a separate question ... $\endgroup$ – Everyone Oct 2 '13 at 1:26
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    $\begingroup$ @Everyone - Yup, better a separate question for the other one. As for the first one, all of these go/no-go decisions will be programmed into the Rosetta and Philae before the lander is deployed. They will target a specific landing site from a specific orbital position, none of which are yet determined. I'm also waiting for more data with my mentioned question, I guess we'll both have to wait till April 2014. ;) $\endgroup$ – TildalWave Oct 2 '13 at 1:30
  • $\begingroup$ Downvoted because Philae does have a propulsion system... $\endgroup$ – Hobbes Mar 25 '14 at 12:03
  • $\begingroup$ @Hobbes Undeleted because it turns out it doesn't. Well, it was supposed to fire a simple upward pointing thruster as its two harpoons would have fired, but since that didn't happen and it was never meant as an attitude control during descent (NASA's term Active Descent System is misleading), I consider the rest of my answer relevant. Philae did land by help of Newton and that alone, to quote one of presenters during the live landing event from ESOC. In fact, it did that three times (so far). I'll probably update it as some point in time to make this clearer, time permitting. $\endgroup$ – TildalWave Nov 13 '14 at 15:29
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The odds of any unexpected event altering Philae's trajectory is negligible. I suspect that the most likely cause for putting it off-target, ironically, would be a propulsion system malfunction, or a leak in a propellant tank!

Not sure what you mean with the last question, "Is it's mass of 100kg on Earth, and Chury's infinitesimal gravity adequate to ensure touch-down on the comet along the planned trajectory?", but Philae is equipped with harpoons to anchor it - otherwise, even if it had thrusters and guidance systems, it would be very difficult to keep it from bouncing off the comet.

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