7 ms·

Philae's landing is commonly represented as a vertical descent. In fact, it seems there was considerable lateral velocity. My guess is that Philae landed tumbl

by udev 12y ago

Philae's landing is commonly represented as a vertical descent. In fact, it seems there was considerable lateral velocity. My guess is that Philae landed tumble-weed style, jumping, and rolling until it hit a wall, in this case, the cliff that shadows it.

TheBiv 12y ago
How did you come to this conclusion? The images shown may just be a matter of perspective and the lander still could've had a vertical descent
chockablock 12y ago
You can't tell that from this sequence of photos, since the orbiter was not viewing the descent from directly 'above' the landing site. After releasing Philae, Rosetta performed a maneuver to establish a new orbit. There are some nice plots of these trajectories here: http://www.bis-space.com/2014/11/12/13849/about-landing-on-a-comet http://www.bis-space.com/2014/11/12/13849/about-landing-on-a...
udev 12y ago
Philae is 1km away from the original landing site. There must've been lateral velocity.
- ChuckMcM 12y ago
  This is my understanding from reading the web site and some of the proposal papers associated with the mission. The plan was that Philae would come in at net 0,0 velocity with respect to the normal of the landing site. That involved putting it into an orbit which intercepted the landing site. This was made a bit more challenging as the top thruster on Philae failed to fire, which meant it was unable to circularize its interception orbit. And that resulted in some horizontal motion at the time of landing. That said, the motion seems to have been well within the safely margin of the harpoon system from what I can read about that subsystem. Since we aren't on the comet we can only guess what happened but one speculation I've read that seems plausible is that the harpoons "fired" but did not "release", which is to say they tried to move and did not. Imparting a delta-v that was vertical with respect to the surface normal of the landing spot. That delta-v, combined with the landers imperfect alignment, contributed to the sideways journey. It did confirm that you really do want something like harpoons to hold you to the comet (and potentially to asteroids if you're doing missions to them).
- theoh 12y ago
  No, the comet is spinning. That is what happened between the first and the second landing sites.
  - ajuc 12y ago
    I thought that too, but then they wouldn't be able to target the first spot so perfectly if they haven't considered that. And assuming 12h day the rotation should be around 2 pi R / 12 h ~= 2 km /h assuming R=4 km. That would result in much bigger velocity than 0.5 km/h when bouncing, and would exceed escape velocity 2 times, just from horizontal part of velocity). So I doubt it's so simple.

by udev 12y ago

TheBiv 12y ago

How did you come to this conclusion? The images shown may just be a matter of perspective and the lander still could've had a vertical descent

chockablock 12y ago

You can't tell that from this sequence of photos, since the orbiter was not viewing the descent from directly 'above' the landing site. After releasing Philae, Rosetta performed a maneuver to establish a new orbit. There are some nice plots of these trajectories here: http://www.bis-space.com/2014/11/12/13849/about-landing-on-a-comet http://www.bis-space.com/2014/11/12/13849/about-landing-on-a...

udev 12y ago

Philae is 1km away from the original landing site. There must've been lateral velocity.

ChuckMcM 12y ago
This is my understanding from reading the web site and some of the proposal papers associated with the mission. The plan was that Philae would come in at net 0,0 velocity with respect to the normal of the landing site. That involved putting it into an orbit which intercepted the landing site. This was made a bit more challenging as the top thruster on Philae failed to fire, which meant it was unable to circularize its interception orbit. And that resulted in some horizontal motion at the time of landing. That said, the motion seems to have been well within the safely margin of the harpoon system from what I can read about that subsystem. Since we aren't on the comet we can only guess what happened but one speculation I've read that seems plausible is that the harpoons "fired" but did not "release", which is to say they tried to move and did not. Imparting a delta-v that was vertical with respect to the surface normal of the landing spot. That delta-v, combined with the landers imperfect alignment, contributed to the sideways journey. It did confirm that you really do want something like harpoons to hold you to the comet (and potentially to asteroids if you're doing missions to them).
theoh 12y ago
No, the comet is spinning. That is what happened between the first and the second landing sites.
- ajuc 12y ago
  I thought that too, but then they wouldn't be able to target the first spot so perfectly if they haven't considered that. And assuming 12h day the rotation should be around 2 pi R / 12 h ~= 2 km /h assuming R=4 km. That would result in much bigger velocity than 0.5 km/h when bouncing, and would exceed escape velocity 2 times, just from horizontal part of velocity). So I doubt it's so simple.