Can Rosetta blow Philae over ?

On Slashdot, I have been involved in a discussion about moving or torquing Philae using the thrusters on Rosetta.

I should say that this will be dangerous and would be unlikely to be attempted until we are close to the end of the mission. It would mean finding Philae, bringing Rosetta down close to the surface, and firing Rosetta’s thrusters right at Philae, all while some long round-trip time from Earth (so, Rosetta would have to do all of this autonomously).

I am also going to ignore for now any issues of damage from nearby thruster firings. That’s something the spacecraft designers would know, and I would only be guessing at.

Rosetta has 24 bipropellant 10 Newton thrusters and is 2.8 x 2 m, not counting solar panels. Philae is 1 x 1 x 0.8 m. Suppose Rosetta fires a thruster at Philae from 3 meters away – Philae is then 1/3 of a radian across, or about 0.1 steradians. Suppose the thruster has a exit angle of 2 pi steradian (i.e., the whole hemisphere away from the spacecraft, which is surely conservative). So, Philae would experience a force of ~ 10 N x 0.1 / 2 pi ~ 0.2 N. It has a mass of ~ 100 kg, so that would impart a thrust of 2 x 10^-3 m/sec^2. (I am assuming Rosetta has a thruster firing on the opposite side too, so it’s not moving away while it is doing this.) That is actually greater than the 67/P gravity, so Philae could move. If firing were done for say 10 seconds, Philae would have a velocity of ~ 1 cm/sec afterwards and maybe a total flight time of 30 seconds.

I would thus conclude that, while it wouldn’t be possible to move Philae far, it should be possible to move Philae some, maybe to torque to land on all three legs on a little flatter terrain and thus get it some more sunshine.


The relation between Science and Science Fiction

There is an interesting article on the Science magazine web site entitled “Physicist who inspired Interstellar spills the backstory—and the scene that makes him cringe. The physicist here is Kip Thorne and “Interstellar” is of course the new movie. (I feel like I grew up with MTW, but that is of course an exaggeration – it was published about 2 weeks after I entered MIT.) Kip Thorne has done a lot of work on General Relativity, but in recent years he has mostly been known for being one of drivers behind the search for gravitational radiation.

What got me to thinking was this statement at the end of the interview :

We learned [that] when you have a fast-spinning black hole, without any accretion disk, and let it just lens the distant sky—a star field—we saw a fantastically beautiful structure that is sort of like a fingerprint, but much more complex. We’ve long known that you’ll get multiple images of each star [around a black hole], due to [the combination of] light rays that come pretty much directly to the camera, [and] rays that go in and circle around the black hole once and come to the camera. But what we found was that on the side of the spinning black hole where space is moving towards us, [you see this beautiful structure].

It was completely unexpected with huge amounts of internal structure in it, regions where the star field appears to be quiescent and other regions where the stars seem to be whirling around in little vortices. To me it’s a lovely kind of discovery in the sense that it is really very beautiful and it arises from a collaboration between a scientist and a group of computer artists. We are submitting a paper about this and about the particular method that Double Negative uses to the journal Classical and Quantum Gravity.

So, thinking about how a black hole might appear to a nearby spacecraft, and actually trying to calculate it for a movie, leads to new research, research which will be very relevant to the attempts to create an Event Horizon Telescope (or EHT) using millimeter wave Very Long Baseline Interferometry.

The EHT is probably the most under-appreciated profound astronomy effort currently underway. (In my experience, VLBI tends to be under-appreciated, which may be why the NSF is thinking of shutting off the VLBA.) The EHT will probably only be able directly observe two event horizons, that of the black hole at the center of the galaxy, Sagittarius A* (Sgr A*), and the back hole at the center of M87, but that could be enough to revolutionize physics.

What doesn’t seem to have penetrated much outside into the wider community is that the EHT is a fundamental test of General Relativity and strong gravity. We really don’t know that black holes exists (although we know that lots of mass is concentrated at SgrA*, we don’t know it has an event horizon, or what sort of event horizon or ergosphere it might have). Maybe gravity and nature do things in some other fashion than the predictions of General Relativity. The EHT will be able to find this out.

So, that’s another post entirely, but what got me here was how a movie (which most scientists would think of as a distraction) has led to real, and maybe very important, science. Can people think of other examples? I know in my own work, many of my best ideas have a Sci-Fi background, of the “how would you start interstellar travel” or “what energy sources would aliens use” type, but those are more like thought-experiments, which have a long history. Can anyone think of example of scientifically motivated entertainment leading to real science? I would love to hear of it.

Philae Status : On its side against a cliff

Here is the status of the Lander as I understand it – Philae bounced 3 times (the first bounce took it up 1 km) and wound up on its side against a cliff. It needs to move, or it drain its batteries by the weekend. So, ESA says they will try this : “We will deploy the MUPUS penetrator for 2/3 of the max. length and then insert it.” The idea (I believe) is to get the craft up on its feet and get it more solar power.

I get the feeling that Philae was just bouncing along (and the comet was rotating under it) until it came up against something vertical that blocked its path. The picture (see below) is pretty spectacular – I had to look at several times to realize it was taken pointing up, not horizontally.

The first panorama from Philae. This is a view up the cliff, not along the surface.  You can see its leg down at the bottom; that is the one sticking up into space.

The first panorama from Philae. This is a view up the cliff, not along the surface. You can see its leg down at the bottom; that is the one sticking up into space.