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.

September 10th Hearing, House Subcommittee on Space – Exploring Our Solar System: The ASTEROIDS Act as a Key Step

I went to this hearing, and frankly was a little disappointed by this aspect of it. The sole witness who dealt with the Asteroid Act was Joanne Gabrynowicz, Director Emerita, Journal of Space Law. I am not a space lawyer, much less a space law professor, but she took a viewpoint along the lines of “nothing should be done until there is an international consensus to clarify the Outer Space Treaty. She flat out said that the Asteroid Act was a “terrible bill” in discussion afterwards. I had discussion afterwards with David Gump (CEO, Deep Space Industries) and Dean Larson (the local Planetary Society rep and the recent author of the OpEd in the Wall Street Journal), together with Henry Herzfeld (Prof. at G. Washington) and i think it is fair to say that none of us agreed with that position.

My take, and believe that at least Dean Larson has a similar opinion, is that Asteroid Mining is clearly legal now but that there is a need for a US Law clarifying the rules for US companies (so that, e.g., two US companies got into an argument over the same minerals on the same asteroid, there would be some law to guide them). The Asteroid Act would do that, so I support it.


Prof. Gabrynowicz and Deep Space Industries CEO David Gump discussing the Asteroid Act after the meeting.

The Asteroid Initiatives Solar Scout CubeSat Explorer

Artist’s rendition of Asteroid Initiatives Solar Scout™ CubeSat Explorer deploying Chipsat Sensors at a Near Earth Asteroid (NEA) a few million kilometers from the Earth.


These sensors (about the size of a credit card) will embed themselves in the asteroid (or go into or
bit around it) and measure the asteroid’s properties, such as the water and metal content of the body, its mass and magnetic field, and even its internal composition. This can be done for a fraction of the cost of similar measurements from a conventional spacecraft – the entire package (Solar Scout + chipsats and their deployment mechanism) would mass about 20 kg at launch. Asteroid Initiatives will make it possible for individuals to own their own sensor to be deployed at a asteroid.

Asteroid Initiatives will substantially lower the cost of asteroid prospecting by leveraging the technology already developed for cubesats, chipsats, and cubesat solar sails, with the Solar Scout being based on the NASA developed NEA Scout.

Before you can do asteroid mining, you have to do asteroid prospecting, and the Solar Scout will make prospecting affordable.

Comet 67P becomes a world.

The European Rosetta spacecraft is steadily closing in on Comet 67P (AKA Comet Churyumov-Gerasimenko). Even though there have been complaints about ESA image release policy (unlike NASA planetary missions, ESA tends not to make everything available immediately), what has been released is sufficient to take the comet nucleus from being a spot of light to an actual world.

DLR last week released a movie of approach images, while ESA today released a shape model with more detail, prepared from higher resolution images (from the OSIRIS camera) than the publicly released NAVCAM images.

ESA Released shape Model for Comet 67P

ESA Released shape Model for Comet 67P

Rosetta plans to orbit Comet 67/P starting next month (August), and then, in November, 2014, place the Philae lander on the surface of the comet. (The 100 kg lander will actually harpoon the comet to make sure it gets a good grip of the surface.) If all goes well, this mission should revolutionize our understanding of comets; I, for one, can’t wait to see the first images from a cometary surface.

The Moon passing in front of the Earth



This photo, from the Epoxi (or Deep Impact) spacecraft from 31 million miles away, shows the Moon transiting (i.e., passing in front of) the Earth. What strikes me is not the Earth (although notice the Sun glinting off of the Indian and Atlantic Oceans) but how dark and alien the Moon looks. The Moon may look white in the night sky, with nothing to compare it to, but it’s really about as dark as an asphalt parking lot, and that really shows in this picture.

This picture was the NASA Astronomy Picture of the Day for Sept 3, 2008.