# Could Rosetta have lassoed Chury to hitch a ride instead of chasing after it for 10 years?

The Rosetta probe had to fly-by several bodies to acquire the necessary velocity to match 67P/Churyumov–Gerasimenko (Chury). Towards this end it was launched almost a decade in advance. Philae will fire a pair of harpoons to anchor itself to the comet.

Thinking aloud - between them, Rosetta and Philae mass around 192 kg. Chury's mass is 3.14×10^12± 0.21×10^12 kg

Call this a thought-experiment ...

Say now Rosetta were to be launched shortly before Chury were at a suitable rendezvous. (As contrasted to 10 years in advance.)

Say further Rosetta were to cold-fire a pair of harpoons to intercept Chury. The harpoons could be attached to a winch aboard Rosetta to reel the craft in towards the comet.

• Could Rosetta have lassoed Chury, and hitched a ride instead of spending 10 years to match velocity, and orbit?
• How much force would such a tether/lasso experience?
• Is any material produced by our contemporary technology capable of withstanding this force whilst being light enough to be launched into deep-space?

PS: The draw-back of the lasso method is it's touch-and-go nature. On the other hand, it may have the potential to afford a relatively quick study for any target-of-opportunity.

• And how much Δv do you expect those harpoons to absorb?? – Loren Pechtel May 24 '14 at 20:28
• @LorenPechtel I wondered about that - the second question should cover it ... – Everyone May 25 '14 at 3:50
• esa.int/esatv/Videos/2013/12/Rosetta_s_Journey_B-Roll/… is an animation of Rosetta's trajectory. Notice the probe is thrown into an orbit with a nearly 5 AU aphelion, similar to the comet's orbit. This orbit is nearly tangent to the comet's orbit at rendezvous. I'd venture to guess the gravity assists provided around 4 or 5 km/s delta V. – HopDavid May 26 '14 at 1:37
• If the rendezvous were at the comet's perihelion from a transfer orbit with a 1 AU perihelion, I'd guess around 10 km/s would be needed from a rendezvous. I doubt there's a harpoon or tether that could endure a 10 km/s impact. If there were, I doubt the harpoon would hook -- the loose comet material would vaporize on impact. – HopDavid May 26 '14 at 1:43

Interesting question. There's a few problems with using a harpoon for capturing something of this nature.

1) Tether strength - the tether material is going to be under a reasonable amount of tensile stress when Rosetta is pulling itself in, but nothing compared to the tensile shock it will experience if you have any serious delta V between Rosetta and the target.

2) Targeting/Accuracy - if the two orbits are out of plane with one another and they are going some reasonable speed then you'll only have a very small window in which to fire your harpoon. A cross speed of 1000m/s means that the 4km target will pass in 4 seconds.

3) Harpoon speed - Assuming you have targeted your harpoon correctly, you still need it to reach the target. If there is a few km/s between your speed and that of your target (assuming your target is going faster) then your harpoon has to be fired at least as fast as that. On top of that it's going to be travelling the distance between your spacecraft and the target, so you had better be close or have a LONG tether.

4) Barb strength - so lets assume you've managed to hit your target and your tether isn't going to give way. The barbs that you have planted deep into the target are now carrying the shock. This will typically be compressive so that's not as bad as it could be.

There's a lot that can go wrong, mainly because you didn't match the speed of the target, but if you could do that then why would you need a harpoon!

• Don't forget g forces. If the speed difference is huge, then it would mean a large jolt, at both ends. Not to mention comets aren't very cohesive... – PearsonArtPhoto May 26 '14 at 22:10
• Another issue--given the Δv involved your harpoons are going to have to be made of unobtainium to avoid being vaporized. – Loren Pechtel Nov 18 '14 at 5:43
• @LorenPechtel Not really the case at all. It all comes down to Δv as you've said, so assuming you have enough information about the target object you can design for your harpoon to strike the target at a more much reasonable speed (~100m/s). – ThePlanMan Nov 22 '14 at 3:32
• @FraserOfSmeg And how in the world do you propose to do that? Had Rosetta gone the straight route the encounter velocity would be many miles per second. Had you equipped the harpoons with rockets powerful enough to make a reasonable approach velocity you wouldn't have actually gained anything--that energy is going to show up somewhere, if it doesn't vaporize the harpoons it will vaporize something else. The only way to survive such velocity changes is to ensure the energy shows up in something that's not your spacecraft. (Usually that means your rocket exhaust.) – Loren Pechtel Nov 22 '14 at 3:40
• @LorenPechtel isn't this outlined in my answer: 'nothing compared to the tensile shock it will experience if you have any serious delta V between Rosetta and the target' etc.. I assumed your comment alluded to something I hadn't covered? – ThePlanMan Nov 22 '14 at 3:47

Comet Chury moves at a speed of 37 km/s. If Rosetta were launched just in time for a rendezvous near Earth, it'd be traveling at maybe 12 km/s (relative to Earth) at a large angle relative to Chury.
Let's just focus on Chury's speed for the moment and ignore the rest. Rosetta flies by Chury at an angle of let's say 90° and fires its harpoon. Now it has to build up 37 km/s in the direction that Chury travels. If we allow Rosetta one second to do this, it will be subjected to $37 km/s^2$ of acceleration, or 3800 G. Rosetta would need to be built like an artillery shell to withstand this force.
Rosetta would also need to reel out on the order of 37 km of bungee cord (in one second), with a tensile strength of 3800 x 200 = 760 tons.
So not only is this an impossible task, it's 4 orders of magnitude away from being possible.

You could use a longer cable and/or reel it out beforehand, but that's just rearranging deckchairs. The big problem is the G-force. To lower the G-force by a factor of 1000, you need 1000 times the length of cable: the two are inversely proportional.
The only way around the problem is by getting Rosetta to match Chury's speed and direction, and then you're back to either ESA's original approach using gravitational slingshots, or you could use a Saturn V-class vehicle and launch Rosetta at the required speed. The choice between those is a matter of budget.

Hitchhiking a comet by tether is a concept which actually today has been awarded \$100,000 to "conduct a nine-month initial definition and analysis study of their concepts", by NASA. This is part of the NIAC program (NASA Innovative Advanced Concepts). So at least a PhD in Aeronautics and Astronautics, MIT 2012, will spend some time to find out what questions one needs to ask about this.

The lander had already been modified to adapt to the new comet since C46 was lost in terms of the launch window opportunities. Technology might have become better in the nine or so years but in 2002/2003 there would have been little advance that would allow the mission director to guarantee a significant better chance of success by postponing for say another eight years, and have approval from the funders, knowing the revised trajectory detail is far more risky.

My original point however was with the difference in velocities, a high(er) powered harpoon would be required to launch the grappling claw, and with that comes all sorts of new calculations, not only the movement of the platform once the harpoon has been fired, but also the trajectory of the craft on the way to C47, and ultimately the mass of equipment and fuel at lauch.

I would guess there is also a reduced risk of Rosetta sustaining damage through smaller particles travelling with Chury by coming up relatively slowly alongside.