Mission to mega-comet 2014 UN271

[RCgothic]

So there's a giant comet we discovered a few years back is now on the way to making its closest approach to the sun somewhere just beyond the orbit of Saturn in Jan 2031.

This guy calculated some DV figures, for flyby or rendezvous:

 

So I'm interested.

Firstly are these figures right?

2033 is somewhat after perihelion. Could we get a mission to the comet before perihelion in 2031? What would that take? And do we even want to get there before? Does the period of maximum activity lag perhelion much the same way the hottest days of the year lag summer solstice?

Secondly, a minimum C3 of 113 is a big ask, and minimum 12.5km for a rendezvous would be the largest ever deep space DV manoeuvre. How would you guys assemble a mission to the comet? Flyby or rendezvous? Is rendezvous even possible with current launch vehicles?

I have some ideas of my own that I'm currently calculating, but thought I'd start a discussion before steering in any particular direction. :-)

[kerbiloid]

We should carefully nuke it to keep in available orbit. Then patiently study.

[RCgothic]

Calculating intercepts is tricky.

My first thought is that 2014 UN271 is super-inclined relative to earth's orbit. This basically proscribes taking full advantage of earth's velocity around the sun unless aiming for the ascending or descending node.

At a current solar radius of 19-21AU UN271 is already inside the descending node (~79.6AU), and the ascending node at ~10.9AU occurs after perihelion. I wonder if this is why they're targeting 2033 for a rendezvous (after perihelion).

There must be easier comets to visit. Nevertheless, I will persist.

 

 

[mikegarrison]

Perihelion is outside Saturn's orbit? I'm sorry, but why send something out that far just to look at what we think is a comet?

[kerbiloid]

It's presumably a comet from the hypothetic Oort cloud.

[mikegarrison]

34 minutes ago, kerbiloid said:

It's presumably a comet from the hypothetic Oort cloud.

Aren't they all?

[RCgothic]

Yes, it's thought be be a large Oort cloud object, ~60km in diameter, that we've spotted in good enough time to *potentially* go and have a look.

But we probably won't. It'd be a major mission launching in about 7 years and as nothing has yet been prepared chances are slim to none.

It just caught my interest, so I'm seeing if I can calculate a hypothetical mission, and I'm interested in what other people come up with.

5 minutes ago, mikegarrison said:

Aren't they all?

Short period comets are not, at least as far as I'm aware.

Long period comets (from out by the Oort cloud) are hard to get a good look at because they can't be predicted from historical observations. They have to be spotted in enough time to plan a mission.

Edited July 19, 2021 by RCgothic

[kerbiloid]

3 minutes ago, mikegarrison said:

Aren't they all?

The short-periodic aren't.

Say, Halley isn't.

[RCgothic]

My current issue is that I'm struggling to reconcile the stated period with current position, semi-major axis and eccentricity.

With a time until periapsis of ~9.5 years and a period of 3 million years, the mean anomaly (M) should be basically 360 deg. Eccentricity is almost 1, therefore the eccentric anomaly M= E - e*sin(E) should give E of basically 360deg as well.

This would put the comet well inside the stated position of 19-21AU.

Also I can't get true anomaly and eccentric anomaly to agree for the given values.

@sevenperforce interested in your thoughts.

[sevenperforce]

52 minutes ago, RCgothic said:

Also I can't get true anomaly and eccentric anomaly to agree for the given values.

@sevenperforce interested in your thoughts.

I haven't had a chance to look very closely at the orbital parameters, but as far as mounting an extremely high-dV mission is concerned, I think the best we can reasonably do is build something like Dawn (but with bipropellant hypergolics and probably 6-8 krypton thrusters rather than 3 xenon thrusters), mount it on top of a Centaur SEC, and then launch that entire stack to LEO on a Starship. 

Retank the Starship from a waiting tanker and burn nearly to depletion on the intended trajectory, then release the Centaur, which executes the next burn, then release the rendezvous spacecraft. The Starship would turn around and retroburn in order to reach a high-elliptical Earth orbit so it can return.

If we assume a 3-tonne spacecraft (a little over twice the size of Dawn) with similar total dV (around 11 km/s), then a fully-tanked Centaur SEC is going to be able to give it 7 km/s. That whole assembly is going to be about 26 tonnes. A full-fueled Starship in LEO can send a 26-tonne payload out with about 10.5 km/s with 30 tonnes of reserve propellant for landing, although the need to retroburn will likely lower that to around 8-9 km/s. If we use one of Elon's described stripped-down expendable Starships, it would be able to push 13 km/s.

So this approach can get us between 26 and 31 km/s worth of dV. Is that enough for a rendezvous?

[mikegarrison]

Short period comets aren't in the Oort cloud *now*, but I thought it was assumed that they came from there and have just been randomly perturbed into orbits that are now much closer to the sun.

[kerbiloid]

1 hour ago, mikegarrison said:

Short period comets aren't in the Oort cloud *now*, but I thought it was assumed that they came from there and have just been randomly perturbed into orbits that are now much closer to the sun.

The Oort cloud itself is currently a mathematical abstraction based on the comet orbit statistics.
There is no direct evidence of its existence, they just presume that it exists due to mass similarity of orbital elements of the observed comets.
So, the short-periodic comets do not add something to this picture, while the comet from outer regions is the comet directly from the Oort cloud if it exists.

[RCgothic]

1 hour ago, sevenperforce said:

I haven't had a chance to look very closely at the orbital parameters, but as far as mounting an extremely high-dV mission is concerned, I think the best we can reasonably do is build something like Dawn (but with bipropellant hypergolics and probably 6-8 krypton thrusters rather than 3 xenon thrusters), mount it on top of a Centaur SEC, and then launch that entire stack to LEO on a Starship. 

Retank the Starship from a waiting tanker and burn nearly to depletion on the intended trajectory, then release the Centaur, which executes the next burn, then release the rendezvous spacecraft. The Starship would turn around and retroburn in order to reach a high-elliptical Earth orbit so it can return.

If we assume a 3-tonne spacecraft (a little over twice the size of Dawn) with similar total dV (around 11 km/s), then a fully-tanked Centaur SEC is going to be able to give it 7 km/s. That whole assembly is going to be about 26 tonnes. A full-fueled Starship in LEO can send a 26-tonne payload out with about 10.5 km/s with 30 tonnes of reserve propellant for landing, although the need to retroburn will likely lower that to around 8-9 km/s. If we use one of Elon's described stripped-down expendable Starships, it would be able to push 13 km/s.

So this approach can get us between 26 and 31 km/s worth of dV. Is that enough for a rendezvous?

I'm not hugely familiar with ion thrusters, but I would have thought that what worked well for Dawn wouldn't work as well for this application?

I think Krypton thrusters certainly have a high enough ISP, however this application may require a lot of point impulse at rendezvous and krypton thrusters have even less thrust than xenons. How long would it take to alter trajectory by 12.5km/s (or more, given the inefficient long burn)?

 The other concern I have is power. At 11AU the solar power available is 1/10th of that available to Dawn at 3AU, and RTGs don't provide very much power either. 

Starship + Centaur would definitely have enough power for the initial trajectory for a spacecraft 3t wet.

A science package equivalent to Dawn, 750kg ish dry seems reasonable for a rendezvous. Might want more for a flyby trajectory to maximise data collected in a short encounter.

[RCgothic]

I found the error in my maths, I was out by a factor of 2pi on my conversion between time elapsed and mean anomaly, so that all makes sense now.

So, getting to the comet 1y before periapsis assuming a launch in early 2026 (big ask) requires a departure velocity of 41.2km/s, and it will arrive with an orbital velocity of 9.1km/s.

At arrival, the comet has an orbital velocity of 12.8km/s and the angle to the intercept orbit is 87.3deg. The DV for rendezvous is therefore 15.4km/s. Ouch.

Departure is at 53.0 deg to earth's velocity, which is 29.8km/s. Therefore need an excess departure velocity from earth of 33.3km/s. Earth's escape velocity is 11.2km/s, so that's 44.5km/s in LEO, or a DV of 36.7km/s. C3 of 1110km2/s2. Oh, and that'd probably need to be from a polar parking orbit. OUCH.

Ok, so that's definitely the reason they're not attempting an interception before periapsis!

[YNM]

3 hours ago, RCgothic said:

Ok, so that's definitely the reason they're not attempting an interception before periapsis!

Yeah, ideally you want the object to be moving in the same direction as you do. Imagining more of an Ulysses-style ejection orbit, maybe.

Honestly the only way out to me is probably to visit it while it recedes. It's further out than Saturn or Jupiter so I don't think there's a lot there that's interesting to observe as it recedes from the Sun... unless if it's internally heated or something which would mean we could've visited it at anytime.

Edited July 20, 2021 by YNM

[RCgothic]

I've re-run my calcs for a trajectory similar to the originally proposed mission, and it broadly agrees. C3 of 120km2/s2 for earth departure (11km/sDV) and DV of 13km/s at rendezvous at the ascending node in ~2032.

Uses the full advantage of earth's velocity, makes such a huge difference.

I think the DV of 13km/s at ~11AU is going to be the difficult bit. Limited power for ion thrusters from RTG/solar and high-ish TWR required.

A flyby would definitely be easier 

[RCgothic]

Scoping of thruster power:

A constant power of ~8kw required at an exhaust efficiency of 70% to add an energy of 84.5 billion joules in a six month burn. The current MMRTG produces 110W.

The space station's new ROSA solar arrays produce 20kW from ~82m2. At 11AU instead of 1AU they'd produce just 165W each.

So power would definitely be a challenge for an ion powered craft.