NASA's Next Big Mission: Dragonfly to Saturn's moon Titan

[James Brown]

Low-frequency radio waves can penetrate ice. Although I don't know if that's sufficient for the transmitting of data.

 

[bilby]

Low-frequency radio waves can penetrate ice. Although I don't know if that's sufficient for the transmitting of data.

Depends how slow you are prepared to go; and perhaps more importantly in this context, how large an aerial array you can have.

 

[DBT]

An orbiter could sample Europa's ocean from its plumes. If there is life to be found in its oceans, perhaps signs of its presence could be found in these samples. An orbiter could alsp measure ice thickness in fissures, chaotic regions, etc, which would get the ball rolling for more ambitious programs such as firing probes into fissures......

 

[skepticalbip]

Low-frequency radio waves can penetrate ice. Although I don't know if that's sufficient for the transmitting of data.

Actually ice is fairly transparent to even high frequency radio waves although water is not. Easily confirmed at home by placing a really cold and dry ice cube in the microwave oven (~2.4 MHz). It will not melt. Wet the surface of that ice cube (a thin layer of water) and the water on the surface of the ice will absorb the microwaves, heat up, and melt the ice cube.

 

[Bomb#20]

I rather like this idea. A modified RTG could do a great job - and they are already designed to withstand crashing back to Earth in the event of a launch problem, so a fairly hard landing could be acceptable.

I suspect the big problem would be communication back to the orbiter - several km of ice is a pretty effective shield. And pulling a cable down from a 'base station' which remains on the surface would have issues as the water in the hole refreezes. Perhaps some kind of 'snow blower' - a pump that sprays the meltwater out of and away from the hole as it is drilled - could solve that problem; Though your probe would likely be instantly frozen in place once it breaks through the ice.

Seems to me existing technology could handle this problem.

https://en.wikipedia.org/wiki/Wire-guided_missile

The unspooling happens in the melted water around the radioactive sphere; the trailing wire is frozen in place but it no longer needs to move.

 

[Derec]

Looks like an interesting mission. I wonder why they chose that rather than a lander on Europa (or a close flyby) to sample the material ejected from the subsurface ocean, with perhaps a chance at finding signs of life?

 

[Derec]

What about a radioactive sphere landed on Europa? It melts the ice and sinks down to the waters below. Then it opens its sensors and sees what it can see, transmitting the results to the orbiter overhead.

I think it would be doable.
But Europa is very cold, you'd need a lot of RTG power to melt through. Also, it has a weak surface gravity, a little weaker than the Moon. Thus it might take a while for the sphere to sink all the way through the ice, if it is really 10km thick or more.

 

[Derec]

Low-frequency radio waves can penetrate ice. Although I don't know if that's sufficient for the transmitting of data.

Low-frequency here means 120-240 MHz according to the article. That is not too bad for communications, esp. since the probe can use all the bandwidth and is not limited to a narrow channel like we are on Earth.

It's nothing like very low frequency (VLF) and extremely low frequency (ELF) bands used for communicating with submerged submarines where such low frequencies are necessary because conductive saltwater absorbs higher frequency radio waves too much. At these frequencies communication speeds are indeed very slow.

 

[Derec]

Low-frequency radio waves can penetrate ice. Although I don't know if that's sufficient for the transmitting of data.

Actually ice is fairly transparent to even high frequency radio waves although water is not. Easily confirmed at home by placing a really cold and dry ice cube in the microwave oven (~2.4 MHz). It will not melt. Wet the surface of that ice cube (a thin layer of water) and the water on the surface of the ice will absorb the microwaves, heat up, and melt the ice cube.

According to the article, ice tends to reflect higher frequency waves, not transmit them.
Also, microwave ovens work at ~2.4 GHz - not MHz.

 

[skepticalbip]

Low-frequency radio waves can penetrate ice. Although I don't know if that's sufficient for the transmitting of data.

Actually ice is fairly transparent to even high frequency radio waves although water is not. Easily confirmed at home by placing a really cold and dry ice cube in the microwave oven (~2.4 MHz). It will not melt. Wet the surface of that ice cube (a thin layer of water) and the water on the surface of the ice will absorb the microwaves, heat up, and melt the ice cube.

According to the article, ice tends to reflect higher frequency waves, not transmit them.

Water also reflects radio waves but it is also a damn good absorber of radio waves. Ice will certainly reflect radio waves but is also fairly transparent to most frequencies. This is why radar can be used to map the terrain of the Earth under the glaciers but not the ocean bed under the water. Incident angle and frequency is always important to consider.

Also, microwave ovens work at ~2.4 GHz - not MHz.

You are, of course, right. That was a brain fart on my part. An ~2.4 MHz oven would need to be much larger than the house.

 

[Loren Pechtel]

Looks like an interesting mission. I wonder why they chose that rather than a lander on Europa (or a close flyby) to sample the material ejected from the subsurface ocean, with perhaps a chance at finding signs of life?

Europa has a big problem: it's much more difficult to land on than Titan is. That's because Titan has a thick atmosphere and Europa has essentially no atmosphere - Detection of an oxygen atmosphere on Jupiter's moon Europa | Nature - surface pressure about 10^(-11) Earth's.

So for Europa, one will have to match velocities and go into orbit around it, and then drop a lander. Going into orbit around Europa will require several km/s of delta-V, and one will need a nuclear-powered electric rocket engine for that. The last bit of the way is about 1.4 km/s of delta-V, comparable to landing on our Moon.

But for Titan, there is no such difficulty. All that is necessary is being able to hit the moon's atmosphere at several km/s and survive doing so -- an ability that has been abundantly demonstrated for Venus, the Earth, Mars, Jupiter, and even Titan itself.

So degree of difficulty is the reason for NASA taking the option of Titan over Europa?

Except it's not really something you can just try harder.

Delta-v is the ultimate ruler of spaceflight. The size of your rocket goes up exponentially with increasing delta-v. If you burn half the mass of your rocket (using the best engines we have, hydrolox) you get about 3,000 meters per second of delta-v. Note that to get another 3,000 m/s you need to burn half of it again. Thus if you add 3,000 meters per second to your mission you either need to halve your payload or double the size of the rocket on the pad. They're already using pretty big boosters for deep space missions, doubling the rocket on the pad simply isn't an option. Thus an additional 3,000 meters per second means halving the payload size. Reality is even worse as hydrolox effectively can't be used in deep space due to the refrigeration requirements--that 3,000 m/s is reduced to 2,100 m/s for the best fuel that doesn't require bringing a cryogenic refrigerator along. Just landing on Europa is going to use half your rocket's mass--which in practice means your Europa lander is half the size of a Titan lander--and that's not counting whatever it costs you to match orbits. (This has no simple answer due to the possibility of gravity assists off Jupiter's moons--but on the flip size playing cosmic billiards in the Jovian system means bringing along a bunch of radiation shielding.)

 

[Loren Pechtel]

NASA rather euphemistically refer to 'lithobraking' to describe a spacecraft being decelerated by contact with a solid surface.

Are they sure they don't mean lithobreaking?

It's not just an euphemism. Lithobraking missions have already been flown--Spirit and Opportunity both had a lithobraking aspect to their landing--the final deceleration was done with solid boosters--no ability to control them. The landers were cut loose and dropped onto Mars from 15 meters up.

 

[Terrell]

I hope NASA can launch this mission on schedule & have it fully successful. I'd love to see what else can be found on Titan.

 

[DBT]

So degree of difficulty is the reason for NASA taking the option of Titan over Europa?

Except it's not really something you can just try harder.

Delta-v is the ultimate ruler of spaceflight. The size of your rocket goes up exponentially with increasing delta-v. If you burn half the mass of your rocket (using the best engines we have, hydrolox) you get about 3,000 meters per second of delta-v. Note that to get another 3,000 m/s you need to burn half of it again. Thus if you add 3,000 meters per second to your mission you either need to halve your payload or double the size of the rocket on the pad. They're already using pretty big boosters for deep space missions, doubling the rocket on the pad simply isn't an option. Thus an additional 3,000 meters per second means halving the payload size. Reality is even worse as hydrolox effectively can't be used in deep space due to the refrigeration requirements--that 3,000 m/s is reduced to 2,100 m/s for the best fuel that doesn't require bringing a cryogenic refrigerator along. Just landing on Europa is going to use half your rocket's mass--which in practice means your Europa lander is half the size of a Titan lander--and that's not counting whatever it costs you to match orbits. (This has no simple answer due to the possibility of gravity assists off Jupiter's moons--but on the flip size playing cosmic billiards in the Jovian system means bringing along a bunch of radiation shielding.)

The idea wasn't really for a lander on Europa, but an orbiter that fires a probe into a fissure, which transmits information to the orbiter and earth. Presumably an active fissure that is venting.

 

[Gun Nut]

Looks like an interesting mission. I wonder why they chose that rather than a lander on Europa (or a close flyby) to sample the material ejected from the subsurface ocean, with perhaps a chance at finding signs of life?

“ALL THESE WORLDS ARE YOURS - EXCEPT EUROPA. ATTEMPT NO LANDINGS THERE.”

use them together.... use them in peace.