Copyright 2023 Robert Clark
The IceCube Neutrino Observatory was able to drill down 2.5 kilometers rapidly into the ice in Antarctica using a relativly simple method using heated, pressurized water. Can we adapt this to reach the subice oceans on Europa?
Here is a published description of the drilling method:
IceCube Enhanced Hot Water Drill functional description.
Published online by Cambridge University Press: 26 July 2017
T. Benson, J. Cherwinka, M. D
https://www.cambridge.org/core/journals/annals-of-glaciology/article/icecube-enhanced-hot-water-drill-functional-description/F2A93B250A9FDB4E8A94101787BA3A68
Published online by Cambridge University Press: 26 July 2017
T. Benson, J. Cherwinka, M. D
https://www.cambridge.org/core/journals/annals-of-glaciology/article/icecube-enhanced-hot-water-drill-functional-description/F2A93B250A9FDB4E8A94101787BA3A68
The specifications:
The total power for drilling a 60 cm wide hole was 5 MW. The great majority of this was the thermal power of 4.7 MW for melting the ice.
For just sending a small probe beneath the ice we'll make the hole width smaller by a factor of 10 to 6 cm, 2.4 inches. A probe of greater capabilities will be obtained by simply making it longer, while keeping it the same width.
The required power will scale by the cross-sectional area to 50,000 watts, 50 kW.
In this blog post I noted the Falcon Heavy can send a 1 ton spacecraft to the surface of Europa using currently existing in-space stages:
Low Cost Europa Lander Missions.
https://exoscientist.blogspot.com/2015/02/low-cost-europa-lander-missions.html
https://exoscientist.blogspot.com/2015/02/low-cost-europa-lander-missions.html
The Europa Clipper mission currently planned for launch in 2024 on the Falcon Heavy is an orbiter mission though; it will not have a lander. However, the cost of the Falcon Heavy is only ca. $100 million, so a second launcher for the lander mission can easily be purchased. By the way, an interesting and much appreciated phenomenon I've observed about NASA science missions is the time between announcement and actual mission launch seems to much more rapid than before. So it's likely the lander mission can be launched also within the ca. 2024 time frame.
There have been several proposals to drill through the Europan ice. But they frequently suggest doing it over long time frames, several months. For instance, the proposal discussed at about the 8:50 point in this video would take up to a year:
The IceCube drill though took only 30 hours to go through 2.5 kilometers of ice in Antarctica. For a 15 kilometer thick ice shell on Europa that would be about 7.5 days. However, because of the fractured surface of Europa the ice shell is believed to be much thinner in some place, perhaps as low as 5 km thick. In that case, the drill may only take 2.5 days to penetrate the ice.
The REASON ice-penetrating radar on the Europa Clipper orbiter can be used to first determine thinnest locations on the ice for the lander to land at.
Supplying the Energy.
But first how to get 50 kW power at Europa? We have a limit of about 1 ton for the lander spacecraft payload to be launched with in-space stages on the Falcon Heavy. First, space fission reactors. Here's a list of space fission reactors that have flown in space:
| Name and model | Used on (# of RTGs per user) | Maximum output | Radio- isotope | Max fuel used (kg) | Mass (kg) | Power/mass (Electrical W/kg) | |
|---|---|---|---|---|---|---|---|
| Electrical (W) | Heat (W) | ||||||
| MMRTG | MSL/Curiosity rover and Perseverance/Mars 2020 rover | c. 110 | c. 2000 | 238Pu | c. 4 | <45 | 2.4 |
| GPHS-RTG | Cassini (3), New Horizons (1), Galileo (2), Ulysses (1) | 300 | 4400 | 238Pu | 7.8 | 55.9–57.8[48] | 5.2–5.4 |
| MHW-RTG | LES-8/9, Voyager 1 (3), Voyager 2 (3) | 160[48] | 2400[49] | 238Pu | c. 4.5 | 37.7[48] | 4.2 |
| SNAP-3B | Transit-4A (1) | 2.7[48] | 52.5 | 238Pu | ? | 2.1[48] | 1.3 |
| SNAP-9A | Transit 5BN1/2 (1) | 25[48] | 525[49] | 238Pu | c. 1 | 12.3[48] | 2.0 |
| SNAP-19 | Nimbus-3 (2), Pioneer 10 (4), Pioneer 11 (4) | 40.3[48] | 525 | 238Pu | c. 1 | 13.6[48] | 2.9 |
| modified SNAP-19 | Viking 1 (2), Viking 2 (2) | 42.7[48] | 525 | 238Pu | c. 1 | 15.2[48] | 2.8 |
| SNAP-27 | Apollo 12–17 ALSEP (1) | 73 | 1,480 | 238Pu[50] | 3.8 | 20 | 3.65 |
| (fission reactor) Buk (BES-5)** | US-As (1) | 3000 | 100,000 | highly enriched 235U | 30 | 1000 | 3.0 |
| (fission reactor) SNAP-10A*** | SNAP-10A (1) | 600[51] | 30,000 | highly enriched 235U | 431 | 1.4 | |
| ASRG**** | prototype design (not launched), Discovery Program | c. 140 (2x70) | c. 500 | 238Pu | 1 | 34 | 4.1 |
Note to get to 50 kW would require several hundred kg just for the power system alone, leaving little for the rest of the spacecraft operations. One might also not like to leave radioisotopes on a world that may have life.
Solar power? Normally the high power requirements would make this a non-starter for the far away Jupiter system with its reduced solar insolation. But quite key here is the majority of the power needs to be thermal. The inefficiencies of space solar power come from when it has to be converted to electrical power. Jupiter gets about 50 watts per square meter of solar power, well less than the 1,200 W per square meter at Earth. Then you would need 1,000 square meters solar collecting area, about 32m x 32m. But you would not send this to solar electric cells. You would collect it either by a parabolic mirror or Fresnel lens to concentrate it to achieve a temperature of up to 3,000°C. In the low gravity of Europa this would need minimal support structures and might weigh only a few tens of kilos total weight. For instance the Sunjammer solar sail only weighed 32 kg at a 1,200 square meter area.
Burning fuel? The disadvantage here is that higher fuel load would need to be carried to Europa for a longer time the drill had to be run. To estimate the power using a rocket engine to deliver the thermal power. it's at 1/2 times the square of the exhaust velocity per fuel flow rate. Or equivalently, the amount of energy per kilogram propellant is 1/2 times the square of the exhaust velocity. Given a (very) large expansion ratio the exhaust velocity of a hydrolox engine can be ca. 4,800 m/s. So the energy per kilo of propellant would be 11.5E6 J/kg. If we ran our 50 kW thermal power system for 7.5 days for a 15 km thick ice shell, it would take 50,000*7.5*24*3,600 = 32.4E9. This would take 2,800 kg of propellant, far too much.
Estimates of power requirement for Europa.
The above was assuming the IceCube drilling method could be adapted to Europa, specifically the energy requirements. This page actually does the calculation from first principles for Europa:
Calculating the Required Energy of Drilling into Europa.
July 9, 2020 by Engineer Calcs
It discusses three methods: 1.)breaking up the ice and simply carrying the broken up ice to the surface, 2.)bringing the -160°C ice on Europa only up to a easier to achieve sublimation temperature of -75°C rather than to the melting point of 0°C, and allowing the ice to sublimate to a gas in the vacuum of Europas surface, and 3.)bringing the ice up to the 0°C melting point and using analogous methods as to the IceCube experiment.
The first method requires surprisingly low amounts of energy. For his calculation, the author takes the width as 1-meter and the depth 25 kilometers. The enegy requirement is simply calculated by the potential energy of raising that much mass of ice the required height in the low gravity of Europa. He calculates 5.92E11 Joules. However, our case will be smaller in using only a 6 cm wide drill hole and assuming at most 15 km height. Also, in actuality not the entire column needs to be raised the full height. The ice near the top for example needs to be raised hardly at all. The height and the energy needed to raise is a linear function of the depth of that portion of the ice. Then the energy can be calculated by using just the middle point of the height, i.e., only 7.5 km for our 15 km drill hole. Then the energy is:
5.92E11*(6/100)^2*(7.5/25) = 6.4E8 Joules.
Using our 50,000 watt solar generator, that would only take 3.4 hours to generate the required energy(!) One question though, the energy needed in this case would be mechanical rather than thermal in mechanically raising the ice to the surface. What would the conversion efficiency be? Because of the quite high temperature of 3,000°C, by the Carnot Law the conversion efficiency could be quite high, certainly over 90%.
Because the energy requirements for the mechanical lifting method are comparatively low we might even allow the fuel burn method of supplying the power rather than using solar thermal. Our needed total energy now is 6.4E8 joules. Using the calculated energy we used for a hydrolox engine of 11.5E6 J/kg of propellant, that would only require 6.4E8/11.5E6 = 57 kg.
However, a problem with hydrolox with long space missions is boiloff. There is research in reducing it but whether it can be reduced enough for a misson of 3 to 4 year travel time through space is uncertain. So we'll estimate it using storable hypergolic propellants, same as used for the propulsion stages at Jupiter. In vacuum these can have 3,200 m/s exhaust velocity. This can give (1/2)*3,200^2 = 5.12E6 J/kg energy. Then it would take 125 kg of the storable propellant. This would still be doable for a 1,000 kg spacecraft payload.
The author of the "Calculating the Required Energy of Drilling into Europa" page does not prefer the mechanical lifting method, preferring to use the melting ice method eventhough it would take 2 orders of magnitude more power. This is because with a mechanical lift it may have a tendency of breakdwon operating in such low temperatures over long time.
However, as the previous calculation using the solar thermal generator above showed it might be it would only take on the matter of hours. This would certainly reduce the likelihood of breakdown.
Note also using the fuel-burn method of generating power, the moving exhaust itself could be used to raise the ice to the surface, thus elimating the issue of mechanical breakdown of the lifting method.
There is another issue though for raising the ice to the surface without expending extra energy to melt it. Ice is brittle so likely would require low energy compared to the energy needed to raise to the surface to break it up. This is the case on Earth. However for the extreme level of ice compaction of Europa the ice would be denser so might be harder to break up. Experiments would need to be done to determine this.
That discussion was for the 1st method of drilling down of mechanically raising the ice to the surface.. For the second method of only raising the ice to sublimatioin temperture of -75°C at first glance that might appear to be preferable. However, the energy of sublimation, the energy required to sublimate from ice to gas is so high, about 10 times higher than that just to melt it, it winds up taking more total energy than raising the temperature of the ice to the melting point of 0°C.
So we'll consider the 3rd method of melting the ice and then using analogous methods as in the IceCube experiment. The calculation of the energy in this case is 1.2E13 but this was for a 1-meter wide drill hole and for a 25 km drill depth. So for our 6 cm wide drill hole down only 15 km the energy required would be: 1.2E13*(6/100)^2*(15/25) = 26E9. Using 50 kW solar thermal power, this would only take about 6 days.
Actually, in his calculation the author of the "Calculating the Required Energy of Drilling into Europa" page used a constant temperature of -160°C for the ice that had to be raised to 0°C for melting. But since at the water level below the ice, the temperature will already be at about 0°C, perhaps a little less for salt freezing point depression, the temperature will gradually decrease linearly from 0°C to -160°C at the surface. So the calculation needed for the energy needed for the temperature change can be done by using the average of -80°C. This cuts the energy required for raising the ice to the melting point by about half. However, the energy for the phase change from ice to liquid will still be the same, resulting in the total energy being in the range of 75% of the estimated energy above.
Also, importantly, in comparison to the IceCube experiment drilling method, which took place in Antarctica at -50°C, an average temperate of -80°C on Europa is only moderately far from -50°C, compared to the -160°C temperature at the surface of Europa, which gives us confidence the energy requirements would be in line with those of the IceCube experiment.
The author of that page prefers this melting method and we now have a good point of comparison to the IceCube experiment to have good confidence that it can actually work. As I mentioned before a great point in its favor is the quite short drilling time of only a fews days compared to the months to a year estimated for other proposals.
Still, I am intrigued by the low energy requirements of just mechanically raising of ice to the surface without melting, especially if using the exhaust from fuel-burn method of raising the ice to the surface, since this could shorten the drill time down even further to hours instead of days.
Robert Clark
