69

That is precisely it. Plutonium-238, which is used in the creation of radioisotope thermoelectric generators (RTGs) is very difficult to come by. There are plenty of news articles on this, from Popular Science to Space News. Basically, it comes down to the fact that plutonium-238 is in short supply now, and it is difficult to make more because of nuclear ...


37

The small separated cells had several functions. Six individual units, the "solar aspect cells", were each oriented and attached as if they were on the sides of a cube to identify the spacecraft's attitude. (This is probably the 2nd and 4th pictures above) This was a time when onboard electronics, and particularly computation, was expensive, so these were ...


36

Another interesting note is that this mission more than any other mission to the outer solar system can use solar power. Why? Juno is in a polar orbit, and will continually be in the sun. Solar panels are also becoming more powerful than they have previously. Between the two of these, solar was a more attractive option than it has been in the past. If it was ...


24

The long boom holding the panels can be retracted (the term used instead of "folding"). It is not done lightly, for fear of not being able to reverse it. It has only been done a few times, once when a truss (the P6 truss) was relocated, and once to work out an issue. During the extension of the boom during the relocation task, the panels tore, ...


22

The high power levels of Shuttle and its need to return to an aerodynamic configuration would have necessitated a very large array that would need to be deployed and retracted each mission. This would have complicated the design considerably. reduced the maneuverability of the Shuttle, and introduced other failure modes. Although I am not as familiar with ...


22

Reasons not to provide mechanical means to clean solar panels on Mars: and this is the primary reason: Wind on Mars occasionally blows the dust away. This means dust is not a major issue, but a minor one. Spirit and Opportunity functioned for years despite not having dust removal equipment. Mechanical operations are expensive: you've just added a series of ...


20

Solar panel technology seems to have caught up with power requirements on the satellite. Since price of components is really no object when building a system like this, super expensive panels with efficiency ratings of up to 40% can be used. The trick to engineering something properly is using just the right amount of materials, as the old maxim goes "Any ...


19

Good catch noting that Cygnus has the same solar panel design! Orbital ATK, developer of Cygnus, builds these panels under the "Ultraflex" and "Megaflex" brands, and did indeed supply them to JPL for two Mars missions: Mars Phoenix Lander Mars InSight The Mars Polar Lander program successfully qualified the panels for flight; but were ...


19

Your delta-v analysis doesn't account for the landing delta-v. On Mars, only a fraction of a km/s has to be done propulsively, on Mercury the entire landing will be propulsive. You also don't account for transit time. BepiColombo launched in 2018 and won't be able to enter orbit until the end of 2025. MESSENGER similarly launched in 2004 and didn't enter ...


16

The Opportunity, Spirit (RIP), and Curiosity rovers all have high-gain antennas that point at Earth when in use, using a two-axis gimbal. They are used mostly to receive the command loads every sol from Earth. The rovers also have low-gain UHF antennas, basically like a car antenna but shorter, for communicating with relay orbiters around Mars. That path ...


16

Two alternatives (RTG or solar) were investigated in the Mars Science Laboratory Environmental Impact Statement. In summary, solar arrays would limit the MSL to operate between 5°N and 20°N, and for one Martian Year only if at exactly 15°N, whereas RTGs would permit it to operate for at least one Martian year anywhere from 60°S to 60°N. From page 2-1 (PDF ...


16

No, the power collected by solar panels is reduced by the square of the distance from the light source. At the Earth's distance from the sun, the energy of sunlight is about 1300 watts per square meter, of which something like ~30% can be converted to electricity by solar panels. Once the sun is far enough away to be "just another star", the total ...


14

I had the opportunity to tour JPL a few months ago and asked this exact question to our tour guide. The solar panels on it are enormous and typically, spacecraft going beyond the asteroid belt are equipped with RTGs, so why doesn't Juno have one? He told us that the US was on very short supply of Plutonium-238 at the time and that they would have had to ...


14

The Mariner 3 and 4 Mars flyby probes had angled vanes at the end of its solar panel arms which provided passive stabilization of the spacecraft from solar radiation pressure:


14

Could Stirling Engines work on sunlight alone? With the heated side facing the sun and the rest in its own shade, could two contra-rotating Vacuum Stirling Engines and flywheels, in tandem, accumulate torque, to power a reaction wheel? Yes. Can the heat from an ion engine be used to... power a Stirling engine? Yes! ...provide forward momentum this ...


13

They will likely be relying on two things: Since the rate at which dust will accumulate on the solar panels is pretty well known, they can estimate a lifespan of the rover. All space probes have some form of estimated lifespan, and the primary science mission is generally planned to fit within that low timeframe. However, as we've seen from Spirit and (...


13

The planned orbit for the JWST is quite a large halo orbit around Sun-Earth L2. It's very roughly elliptical, with dimensions of about +/- 350,000 km "vertically" (perpendicular to the Earth's orbital plane) and about +/- 750,000 km "horizontally" (in the Ecliptic plane). You can see a drawings in (for example) James Webb Space Telescope Initial Mid-Course ...


13

This may belong to Astronomy SE, but the $29.5$ Earth day figure, or more accurately the time in the third reference, is what you should be planning on when you or at least your instruments go to the Moon. This represents the actual cycle between daylight and darkness, the solar day. When one clicks on the references cited in the question, the first and ...


13

The ISS does not have any sort of electric propulsion system. It uses ordinary chemical rockets, either on visiting vehicles or its own rockets which are refueled by such vehicles. (https://en.wikipedia.org/wiki/International_Space_Station#Orbit) Its solar panels are for running its experiments and other equipment. There is only one rocket that uses lithium ...


12

There are at least two problems with solar photovoltaic cells (not considering concentrators) in the outer solar system: the low power of the sun, and the low temperature of the cells. For the Cassini mission to Saturn (9–10 AU from the Sun), NASA investigated solar as an alternative. They calculated the surface area that would be required, and concluded ...


12

Settling dust is not a problem. Keep in mind that dust in space does not settle on surfaces like it does on Earth--it's not slowly sinking, it hits the surface at relative speeds measured in km/s. It's basically a micro-meteoroid bombardment and the allowed degradation is derived from the performance requirements over the design lifetime of the solar array. ...


11

Fuel cells produce water. The output of the Apollo fuel cells (PDF on Apollo power supply system design) was used as drinking water and as a coolant in the environmental control system. If you use solar cells, you need rechargeable batteries to supply the spacecraft when it's on the night side of the planet. I suspect the choice came down to the total weight ...


11

Liquid Hydrogen is difficult to deal with. The temperature must be 33 K or lower. Liquid Oxygen requires 90K, and Liquid Methane is similar. The temperature requirements are far less as such. The surface of Mars varies between 140K to 300K. The values for storing Methane/ Oxygen are much closer. Methane also requires less hydrogen than the LH2/LOX rocket. ...


11

Well, we don't really know how much energy a Mars base would need, but we can make some rough estimates. Bases like McMurdo and Mawson have power capacities of several hundred to several thousand kilowatts. Now, we're talking about a Mars base, presumably we're going to need a little more power than that. Let's estimate that Mars Base One will require 10,...


11

Having worked extensively on the solar arrays over the last five years, I can say they do receive about a third of their power from albedo exposure to the back side. This is further confirmed from a "funny" I noticed in 2014 when doing testing on ISS solar cells. When trying to measure a solar cell's illuminated short-circuit current, I measured a value two ...


11

In theory, yes, if the accelerator and the spacecraft are of the same mass, they'll gain the same amount of velocity when they pass, and so they'll meet at a higher altitude on the opposite side. If they aren't the same mass, then the lighter one gains more speed than the heavier, and they won't meet up again. In practice, I don't think it's workable. The ...


11

The drawing shows a Crookes Radiometer. They seem to spin nicely in even a little bit of sunlight. The common explanation is light pressure on the black vanes. Unfortunately, they don’t work that way. Photons bouncing off the white side transfer more momentum than ones absorbed by the black: of the mechanism was light pressure, they’d turn the other way. ...


10

As 2012rcampion noted you are using the wrong metric of cost in your efficiency figures. Triple junction solar panels are extremely inefficient wrt weight when compared to single junction. To understand why this is you need to understand what a triple junction PV is. To do that I need to explain the major inefficiency of standard single junction PV is ...


10

No, it doesn't make sense, for two key reasons. First of all, it is a loss of redundancy. If one of the cells fails, then the entire string fails. You really don't want that to happen... If you build in a shunting circuit of some kind to manage a failed cell, then the voltage could fluctuate, causing additional issues. Secondly, solar cells placed in ...


10

There isn't enough solar energy to power a spacecraft once you get a certain distance from the sun, even if you had 100% power conversion efficiency. The intensity of sunlight diminishes inversely proportional to the square of the distance from the Sun. The intensity of sunlight at Mars is half what it is at Earth, and at Jupiter it's only 5%. At the Earth's ...


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