Astronomy Without A Telescope – Solar Or RTG?

The 'edge of the envelope' solar powered Juno mission - scheduled for launch in 2011.

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It used to be the case that if you wanted to send a spacecraft mission out past the asteroid belt, you’d need a chunk of plutonium-238 to generate electric power – like for Pioneers 10 and 11, Voyagers 1 and 2, Galileo, Cassini, even Ulysses which just did a big loop out and back to get a new angle on the Sun – and now New Horizons on its way to Pluto.

But in 2011, the Juno mission to Jupiter is scheduled for launch – the first outer planet exploration mission to be powered by solar panels. And also scheduled for 2011, in another break with tradition – Curiosity, the Mars Science Laboratory will be the first Mars rover to be powered by a plutonium-238 radioisotope thermoelectric generator – or RTG.

I mean OK, the Viking landers had RTGs, but they weren’t rovers. And the rovers (including Sojourner) had radioisotope heaters, but they weren’t RTGs.

So, solar or RTG – what’s best? Some commentators have suggested that NASA’s decision to power Juno with solar is a pragmatic one – seeking to conserve a dwindling supply of RTGs – which have a bit of a PR problem due to the plutonium.

However, if it works, why not push the limits of solar? Although some of our longest functioning probes (like the 33 year old Voyagers) are RTG powered, their long-term survival is largely a result of them operating far away from the harsh radiation of the inner solar system – where things are more likely to break down before they run out of power. That said, since Juno will lead a perilous life flying close to Jupiter’s own substantial radiation, longevity may not be a key feature of its mission.

Perhaps RTG power has more utility. It should enable Curiosity to go on roving throughout the Martian winter – and perhaps manage a range of analytical, processing and data transmission tasks at night, unlike the previous rovers.

With respect to power output, Juno’s solar panels would allegedly produce a whopping 18 kilowatts in Earth orbit, but will only manage 400 watts in Jupiter orbit. If correct, this is still on par with the output of a standard RTG unit – although a large spacecraft like Cassini can stack several RTG units together to generate up to 1 kilowatt.

So, some pros and cons there. Nonetheless, there is a point – which we might position beyond Jupiter’s orbit now – where solar power just isn’t going to cut it and RTGs still look like the only option.

Left image: a plutonium-238 ceramic pellet glowing red hot, like most concentrated ceramicised radioisotopes will do. Credit: Los Alamos National Laboratory. Right image: the Apollo 14 ALSEP RTG, almost identical to Apollo 13's RTG which re-entered Earth's atmosphere with the demise of the Aquarius lunar module. Credit: NASA

RTGs take advantage of the heat generated by a chunk of radioactive material (generally plutonium 238 in a ceramic form), surrounding it with thermocouples which use the thermal gradient between the heat source and the cooler outer surface of the RTG unit to generate current.

In response to any OMG it’s radioactive concerns, remember that RTGs travelled with the Apollo 12-17 crews to power their lunar surface experiment packages – including the one on Apollo 13 – which was returned unused to Earth with the lunar module Aquarius – the crew’s life boat until just before re-entry. Allegedly, NASA tested the waters where the remains of Aquarius ended up and found no trace of plutonium contamination – much as expected. It’s unlikely that its heat tested container was damaged on re-entry and its integrity was guaranteed for ten plutonium-238 half-lives, that is 900 years.

In any case, the most dangerous thing you can do with plutonium is to concentrate it. In the unlikely event that an RTG disintegrates on Earth re-entry and its plutonium is somehow dispersed across the planet – well, good. The bigger worry would be that it somehow stays together as a pellet and plonks into your beer without you noticing. Cheers.

Curiosity Rover Takes First Test Drive

Even though there were no wheelies or skid marks, it was an exciting day for the teams working on the next Mars rover. The Mars Curiosity rover (or the Mars Science Laboratory) took its first short drive in the JPL clean room where it is being built. This video was captured from live broadcast on July 23, 2010. Cheers and commentary provided by mission team members who watched the event from a viewing gallery above the clean room floor. In this clip the rover drives backward for the first time.

Launch Dates Narrowed for Mars Science Lab

This artist's concept from an animation depicts Curiosity, the rover to be launched in 2011 by NASA's Mars Science Laboratory, as it is being lowered by the mission's rocket-powered descent stage during a critical moment of the "sky crane" landing in 2012. Image Credit: NASA/JPL-Caltech

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Mission planners have narrowed the field for possible launch dates for NASA’s next generation rover to Mars, the Mars Science Laboratory, nicknamed Curiosity. Taking into account orbital mechanics, planetary alignment, and communications issues, MSL’s launch will occur between Nov. 25 and Dec. 18, 2011, with landing will taking place between Aug. 6 and Aug. 20, 2012. The actual landing site is still being decided, between four different locations on Mars (read about the four sites here.)

“The key factor was a choice between different strategies for sending communications during the critical moments before and during touchdown,” said Michael Watkins, mission manager at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “The shorter trajectory is optimal for keeping both orbiters in view of Curiosity all the way to touchdown on the surface of Mars. The longer trajectory allows direct communication to Earth all the way to touchdown.”

Landing on Mars is always very difficult, and NASA has put a high priority on communication during Mars landings, especially after a landing failure in 1999. Therefore, the flight schedule allows for favorable positions for the Mars Odyssey and the Mars Reconnaissance Orbiter, currently orbiting Mars, which can both obtain information during descent and landing of MSL.

The simplicity of direct-to-Earth communication from Curiosity during landing has appeal to mission planners, but the direct-to-Earth option allows a communication rate equivalent to only about 1 bit per second, while the relay option allows about 8,000 bits or more per second.

“It is important to capture high-quality telemetry to allow us to learn what happens during the entry, descent and landing, which is arguably the most challenging part of the mission,” said Fuk Li, manager of NASA’s Mars Exploration Program at JPL. “The trajectory we have selected maximizes the amount of information we will learn to mitigate any problems.”

Curiosity will use several innovations during entry, descent and landing in order to hit a relatively small target area on the surface and set down a rover too heavy for the cushioning air bags used in earlier Mars rover landings. MSL will use employ of the largest parachutes ever used in a space mission to land a car-sized rover on the Red Planet. Most interesting is the final phase of landing, where a “sky-crane,” a rocket-powered descent stage will lower Curiosity on a tether for a wheels-down landing directly onto the surface.

Even though Curiosity won’t be communicating directly with Earth at touchdown, data about the landing will reach Earth promptly. Odyssey will be in view of both Earth and Curiosity, in position to immediately forward to Earth the data stream it is receiving during the touchdown. Odyssey performed this type of “bent-pipe” relay during the May 25, 2008, arrival of NASA’s Phoenix Mars Lander.

Curiosity will rove extensively on Mars, carrying an analytical laboratory and other instruments to examine a carefully selected landing area. It will investigate whether conditions there have favored development of microbial life and its preservation in the rock record. Plans call for the mission to operate on Mars for a full Martian year, which is equivalent to two Earth years.

More information about NASA’s Mars Science Laboratory.

Source: JPL