<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>philipkglass</author><text>Nuclear thermal rockets have low specific impulse compared to electrical propulsion, but better ISP than any chemical rockets. They also provide orders of magnitude more thrust than electrical propulsion does. I don&amp;#x27;t think that they are in the sweet spot for uncrewed probe missions, but the combination of higher-than-chemical ISP and higher-than-electrical thrust seems like a good match for crewed missions to Mars or beyond.&lt;p&gt;Regarding the many comments in this discussion about accidents during initial launch: if nuclear thermal rockets are fueled with uranium 235, and don&amp;#x27;t reach criticality until &lt;i&gt;after&lt;/i&gt; the risky chemical-booster ascent out of atmosphere, they should be quite safe. Uranium 235 has a 700 million year half life, hence very low radioactivity. The fission products from a live reactor of course include very-short-lived, very-high-radioactivity nuclides. But you can have nuclear power in space with extremely low risks to the terrestrial environment if you use it only for missions that leave Earth orbit, and turn on the reactor only after the risky initial ascent on chemical boosters.&lt;p&gt;That said, I agree that it would be expensive to develop nuclear thermal rockets. It&amp;#x27;s very expensive to test them on Earth, since the exhaust carries away some small amount of fission products and people don&amp;#x27;t just tolerate such radioactive pollution like back in the 1960s. You&amp;#x27;d need some very expensive&amp;#x2F;elaborate test facility that can capture the hot, radioactive exhaust, or you&amp;#x27;d need to do all the actual testing in space (also expensive&amp;#x2F;elaborate).</text><parent_chain><item><author>Gravityloss</author><text>It&amp;#x27;s not so easy to make space nuclear power light and reliable. Most likely a nuclear powered spacecraft with electric thrusters would be a dog.&lt;p&gt;Nuclear thermal rockets aren&amp;#x27;t that great either, the temperature and thus the specific impulse is not very high, and they have high dry mass and would be really expensive to develop and hard to make reliable.&lt;p&gt;In space you have 1.3 kW per square meter sun power constantly without the atmosphere or clouds in the way, and solar cells improve constantly.</text></item><item><author>nickik</author><text>The hole of Space exploration is really hindered by the approach to nuclear power. Many missions that could fly with nuclear batteries don&amp;#x27;t use them. Rosetta would have been a far better mission with one.&lt;p&gt;On a larger scale we should have nuclear reactors in space. If we ever want a mars colony, nuclear is way, way better then solar. Elon Musk is gone try with solar but he really doesn&amp;#x27;t have any other options.&lt;p&gt;NASA is the one place there can actually be progress on this stuff, no private company can reasonable attempted this stuff because of regulation. We need more nuclear batteries, we need small reactors for space and we need nuclear thermal rockets engines.</text></item></parent_chain></comment><story><title>Thruster for Mars mission breaks records</title><url>http://ns.umich.edu/new/releases/25192-thruster-for-mars-mission-breaks-records</url></story>
<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>snerbles</author><text>At Earth orbit, yes.  Recall that the inverse square law applies for light emitted by the sun.&lt;p&gt;On Mars, solar power per square meter is a bit over 550W.</text><parent_chain><item><author>Gravityloss</author><text>It&amp;#x27;s not so easy to make space nuclear power light and reliable. Most likely a nuclear powered spacecraft with electric thrusters would be a dog.&lt;p&gt;Nuclear thermal rockets aren&amp;#x27;t that great either, the temperature and thus the specific impulse is not very high, and they have high dry mass and would be really expensive to develop and hard to make reliable.&lt;p&gt;In space you have 1.3 kW per square meter sun power constantly without the atmosphere or clouds in the way, and solar cells improve constantly.</text></item><item><author>nickik</author><text>The hole of Space exploration is really hindered by the approach to nuclear power. Many missions that could fly with nuclear batteries don&amp;#x27;t use them. Rosetta would have been a far better mission with one.&lt;p&gt;On a larger scale we should have nuclear reactors in space. If we ever want a mars colony, nuclear is way, way better then solar. Elon Musk is gone try with solar but he really doesn&amp;#x27;t have any other options.&lt;p&gt;NASA is the one place there can actually be progress on this stuff, no private company can reasonable attempted this stuff because of regulation. We need more nuclear batteries, we need small reactors for space and we need nuclear thermal rockets engines.</text></item></parent_chain></comment><story><title>Thruster for Mars mission breaks records</title><url>http://ns.umich.edu/new/releases/25192-thruster-for-mars-mission-breaks-records</url></story>