With reference to radioisotope thermoelectric generators (RTGs), consider the following statements: 1. RTGs are miniature fission reactors. 2. RTGs are used for powering the onboard systems of spacecrafts. 3. RTGs can use Plutonium-238, which is a by-product of weapons development. Which of the statements given above are correct?

Let's analyze each statement regarding Radioisotope Thermoelectric Generators (RTGs):

1. Statement 1 is incorrect. RTGs are not miniature fission reactors. Fission reactors generate power through controlled nuclear chain reactions. RTGs, on the other hand, produce electricity by converting the heat generated from the radioactive decay of a suitable radioactive material (like Plutonium-238) into electricity using thermocouples. There is no controlled chain reaction involved.

2. Statement 2 is correct. RTGs are indeed widely used to power spacecraft, particularly those operating in deep space where solar power is insufficient (e.g., Voyager, Cassini, Perseverance rover). They provide a reliable, long-lasting power source for onboard systems, instruments, and communications.

3. Statement 3 is correct. RTGs typically use Plutonium-238 (²³⁸Pu) as their heat source. Plutonium-238 is a specific isotope of plutonium, which is not fissile and therefore not directly used in nuclear weapons (weapons primarily use Plutonium-239). However, ²³⁸Pu can be produced in nuclear reactors by irradiating Neptunium-237. The production of plutonium isotopes, in general, is related to nuclear programs, including those for weapons development or nuclear power, and thus ²³⁸Pu can be considered a by-product or associated with such programs, though it's distinct from weapons-grade material itself. The U.S., for instance, has restarted production of Pu-238 at national laboratories for RTG needs, using processes related to nuclear material handling.

Therefore, statements 2 and 3 are correct.