This is an interesting study that could get 20kg into Pluto orbit. It uses gridded ion engines coupled to multiple Radioisotope Thermal Generators.<p>PDF:
<a href="http:&#x2F;&#x2F;www.esa.int&#x2F;gsp&#x2F;ACT&#x2F;doc&#x2F;PRO&#x2F;ACT-RPR-PRO-ISTS2004-Pluto.pdf" rel="nofollow">http:&#x2F;&#x2F;www.esa.int&#x2F;gsp&#x2F;ACT&#x2F;doc&#x2F;PRO&#x2F;ACT-RPR-PRO-ISTS2004-Plut...</a>