radiation around 60 micrometres in wavelength. This wavelength of light is impossible to observe on the Earth's surface, but only from space using, e.g. the Spitzer Space Telescope. For ground-based observations, astronomers observe the tail of the black-body radiation in the far infrared. This far infrared radiation is so dim that the thermal method is only applicable to the largest KBOs. For the majority of (small) objects, the diameter is estimated by assuming an albedo. However, the albedos found range from 0.50 down to 0.05, resulting in a size range of 1,200–3,700 km for an object of magnitude of 1.0. Notable objects Exploration The only mission to date that primarily targeted a trans-Neptunian object was NASA's New Horizons, which was launched in January 2006 and flew by the Pluto system in July 2015 and 486958 Arrokoth in January 2019. In 2011, a design study explored a spacecraft survey of Quaoar, Sedna, Makemake, Haumea, and Eris. In 2019 one mission to TNOs included designs for orbital capture and multi-target scenarios. Some TNOs that were studied in a design study paper were , , and Lempo. The existence of planets beyond Neptune, ranging from less than an Earth mass (Sub-Earth) up to a brown dwarf has been often postulated for different theoretical reasons to explain several observed or speculated features of the Kuiper belt and the Oort cloud. It was recently proposed to use ranging data from the New Horizons spacecraft to constrain the position of such a hypothesized body. NASA has been working towards a dedicated Interstellar Precursor in the 21st century, one intentionally designed to reach the interstellar medium, and as part of this the flyby of objects like Sedna are also considered. Overall this type of spacecraft studies have proposed a launch in the 2020s, and would try to go a little faster than the Voyagers using existing technology. One 2018 design study for an Interstellar Precursor, included a visit of minor planet 50000 Quaoar, in the 2030s. Extreme trans-Neptunian objects Among the extreme trans-Neptunian objects are three high-perihelion objects classified as sednoids: 90377 Sedna, , and 541132 Leleākūhonua. They are distant detached objects with perihelia greater than 70 au. Their high perihelia keep them at a sufficient distance to avoid significant gravitational perturbations from Neptune. Previous explanations for the high perihelion of Sedna include a close encounter with an unknown planet on a distant orbit and a distant