Very red featureless asteroids ( spectroscopic D-types ) are expected to have formed in the outer solar system far from the sun .
They comprise the majority of asteroids in the Jupiter Trojan population , and are also commonly found in the outer main belt and among Hildas .
The first evidence for D-types in the inner and middle parts of the main belt was seen in the Sloan Digital Sky Survey ( SDSS ) .
Here we report follow-up observations of SDSS D-type candidates in the near-infrared .
Based on follow up observations of 13 SDSS D-type candidates , we find a \sim 20 % positive confirmation rate .
Known inner belt D-types range in diameter from roughly 7 to 30 kilometers .
Based on these detections we estimate there are \sim 100 inner belt D-types with diameters between 2.5 and 20km .
The lower and upper limits for total mass of inner belt D-types is 2x10 ^ { 16 } kg to 2x10 ^ { 17 } kg which represents 0.01 % to 0.1 % of the mass of the inner belt .
The inner belt D-types have albedos at or above the upper end typical for D-types which raises the question as to whether these inner belt bodies represent only a subset of D-types , they have been altered by external factors such as weathering processes , or if they are compositionally distinct from other D-types .
All D-types and candidates have diameters less than 30km , yet there is no obvious parent body in the inner belt .
Dynamical models have yet to show how D-types originating from the outer solar system could penetrate into the inner reaches of the Main Belt under current scenarios of planet formation and subsequent Yarkovsky drift .