paying attention t bennu.take it-- it's important to understand this risk and to isk.gate the >> reporter: of course even smaller fragments ofid astehit earth all the time in the form of meteorites. s so why is entifically ?ecessary to return a piece of one still in spa >> what actually survives passes marough the earth's atmosphere that hit the grounnot be representative of what material it's out there. most of the meteorite, 99% of its mass probably burns off. so, it's a good chan of what you're actually finding is just the strongest material. so, by actually going to an asteroid and bringing back nderything we can from at least this little area aurface. you end up with an unbiased sample. >> reporter: if all s planned, osiris-rex should leave bennu with that sample in march of 2021, returning it to the utah desert in september of 2023. and for scientists that's when it's game on. the precious cargo will keep them busy for many years, trying to understand the origin and possiblebiquity, of life. for the pbs