Starting with the answer to what is Bennu for those who haven’t heard; Bennu (provisional designation 1999 RQ36) is an Apollo asteroid that was discovered back in 1999 and was named by a third grader, Michael Puzio, in a contest in 2013. Puzio felt that the Osirris-Rex spacecraft which will “visit” Bennu looked a bit like the Egyptian mythological bird Bennu and so the name was selected. The Apollo asteroids are asteroids with Earth-Crossing orbits and so present a very real threat. It only takes an asteroid the size of 1km to kick up so much dust into the atmosphere that sunlight would blocked for several years resulting in a global agricultural catastrophy – and these events happen every 100,000 years or so. Of course if the asteroid were to be 10km, most animal life on the planet would be destroyed. A touch larger and you get the general idea since most animal life was already gotten, expect the rest to follow! For years now (and I wish it were more) a program called Spacewatch now does just that, watches space and keeps us safe from Earth-crossing asteroids. When it was first begun, the total known was around a dozen. Now that it is hovering around several hundred.
The size of Bennu? While NASA will change the exact dimensions when Osirris-Rex spacecraft gets more details, Bennu has a girth of approximately .5km (bit larger than the usual Earth-approaching asteroid) and it orbits the Sun every 1.2 years (or 436 days) passing close to the Earth every 6 years.
8 impacts predicted – Between the years of 2169 & 2199 there are many possible impacts by Bennu but it is difficult to predict the actual effects not knowing Bennu that well. The best chance of a direct hit to Earth (using the Palermo scale – a scale the risk of potential impacts), is in the 2175-2196 timeframe.
So that means NASA grabs the sample from the Asteroid surface, then leaves for home. The sample is released and comes down solo while Osiris-Rex heads for the Sun – and that actually is the plan for Bennu as well, to head to the Sun, same as its orbit always takes it. The main difference is this time Osiris-Rex will be there and monitoring the situation. For the first time scientists will actually measure in real time the Yarkovsky effect. (A force acting on a rotating body in space – put simply the Sun’s anistropic emission of thermal photons which in turn influences asteroids such as Bennu.) The reason this is so important is when calculating what will and will not hit Earth we need to understand the small push caused by the Sun on the asteroid and then add on, based on the asteroids shape and other physical characteristics, any wobble or other rotation, and understand what all these put together mean to the heading to Earth to destroy it. (Seems important, no?)
So perhaps this is a rather easy explanation of all that will be going on, but think of the two important issues to be explained. The results of the tests of the samples which will have returned to Earth, to the Utah dessert, will hopefully determine the asteroids chemical composition. Sample scientists will be looking for organic compounds such as sugar or amino acids – the building blocks of life.
Actually each piece of this mission includes awesome and really difficult maneuvers, and as it continues, what it achieves will be tremendous, and then maybe we can see if Bennu will still be a threat in 2175 and if some sort of intervention is necessary.
**Thought – I would imagine by observing the Yarkovsky effect, this might provide a final option for changing the trajectory of worrisome asteroids. Perhaps covering them with something that would result in a concentration of the thermal photons might be an option just before the asteroid makes a pass of the Sun. This is an intriguing mission, and simultaneously kind of dark in that we might be viewing the preamble to our own destruction.