Bennu was discovered in 1999 and targeted for exploration due to its near-Earth orbit, size, and composition.
Near-Earth is something of a relative term: anything within 120 million miles is technically “near-Earth.” This is calculated as a particular range based on the distance between the Earth and Sun (93 million miles). For an asteroid to be considered a practical target it must orbit anywhere between 148 million and 74 million miles from Earth. While Bennu orbits well within that range, it still took OSIRIS-REx two years to complete its approach.
While doomsday prophets frequently cite Bennu as our species’ inevitable end, the planet only approaches Earth every six years and has a 1-in-2,700 chance of making landfall sometime late in the 2200s. Although those odds are still better than the lottery, Venus is actually the planet most likely to bear the brunt of an impact. It’s also possible that the asteroid may be reduced to ash by the Sun.
Bennu is roughly the size of the Empire State Building, although scientists speculate it’s a fragment from a much larger asteroid the size of Connecticut. The asteroid’s larger size means it rotates more slowly, throwing off less debris and making it a safer target for spacecraft to approach.
The asteroid’s density is surprisingly light: only 30% more than water. This leads scientists to believe that Bennu is not one single object but rather a collection of boulders held together by sheer force of physics. Scientists also believe it has a high concentration of carbon, meaning it is a “primitive asteroid” – one that has remained unchanged over 4 billion years and contains organic molecules that could shed light on how life formed on Earth.
The OSIRIS-REx mission launched in September 2016. Its name is perhaps NASA’s most complicated acronym, but it does provide an outline for the mission’s main goals: Origins, Spectral Interpretation, Resource Identification, Security, and Regolith Explorer.
Origins simply mean we will collect a sample of and discover the exact minerals that make up Bennu and the organic compounds contained within.
Spectral interpretation compares data collected from the asteroid itself with information we have learned from our telescopes on the ground, hopefully identifying where our interpretations are correct and revealing our miscalculations to allow for greater ground-based accuracy in the future.
Resource identification will analyze the rock samples on a molecular level and give insight into its history and evolution based on our understanding of geological processes here on Earth.
Security is an easy one: how likely is Bennu to crash into Earth, and which of its properties is most likely to contribute to that? It will also analyze something called the Yarkovsky Effect, which is how heat can change an asteroid’s orbit over time.
Regolith explorer is also a straightforward agenda. OSIRIS-REx will essentially collect data on the ground around where it collects these rocky samples – what does it look like? How much light does it reflect? Is it rocky or smooth?
Scientists hope that these five factors combined will provide critical insight into our solar system’s early history and formation. If we can discern how life formed on Earth, it could reveal how we might one day find it on other planets.
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