ATLAS has discovered its first virtual impactor, or VI, an asteroid designated 2016 SJ35. The asteroid was discovered on September 28th and classified as a VI a few days later, when enough observations were available to calculate a useful approximate orbit. Based on current data, it has a four-in-a-million chance of hitting Earth between the years 2086 and 2109. This probability was calculated by scientists at NASA’s Jet Propulsion Laboratory (JPL), using measurements from ATLAS and other observatories. You can view  detailed results from JPL’s calculation for 2016 SJ35  here, and those for other VI’s here.
With an estimated size of 50 meters, 2016 SJ35 does not pose a global threat. In the unlikely event that it does hit the Earth, it will cause significant damage over a region no more than 100 km across. Thanks in part to ATLAS’ discovery, a precise impact prediction will be available years in advance, allowing everyone in the impact zone to evacuate safely. Most likely, however, further observations will show that 2016 SJ35 is not going to hit the Earth at all.
To understand what it means for 2016 SJ35 to be a VI, consider the idea of an ‘approximate orbit’. When an asteroid is first discovered, very little is known about its orbit. Using the laws of gravity and the known parameters of the Solar System, we can calculate a huge variety of possible orbits that would match the initial measurements. We don’t know which one of these orbits the asteroid is really on. As astronomers continue to observe the asteroid, their new measurements rule out most of these preliminary orbits. We can say, “Well, if it were on that orbit, we wouldn’t have observed it there.” Vast numbers of orbits could still fit the data, but now all these possible orbits are similar. We can predict where the asteroid will be in the distant future with some accuracy, because all the possible orbits predict it to be in about the same place.
The orbits don’t all predict exactly the same place, so there is some range of possible positions, called the ‘uncertainty’ of the prediction. This is what we mean by an approximate orbit: we don’t know exactly what the orbit is, and we don’t know exactly where the asteroid will be in the distant future, but we can make a good guess. Even more importantly, we know what we don’t know. We have calculated the uncertainty of the prediction, so we know how far off our guess could be.
The definition of a VI is that some of the possible orbits still hit the Earth. In other words, if we ask “Where will 2016 SJ35 be at a specific time in the year 2086?”, one of the possible answers is, “Right where Earth will be at that same moment!” Another way of saying this is that the uncertainty of the prediction for 2016 SJ35 overlaps the known position of Earth on that date. However, most of the possible orbits don’t predict a collision with Earth: only four out of a million of them do.
Astronomers all over the world will continue to make measurements of 2016 SJ35 because of its very interesting status as a VI. These new measurements will rule out more possible orbits, almost certainly including the orbits that impact Earth. When this happens, 2016 SJ35 will no longer be a VI. If, on the other hand, the new measurements don’t rule out the orbits that hit the Earth, they will rule out other orbits. If this continues as more measurements are made, the calculated probability of an Earth impact will increase until we know for sure that 2016 SJ35 is going to hit the Earth. In that case, further extremely precise measurements will enable us to calculate exactly when and where the impact will happen, so that our great-grandchildren can be ready.
The question of whether or not this asteroid can hit the Earth is likely to be answered soon! We will post updates as new measurements become available.