The Sentinel orbit is a long way from Earth, so it won’t observe the same objects that ground-based telescopes are looking at.

But it will find objects that are a large distance from Earth. The Nominal Sentinel Orbit is the Sentinel orbit, which is at the same distance from the Sun as Venus. There, at an independent orbit around the Sun, we are looking back towards the Earth’s vicinity. We call it the Nominal Sentinel Orbit based on the result of modeling work done with by mission scientist Dr. Mark Buie.

We investigated in other orbits, which were either closer or farther from the Sun. We also had different observing schemes we looked at, or different intervals and different parts of the sky. And while there some differences, there was no position that gave us overall better performance than the Nominal Orbit, so it was reassuring to demonstrate that Sentinel will be in the right place for an asteroid detection mission.

The subset illustrated is for objects that are greater than 40 meters. Forty meters is the size of the Tunguska asteroid that hit Siberia in 1908. Had it impacted in over a populated area, it would have created substantial devastation. This object is a proxy for the objects that people involved in planetary defense need to worry about.

Impactors is a term that we use for subset of Near-Earth Objects (NEOs) or asteroids. The Impactors are on orbits that will intersect the Earth in 100 years. While we don’t currently know of any objects that are on an impact path with Earth in 100 years or any other time scale, what we do is have the computer look at the whole range of all possible orbits. Then the computer sorts out for us in these hypothetical orbits—the ones that will hit the Earth in the next 100 years.

It tells us what are chances are of seeing an object if there was one on an impact path within the next 100 years. In 10 years, the combination of Sentinel (plus ground-based observations, represented by LSST which will come online in 2022) can find 81 percent of the objects that are on impact paths with Earth. This is actually quite a high number and we are confident that we would get to 90 percent on objects greater than 140 meters, which was the Congressionally defined objective in an authorization bill in 2005.

There are multiple ground-based searches right now. What’s important to realize about ground-based searches is that they’re all basically in the same location in the solar system. What one search can look at, another search can also look at. The coordination with ground-based searches will involve dividing and conquering the night sky.

Each survey can have an area that is studied each night and each time in the night, and that base search is their primary field with other observatories collaborating with them or elsewhere. Sentinel be observing from a different vantage point even when we’re closest to the Earth because it will be 30 million miles away.

But back to what we will observe and why the 40-meter benchmark is important. That’s an important part of our message: When you’re concerned about things that are going to hit the Earth (which is Planetary Defense by definition) then the broadest category of NEO 140 meter objects are not the inherent risks. On an average interval of 10,000 years, there’ll be an impact from a 140-meter object, and 10,000 years is a long time compared to say the 10 years that we think Sentinel will operate.

And it’s also a long time compared to the interval between 40-meter impactors, which is maybe several hundred years. So the next impact to the Earth has will cause injuries and casualties and damage—major damage—is almost certainly going to be smaller than than 40 meters. From a planetary defense point of view, what we need to focus on is exactly what is demonstrated in this chart, which are objects in 40-meter size that are on orbits that are an impact with the Earth.

Note: This essay was excerpted from a recent talk by Dr. Harold Reitsema in a quarterly call with our donors. If you’d like to take part in a future discussion, please contact us at info@b612foundation.org on how to be included.