Q. The Sentinel Space Telescope will map the inner solar system. How is such a map different from what is commonly thought of as a map?
A. Because the solar system is dynamic and subject to the laws of orbital mechanics, the individual pieces of territory (asteroids and planets) are continuously in motion. A comprehensive map must therefore show not only where things are now, but also where they are going (think of the Marauder Map from Harry Potter, but much bigger, including a half million elements).
If the positions and velocities of asteroids are accurately enough measured, this also means that the locations of each of the asteroids can also be calculated for about 100 years. The Sentinel Mission will create such a comprehensive dynamic map of our inner solar system.
Q. What role do maps play in exploration and why do we need such a map of the inner solar system?
A. Mapping the inner solar system is the first step to opening up the frontier and protecting the future of humanity on Earth. Just as the U.S. geological surveys and the mapping expedition of Lewis and Clark were instrumental in the development of the American frontier, the map of Near-Earth Asteroids (NEAs) will be instrumental in pushing forward the frontier as humanity opens up the inner solar system.
The inner solar system is mostly uncharted territory as we have mapped the orbits of only about one percent (1%) of the Near-Earth Asteroids larger than the one that struck Tunguska in 1908. If we want to preserve life on Earth, and open up the inner solar system to scientific and economic endeavors, we need an accurate map of the locations and trajectories of asteroids. We need to know which ones approach Earth and are therefore accessible, and which ones threaten Earth and therefore must be deflected.
Q. How much advance notice will we have of an impending impact with Earth?
A. The map that the Sentinel Space Telescope will create will give Earth decades of advance notice of an impending impact—so that deflection becomes relatively easy. There are several promising technologies, including kinetic impactors, gravity tractors, and nuclear standoff explosions. The urgency in completing the map arises because there could be an impact in the next few decades, when the task of deflecting an asteroid becomes extremely difficult, to the point where it could become almost impossible (depending on the size of the asteroid) using current technology. Every year delayed in completing the Sentinel Space Telescope increases our chances of having no available options. Why take this risk?
Q. Where does the Sentinel Space Telescope need to be located for optimal mapping of asteroids?
A. The optimal location for tracking Near-Earth Asteroids, and for creating the Sentinel Map, is from a location between the Earth and the Sun, from where a space telescope can scan Earth’s orbit while continuously looking away from the Sun. The optimal wavelength to find asteroids is in the infrared, where asteroids stand out from the cold dark background. This strategy has been analyzed and affirmed by both the National Academies report “Defending Planet Earth” and the NASA Advisory Council report on Planetary Defense.
We will launch the Sentinel Space Telescope into orbit around the Sun, near the orbit of Venus. The advantage of this orbit means that Sentinel will be able to scan the opposite side of the Sun from Earth. However, it also adds to the technical challenge in that the distance from Earth to the Sentinel Space Telescope will vary between 30-170 million miles. This is hundreds of thousands of times further than the Hubble Space Telescope, which was placed in orbit only 350 miles from Earth.