1I/2017 U1
A/2017 U1 is most likely of interstellar origin. Approaching from above, it was closest to the Sun on Sept. 9. Traveling at 27 miles per second (44 kilometers per second), the comet is headed away from the Earth and Sun on its way out of the solar system. NASA/JPL-Caltech

Map of the positions of known near-Earth objects (NEOs) at points in time over the past 20 years, and finishes with a map of all known asteroids as of January 2018. Asteroid search teams supported by NASA's NEO Observations Program have found over 95 percent of near-Earth asteroids currently known. There are now over 18,000 known NEOs and the discovery rate averages about 40 per week NASA/JPL-Caltech.

NASA already has a planetary defence programme in the form of its Center for Near-Earth Object Studies and it is twenty years old now. Established in 1998, it involved astronomers from all over the world and it was set up because of a threat identified that year.

According to a release put out by NASA, observational data on an asteroid, named 1997 XF11 put it on a collision course with Earth. Astronomers believed that believed that the asteroid- about a kilometre wide could strike Earth in 2028. There was a small chance that it could crash into Earth.

The Minor Planet Center (MPC), in Cambridge, Massachusetts, put out this message, says a NASA report. It was important because MPC is the worldwide repository for asteroid observations and they also carry out initial determination of asteroid orbits. In spite of this message being intended to reach only a few eyes, namely a small community of astronomical scientists that hunt and track asteroids. It was a call to make more observations and collect information. The news, however, spread fast.

Around this time, the report notes that several media outlets did not really understand this internal announcement and they mistakenly went about highlighting the chances of Earth going to get hit.

As it turns out, Earth was not in the line of 1997 XF11, it is not going to hit in 2028. Astronomers came to this conclusion after performing more precise orbit analysis with the then available asteroid observations, notes a report by NASA.

Don Yeomans, who was at that time the leader of the Solar System Dynamics group at the Jet Propulsion Laboratory (JPL), teamed up with colleague Paul Chodas, and the two of them came to the conclusion that the Earth was not going to end in 2028.

"The 2028 impact was essentially impossible," said Chodas, who is now director of NASA's Center for Near-Earth Object Studies (CNEOS), at JPL.

"To this day we still get queries on the chances of XF11 impacting in 2028," Chodas said. "There is simply no chance of XF11 impacting our planet that year, or for the next 200 years."

CNEOS was able to conduct precise orbit calculations for XF11 from MPC observation data which in turn came from observatories around the world that track comets and asteroids.

Over the last 20 years, CNEOS has made it possible for NASA to become the best in the world as far as asteroid data is concerned, notes the report. The space agency and its branches keeping account of pretty much every asteroid and comet—especially the ones that could possibly pass Earth orbit.

CNEOS computes high-precision orbits for all asteroids and comets and also map their positions within the Solar System, this includes both forward in time, and backwards so that there is a clear picture of where they've been in the sky and to detect potential impacts in the future, said Chodas.

"We provide the best map of orbits for all known small bodies in the Solar System."

NASA explains that the first step in calculating impact risk asteroids or even comets is to assess whether or not the object's orbit intersects Earth's if it does, how close will it physically get to Earth?

JPL was already making high-precision orbits calculations for select Near-Earth Objects (NEO) —asteroids and comets with orbits within 195 million kilometres of the Sun. This was reportedly before NASA launched its NEO Observations Program. It has since, over time, upgraded orbit models to cover several thousands of asteroid positions and their orbits.

Observatories from around the world make digital images of the night sky and detect asteroids and comets as moving points of light. Over a time period of days, weeks, and months and then report positions of moving objects in relation to the still, starry background. This is then relayed to the MPC.

CNEOS scientists then gather this data and precisely calculate NEO orbits, predicting its motion in the far future. By comparing it with Earth, they come to know if it is any danger to Earth. CNEOS has a system called the "Sentry" which searches in advance for every future Earth impact possibilities a time period of about a hundred years. This is done for each and every known NEO.

Sentry monitors constantly and uses the latest CNEOS generated orbit models. The results of all the calculations are stored online, says the report. In most cases, chances for impact are actually "extremely small" and in others, the objects are a way to small to cause any real damage. Rocks that are less than 20m in size, would almost certainly disintegrate even if they do crash into Earth.

In case there is some chance of an impact, "we add it to our online 'impact risk' table, and asteroid observers can then prioritize that object for further observation," said Steve Chesley of JPL.

He added, "The more measurements made of the object's position over time, the better we can predict its future path." in most cases, notes the report, the more information that is collected on an NEO, usually puts it out of the risk list.

Alongside Sentry works "Scout" that works in the short term, for space rocks that are identified more recently. It is a quick response system that notifies multiple agencies in case of any imminent threats. Scout famously predicted the impact of 2018 LA over Botswana, notes the report.