Near-Earth Object Lecture 10 - Part 2 PDF

Lecture 10 – Part 2 Near-Earth object (cont.) – Comet Shoemaker–Levy 9 Collided with Jupiter in 1994 – Discovery of asteroids – Predict a potential impact – Options for actively averting the threat  Awareness of the wider public rose The awareness of the wider public of th...

Lecture 10 – Part 2 Near-Earth object (cont.) – Comet Shoemaker–Levy 9 Collided with Jupiter in 1994 – Discovery of asteroids – Predict a potential impact – Options for actively averting the threat  Awareness of the wider public rose The awareness of the wider public of the impact risk rose after the observation of the impact of the fragments of Comet Shoemaker–Levy 9 into Jupiter in 1994.  Comet Shoemaker–Levy 9 Comet Shoemaker–Levy 9 (SL9) was discovered by Carolyn and Eugene M. Shoemaker and David Levy in 1993, while conducting a program of observations designed to uncover NEOs. It was the 9th short-period comet discovered by the Shoemakers and Levy. This Photo by Unknown Author is licensed under CC BY  Multiple nuclei in an elongated region The discovery image gave the first hint that comet SL9 was an unusual comet, as it appeared to show multiple nuclei in an elongated region.  Orbiting Jupiter Orbital studies of the new comet revealed that it was orbiting Jupiter rather than the Sun. This Photo by Unknown Author is licensed under CC BY-SA  Jupiter’s tidal forces pulled apart the comet Calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992. At that time, Jupiter’s tidal forces had acted to pull apart the comet. Original: Theresa Knott at the English WikipediaSVG: Rehua, CC BY-SA 3.0, via Wikimedia Commons  Prediction The best orbital calculations predicted that the comet would collide with Jupiter in July 1994.  Collided with Jupiter Comet SL9 collided with Jupiter in July 1994.  Closely observed by astronomers worldwide It provided the first direct observation of an extraterrestrial collision of Solar System objects. The comet was closely observed by astronomers worldwide. https://www.nasa.gov/centers-and-facilities/goddard/how-historic-jupiter-comet-impact-led-to-planetary-defense/  Coverage in the popular media This generated a large amount of coverage in the popular media. https://www.youtube.com/watch?v=KGRgDM2wp14  Summary Scientists have recognised the threat of impacts since the 1980s. – The impact hypothesis was proposed in 1980. The awareness of the wider public of the impact risk rose after the impact of the Comet SL9 into Jupiter in 1994.  Discovery of asteroids Asteroids are discovered by telescopes which repeatedly survey large areas of sky.  Palomar Planet-Crossing Asteroid Survey The first astronomical program dedicated to the discovery of near-Earth asteroids was the Palomar Planet-Crossing Asteroid Survey. It was initiated by Eleanor Helin and Eugene Shoemaker at the U.S Palomar Observatory in 1973. The program is responsible for the discovery of 95 NEOs.  Spaceguard Survey Plans for a more comprehensive survey, named the Spaceguard Survey, were developed by NASA from 1992, under a mandate from the United States Congress. In 1998, the United States Congress gave NASA a mandate to detect 90% of near-earth asteroids over 1 km diameter (that threaten global devastation) by 2008.  Other “Spaceguard” associations Subsequently, there have been “Spaceguard” associations or foundations (an umbrella term) formed in countries around the world to support the ideas of discovering and studying near-Earth objects. After the impact of Comet SL9, asteroid detection programs all over the world received greater funding.  Near-Earth asteroids larger than 1 km As a result, the ratio of the known and the estimated total number of near-Earth asteroids larger than 1 km in diameter rose from about 20% in 1998 to 93% in 2011.  Define the orbit Once a new asteroid has been discovered and reported, the orbit of the newly discovered object can be defined.  Predict a potential impact Once the initial orbit is known, the potential positions can be forecast years into the future and compared to the future position of Earth. Error eclipse If the distance between the asteroid and the centre of Earth is less than Earth radius then a potential impact is predicted. Phoenix7777, CC BY-SA 4.0, via Wikimedia Commons  Error ellipses The ellipses in the diagram on the right show the predicted position of an example asteroid at closest Earth approach.  First observations At first, with only a few asteroid observations, the error ellipse is very large and includes Earth. The impact prediction is small because Earth covers a small fraction of the large error ellipse.  Further observations Further observations shrink the error ellipse. If it still includes Earth, this raises the predicted impact probability, since Earth now covers a larger fraction of the smaller error region.  More observations Finally, yet more observations shrink the ellipse, usually revealing that Earth is outside the smaller error region and the impact probability is then near zero. In rare cases, Earth remains in the ever shrinking error ellipse and the impact probability then approaches one.  When to raise an alarm This initially very similar pattern makes it difficult to quickly differentiate between asteroids which will not and those which will actually hit Earth. It is difficult to decide when to raise an alarm as gaining more certainty takes time, which reduces the time available to react to a predicted impact. However raising the alarm too soon has the danger of causing a false alarm and creating a Boy Who Cried Wolf effect if the asteroid in fact misses Earth.  Better than 1% chance of impacting NASA will raise an alert if an This Photo by Unknown Author is licensed under CC BY asteroid has a better than 1% chance of impacting. Wolf! This Photo by Unknown Author is licensed under CC BY-SA  Torino scale The Torino scale is a method for categorizing the impact hazard associated with NEOs. – It was adopted at a 1999 international conference on NEOs held in Torino, Italy. It is intended as a communication tool for astronomers and the public to assess the seriousness of collision predictions. Hpnx9420, CC BY 3.0, via Wikimedia Commons  0 to 10 value An object is assigned a 0 to 10 value based on its collision probability and the kinetic energy of the possible collision. See page for author, CC BY-SA 3.0, via Wikimedia Commons  None of the asteroids are assigned even a 1 Some NEOs have had temporarily positive Torino rating after their discovery. As of now, none of the near-Earth asteroids are assigned even a 1 on the Torino scale, meaning that none warrant the attention of the general public.  Apophis Apophis is a near-Earth asteroid with a diameter of 370 m. It caused a brief period of concern in December 2004 when initial observations indicated a probability up to 2.7% that it would hit Earth in 2029. This Photo by Unknown Author is licensed under CC BY-SA-NC  Temporarily reaching level 4 It set the record for highest rating ever on the Torino scale, reaching level 4. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth in 2029. See page for author, CC BY-SA 3.0, via Wikimedia Commons  Options for actively averting the threat Scientists involved in NEO research have also considered options for actively averting the threat if an object is found to be on a collision course with Earth. Strategies fall into two basic sets: Fragmentation and delay.  Fragmentation This Photo by Unknown Author is licensed under CC BY-NC-ND Fragmentation concentrates on rendering the impactor harmless by fragmenting it and scattering the fragments so that they miss Earth or are small enough to burn up in the atmosphere. https://www.youtube.com/watch?v=vQWmd8REdaE  Delay Delay exploits the fact that both Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time. Delaying, or advancing the impactor’s arrival can cause it to miss Earth. This Photo by Unknown Author is licensed under CC BY-SA-NC  Direct methods The direct methods, such as nuclear explosives, or kinetic impactors, rapidly intercept the object’s path. Their effects may be immediate. This Photo by Unknown Author is licensed under CC BY Thus, these methods would work for short-notice threats.  Outline Direct methods – Nuclear technology – Kinetic impactor Test with experiment Efbrazil, CC BY-SA 4.0, via Wikimedia Commons  Nuclear technology Simulations have been run analyzing the possibility of using nuclear technology to redirect an asteroid. However, there has not been a practical test studying the possibility. This Photo by Unknown Author is licensed under CC BY  Legal concerns There are legal concerns regarding the launch of nuclear technology into space. In 1992, the United Nations adopted a resolution that provides strict rules regarding sending nuclear technology to space.  Kinetic impactor The impact of a spacecraft is another possible solution to a pending NEO impact. This deviation method has been implemented by DART (Double Asteroid Redirection Test) and by Deep Impact space probe.  Concern about deflection technology Concern has been expressed This Photo by Unknown Author is licensed under CC BY about deflection technology, noting that any method capable of deflecting impactors away from Earth could also be abused to divert non-threatening bodies toward the planet. It has been suggested that deflection technology be developed only in an actual emergency situation.  Summary Near-Earth object – Comet Shoemaker–Levy 9 Collided with Jupiter in 1994 – Discovery of asteroids – Predict a potential impact – Options for actively averting the threat