The overview presented in the previous post has set the stage for our next inquiry into our part of the Solar system. As we wait for the Large Synoptic Survey Telescope to be operational, the vast majority of Near-Earth Objects (NEOs) are presently discovered by a small number of dedicated surveys such as NEOWISE, the Catalina Sky Survey, and Pan-STARRS. The Arecibo Observatory in Puerto Rico that is now permanently closing played a big part in the planetary defence since it contributed to monitoring the close approach of asteroids, along with Goldstone DSS-14 and DARPA’s Space Surveillance Telescope. Coincidently, the Spitzer Telescope, whose mission included the study of asteroids, was also decommissioned earlier this year in anticipation of the launch of the James Webb Space Telescope in 2021.
Approximately 50 % of the objects are discovered before the closest approach and 50 % afterwards, primarily as the objects are approaching from the direction of the Sun and are not observable in the day-lit hemisphere using telescopic surveys.
The current rate of discovery is far exceeding progress in physical characterization that is essential to help inform hazard assessment. The Solar system’s small bodies are often regarded as more primitive relics dating back to the early stages of the Solar system. The Near-Earth Objects, whose number is -- I recall -- at around 24 000, are predominantly replenished by leakage from the main asteroid belt. The threat posed by asteroid families is mitigated by the fact that the large icy-body reservoir of the Kuiper belt is bounded on the inside by Neptune’s ability to keep asteroids at bay, while objects in the main asteroid belt may be subject to minor dynamical erosion and potentially destructive collisional activity.
Dedicated algorithms are in charge of orbit determination and risk assessment for any detected NEO, but their efficiency is limited in cases in which the object has been observed for a short period of time, as is the case with newly discovered asteroids and imminent impactors.
Beyond the mere possibility of an asteroid falling on our heads, there seems to be a legitimate concern regarding the instability of the asteroid families. A disruption within one of them could have repercussions on the future of humankind. The problem is that each time an asteroid experiences a close encounter with another object, it impacts its path and evolution. Although a cataclysmic event on a global scale is unlikely, it is anticipated that small impactors rushing through the atmosphere in the form of fireballs will continue to occur.
Given the current data, it is estimated that a space object sized between 1.5 and 2 km represents a threshold for a global catastrophe, which could result in the death of even a quarter of the world’s population.
A paper by Radosław Bielawski, published this year in Security & Defence Quarterly, observes that asteroids less than 50 to 100 meters across rarely impact the Earth as a single body. They instead explode in the atmosphere. Nevertheless, their detonation can still cause substantial damage. During the entry phase, the atmospheric friction causes objects to decelerate and pick up temperature. Owing to their insufficient size, they enter the atmosphere as fireballs whose fragments may occasionally be found on the ground.
Potentially Hazardous Asteroids (PHAs) are asteroids at a distance equal or less than 0.05 au, which is about twice the distance to the Moon. Apophis appears to hold a special status among potentially hazardous asteroids. On April 13th, 2029, it will pass at about 0.0002561 au, which is 1/10 of the distance to the Moon, less than 24 000 miles from the Earth, according to NEODyS-2 and JPL Small body database. For sure, it will be an amazing opportunity to watch and study a passing asteroid since it will be visible to the naked eye in Europe, Africa, and West Asia.
In a white paper published this year, Richard Binzel of MIT and over 40 scientists urge NASA’s Planetary Defense Coordination Office to plan for the appropriate investigations of Apophis during its flyby of the Earth. An asteroid as large as Apophis coming that close to the Earth is, on average, a once-per-thousand-year event. It is estimated to be 5000 times more massive than the Chelyabinsk meteor that caused damage to nearly 7 500 buildings in 2013.
Apophis with a diameter of about 370 meters will be closer to us than orbiting geosynchronous satellites. While it is expected to pass by the Earth and the Moon safely, weather satellites and NASA observatories will be in the front row. Not only launcher stages and inactive spacecraft that are often left into orbits could pose the risk of impacting and contaminating celestial bodies such as Apophis, but the asteroid could potentially bump into space debris, setting off a chain of reaction.
Furthermore, the crumbling asteroid designated P/2013 R3 -- whose picture I published in my previous post -- got me wondering whether Apophis could break up and throw a few meteorites in our way. P/2013 R3 disintegration may have been caused by the effect of sunlight and internal fractures that were the result of past collisions. A paper suggested that a fireball observed over Kyoto on April 28th, 2018 was indeed produced by the nearby binary near-Earth asteroid (164121) 2003 YT.
Measuring seismic vibrations inside Apophis might warn us of any threat in that regard. We are -- as it was clearly stressed by the authors of the white paper -- still ill-informed of what impact, if any, Earth’s physical interactions could have on Apophis. Could they trigger a change in its orbit? The greatest uncertainty lies in our lack of knowledge of the internal structure of potentially hazardous asteroids. In order to measure seismic activity, plans should be made to place a seismometer on Apophis.
Beyond Apophis, among worldwide efforts to address space threats in the future, a 2019 paper reports the design, construction, and training of a relatively simple neural network aimed at classifying asteroids with the potential to impact the Earth over the coming 20 000 years. Another study evaluates the observability of Near-Earth Objects with the EISCAT 3D radar currently under construction in Northern Scandinavia, in particular for the detection and observation of mini-moons.
The Near-Earth Object Surveillance Mission (NEOSM) is another NASA mission concept in the formulation phase designed to find and characterize the majority of NEOs that could cause severe regional damage. ESA is also developing the infrastructure for the NEOSTEL Fly-eye telescope that will provide by 2030 early warning for hazardous asteroids. Finally, a paper last month outlined a mission concept aiming to perform close flybys of a series of NEOs with a camera and a LIDAR on board of spacecrafts. Could such a concept be implemented by 2029?