Rogue asteroids that could demolish the Earth are not just popular science fiction. Impacts on the Earth by large Near-Earth Objects have caused great damage in the past. Many remain potential hazards for reasons of dynamics in the Solar System, viz, the encounters and resonances that influence the trajectory of these bodies. The probability of such an object colliding with the Earth, though extremely small, is not zero.
Astronomers have for long been concerned about an asteroid named Apophis, a rock of size over 300 meters. Its proper designation is 99942 Apophis (2004MN4). The orbit is elliptical and period 323.6666 days. However, the path of Apophis crosses the Earth’s since its perihelion (q= 0.7459 au; Astronomical Unit: 149597870.7 km) lies inside the Earth’s orbit and the aphelion (Q=1.0992 au) on its outside. The asteroid was discovered on 19 June 2004 by R.A. Tucker, D.J. Tholen and F. Bernardi observing from the Kitt Peak Observatory, USA. Also called Apep, the destroyer, Apophis is an Egyptian god of evil.
There is apprehension that Apophis might be on a collision course with the Earth in future, in its close encounters in 2029, in 2036 and in 2068. Computations of its orbit revealed that on 13 April 2029 it will pass by the Earth from within 40000 km. That brings it within the orbits of Earth’s geosynchronous satellites. However, as Apophis is to pass in a plane inclined at 40 degrees to the Earth’s equator, the threat to the satellites is minimal. As for the Earth, the collision probability, initially estimated to be 2.7%, is very near zero. It is estimated that it shall equal an energy release of 500 Megaton bombs. The impact probability for the 2068 fly-by has been revised up to 1 in 1,50,000.
On 9 January 2013, Apophis passed the Earth from a distance of 14.4596 million km, then its nearest. For the astronomers, that was close and the occasion an opportunity to compute its orbit afresh and quantify the minuscule acceleration on it due to a differential heating of a slowly rotating body in sunlight and the anisotropic emission of thermal energy – the so-called Yarkovsky Effect. That constantly affects the trajectory of the small bodies in space. Here one wanted to see to what extent the orbit would get altered by the time of the predicted close encounters with the Earth in 2029 or in 2036. The effect depends on the attributes of the asteroid – its size, mass, and the orientation of its axis of rotation. The study was conducted at the Deep Space Radar, Goldstone, California and the Arecibo Observatory in Puerto Rico to progressively measure its distance and line-of-sight velocities during the fly-by. The Goldstone and Arecibo radar observations revealed an elongated object, possibly bi-lobate. In the course, European Space Agency’s Herschel Space Observatory and the radar observations estimated its diameter to be 325 to 340 m and a lesser albedo (a measure of reflectivity) at 0.33 than previously thought. This can have bearing on the magnitude of the net Yarkovsky effect. The study further revealed that the probability of an impact in 2036 was very remote. A calculation with the Horizons System of the Jet Propulsion Laboratory reveals that Apophis will fly by the Earth from 38010.6 km on 13 April 2029, at 21:45 UT. That would be a close call from a distance of about 1/10th to the Moon.
The asteroid in its round of the Sun is going to have a close encounter with the Earth also on 5 March next, when it will pass its nearest at 23:30 IST from 16.85 million km. That is some distance but the astronomers are enthused to be able to make extensive optical and radar measurements and a refined estimate of the magnitude of the Yarkovsky acceleration. As the asteroid will pass by, there is likely to be a small change in its spin-state due to the tidal effects of the Earth’s gravitational field. All this will lead to slightly altered values of the orbital parameters from which more exact predictions of future close encounters will be possible.
This is preparatory to the 2029 fly-by when Apophis shall offer an unprecedented opportunity to the astronomers to observe it closely for its characterization and geometry with the ground based and space borne facilities. Then its angular diameter will reach 1.46 arcsec, 350 times greater than what would be on 5 March next, and it shall brighten up for a short span, reaching 170000 times its current brightness to touch visual magnitude 3.4. That means it can then be seen by the naked eye, moving rapidly across the sky. This occasion offers a great opportunity for Planetary Defence Awareness and to educate the policy makers and the public about the asteroid science and what can be done to reduce the future threats and the misinformation.
It will be a rare opportunity to fly a dedicated rendezvous mission to Apophis. There are suggestions to re-purpose NASA’s OSIRIS-Rex, the sample-return mission to the asteroid Bennu that shall be back in September 2023. This can be done through a number of flybys such that the spacecraft returns to Earth in April 2029 just around when Apophis flies by, and, place it in orbit round the latter. However, in a mission intended at landing, sample collection or experimental impact, the spacecraft contact itself can perturb the orbit and deflect the asteroid into an impact path. There are serious apprehensions about such a venture and as put by an astronomer during the “Apophis T-9 Years” workshop (virtual) held in November 2020, “We must, in the spirit of the Hippocratic Oath, first do no harm.” Some scientists had in 2011 suggested launch of a robotic mission, first in May 2028 so as to be on site by February 2029. If successful, this could be followed by a manned mission in March 2029 to intercept the incoming asteroid and land on it in early April for which the navigational guidance from the robotic mission about the accurate ephemerides of Apophis would prove extremely valuable. With a proposed stay of several days on the asteroid, that would be some mission, the most challenging one in the history of mankind. ‘Armageddon’ for real?
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R.C. Kapoor 