Bennu Asteriod

(Futuristic Photons)

Asteroids’ flybys have become frequent this year. Each day we read an article or two where it is mentioned that about one or two asteroids would be passing very closely to us. But we all look for one answer to a really important question in those articles: Are there any chances that it would impact Earth? We sigh in relief after finding the answer, but tighten your seat belts, as we are going to read about a Potentially Hazardous Object that has a high chance of impacting Earth!

101955 Bennu, commonly known as Bennu, is an asteroid in the Asteroid Belt. It was discovered on 11 September 1999 by the LINEAR (Lincoln near Earth Asteroid Research), which has been replaced by Catalina Sky survey. Bennu has a mean diameter of 496 meters, which was interestingly discovered by the Goldstone DSN and Arecibo Observatory, which is a radio telescope in Puerto Rico, operated by the University of Central Florida and several others, under an agreement with US National Science Foundation.

Coming back to our dear Bennu, it has a surface area of approximately 0.782 km2 and when it is kept on weighing machine (just joking :), it weighs around 7.329 x 1010 kg.

The temperature of Bennu ranges from 236K to 279K, with a mean temperature of 260K. Earlier observations from Earth showed that it had probably a smooth surface, but soon, the OSIRIS-Rex mission showed us various boulders with diameter more than 11 m. The images and data showed us that the surface of Bennu was very rough, contrary to what was believed earlier. Bennu also has a ridge along its equator, which is one of its prominent features.

It is a C-Type Asteroid, which means that it is mostly made up of carbon, in addition to various rocks and minerals. C-Type asteroids basically have a low albedo, which is a measure of the diffuse reflection of solar radiation out of the total solar radiation received by an asteroid. Different reflection can be thought of as a ray that is scattered or reflected at many angles other than the normal reflection. Albedo is measured between a scale of 0 and 1, where 0 corresponds to the black body which absorbs all the incident radiation and 1 corresponds to a body which reflects all incident light, although the latter hasn’t been found till date.

Formula for Surface Albedo is Ratio of Radiosity: Irradiance received by a surface

Radiosity is basically radiant flux of radiant energy reflected, transmitted or emitted by a surface per unit area whereas irradiance is the radiant power which is received by a surface per unit area. Carefully the notice the formula along with the explanation given above.

Coming back to our topic, what makes Bennu further interesting is that its rotational velocity is accelerating with time! This happens to YORP effect. In simple words, this basically happens because the radiation and the heat striking the asteroid also have some momentum, like here, it means photons. They have some momentum

P=E/c

Where p is the momentum, E is the energy and c indicates the speed of light in vacuum.

Since Bennu has a very high albedo, therefore it receives some angular momentum, which creates torque .And here comes Newton’s 3rd law of motion into play, the reaction force of photons which are re radiated or emitted from the sides of asteroid create a torque, since the reaction force doesn’t points towards the centre of mass of asteroid, therefore contributing to the spin of Asteroid. But you all must be thinking that dude, aren’t photons very tiny, and won’t their mass be considered negligible? Well you are correct, but this momentum and reaction force never initiates an instant change. But its effects are felt only over the years. This affects the shape of its pole and its spin rate as discussed above.

As a result, the 490m guy is tilted 178 degrees on its axis, and its direction of rotation is opposite with respect to its revolution. This could even mean that Bennu was hit by another Asteroid which caused this change.

But why is Bennu so much talked about? Not because of its is, features, but that it is a Potentially Hazardous Object, listed on Sentry Risk Table. And to top it all, it has the second highest rating on Palermo Technical Impact Hazard Scale. It is basically a scale used astronomers to rate the potential hazard of impact of a near earth object.

It basically measures the hazard as follows:

A rating of zero is like a base mark, which is also called background hazard, and can be thought to have equal likely and unlikely consequences of hitting Earth. This background hazard is defined as risk of random impacts henceforth.
A rating of +2 would indicate the hazard is 100 times greater than a random event happening at that time.
A rating of -2 means that there are no likely consequences of the object hitting Earth.

Objects rated between -2 and 0 are carefully observed since they pose some risk.

Only 2 objects till now have been rated above -2. First one is 1950DA, an asteroid, which had the highest probability of impacting Earth in 2002. It has a rating of -1.42. The next guy is, as you can guess is Bennu, with a rating of -1.69.

This makes Bennu hazardous, and has caused it to come in limelight. It is thought that Bennu would probably impact Earth in the last quarter of 22nd century.

In regard to Bennu, OSIRIS-Rex mission has been launched by NASA, operated by Goddard Space Flight Centre and Lockheed Martin to study about Bennu more closely. Launched by another Atlas V rocket, although its full name is boring, but it would give you some idea about the equipments onboard and its work. The full name is Origins, Spectral Interpretation, Resource Identification, Security and Regolith Explorer. This spacecraft is Bennu-centric and entered Bennu’s orbit on 31 December 2018. It is planned to return to Earth by 3 March 2021 with a sample of a rock from Bennu, weighing around 60 grams.

As the Space exploration would continue, projects like LINEAR and Catalina Sky Survey would be initiated, more and more NEO would be detected, and it is possible that they would have a rating of +2 too on Palermo Scale! But humans would also continue to develop, and would one day develop a missile, which won’t aim to protect country, but would aim to protect humanity from such dangers, which would create a feeling similar to that of nationalism amongst our conscience-s.