Return of the King: OSIRIS-REx’s Homecoming
On September 24, the Astronomy community world over will collectively hold its breath as a tiny capsule 80cm in diameter enters the Earth’s atmosphere. It must survive the almost 30000C temperature of re-entry as it moves through this blanket of air at 13 km/s, and successfully open up its parachutes above land, to ensure that it touches down somewhere in an 800 square-kilometre field in Utah, USA. Fascinating as this procedure will be, it will pale in comparison to what it contains – around 250 grams of dust collected from an asteroid.
What are Asteroids?
Around 4.5 billion years ago, a cosmic gas cloud imploded, compressing over 99.86% of its bulk into three trillionths of its original size. This colossal squeeze ignited a cosmic fireball – our Sun.
Most of the remaining material (around two thirds of what was left over after forming the Sun) accreted to form Jupiter, but the rest of it spread out over the whole solar system, roughly in a single plane. The material formed rocks that kept colliding, merging and breaking apart, for much of the next 4.5 billion years. The larger rocks fell into stable orbits, and are classified today as planets and dwarf planets. The smaller rocks, in orbit around the Sun within the orbit of Jupiter, are called Asteroids. While most of these are found between the orbits of Mars and Jupiter (i.e. the Main Asteroid Belt asteroids), a large number of them known to us also exist around the Earth (Near Earth Asteroids).
Near Earth Asteroids (NEAs) orbit the sun in relatively unstable orbits that may change every few million years. They’re quite close to the Earth, all within 45 million kilometres of the planet. Some of them have orbits that come very close to the Earth’s. or even cross the Earth’s orbit, making it possible for them to collide with the Earth. These are called Potentially Hazardous Asteroids (PHAs). There are around 30,000 NEAs of which 2,300 are PHAs.
One such PHA is Bennu.
Meet Bennu
Discovered in 1999, Bennu, (formally 101955 Bennu) was named by a third grade student in the US in an asteroid-naming contest in 2012. Bennu in Egyptian Mythology is a deity in the form of a heron, linked with rebirth.
Bennu is classified as a C-Type asteroid, where C stands for Chondrite, which is a type of meteorite that has not been modified chemically from its original form. Consequently, C-type asteroids are the most ancient asteroids.
The asteroid is shaped roughly like a ball with a diameter of 500m. At its closest, it would reach around 4,82,000 km from the Earth, which is over one lakh kilometres farther away than the Moon. However, this distance is too close for comfort, since slight modifications can move it significantly closer to the Earth, which could potentially draw it in, resulting in a collision. There is a risk of this happening, sometime between 2178 and 2290.
Why Study Asteroids?
Asteroids, being the randomly moving rocks that they are, are prime candidates in creating the Armageddon that exists in our popular imagination. As such, the study of asteroids is fundamental to our survival.
Beyond that, there are two important reasons to study the composition of Asteroids.
Firstly, the scientific aspect. Asteroids are made of the same stuff that the inner planets are made of. Studying these objects gives us a fair idea of the early Solar System. There is also the theory that life came from outer space (“Panspermia”) and it is worth exploring asteroids for direct or proxy signs of life.
The second reason is economic. Asteroids may contain metals and minerals that are considered rare and precious on Earth, and thus make them prime sites for mining. If there is a business case for mining asteroids, we can expect a rapid progress in space travel technology, much faster than what we currently are witnessing.
Exploring Asteroids
Asteroid exploration has been going on for over three decades now. The Near Earth Asteroid Rendezvous (NEAR) spacecraft orbited and then successfully landed on an asteroid in 2000, imaging the celestial object in great details. But the first sample return was done, surprisingly, by Japan. The Japan Aerospace Exploration Agency (JAXA) sent up the Hayabusa, which reached the Ikotawa asteroid in 2005. It collected a very small quantity – around 1500 grains – of asteroid regolith (i.e. dust), and brought it back to the Earth in 2010. JAXA built on this mission to launch the Hayabusa2, which reached a NHA called Ryugu in 2018. Two years after that, the Sample Return Capsule landed on Earth with 5 grams of samples.
Hayabusa’s success set the goal for NASA’s asteroid sample return mission, the “Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer”, or OSIRIS-REx.
The OSIRIS-REx is part of NASA’s New Frontiers Program, a futuristic exploratory program with missions aimed at exploring the Solar System. The first mission was the New Horizons mission to Pluto, the second was the Juno mission to Jupiter. OSIRIS-REx is the third.
The OSIRIS-REx was designed to visit the NHA Bennu, and recover at least 60g of samples. It took off on an Atlas V Rocket in 2016, and reached Bennu two years later. It spent the next two years in orbit around the asteroid, using this time to image the asteroid in detail and analyse appropriate landing sites. It identified four potential sites, and finally in 2020 it landed at the site termed Nightingale.
The sample collection process itself was interesting. First, a robotic arm called the Touch-and-go Sample Acquisition Mechanism (TAGSAM) extends from the spacecraft. The spacecraft lands on the surface with this TAGSAM. As it reaches the surface, a blow of nitrogen is shot onto the surface. This kicks up the dust (“Regolith”) which then enters the sample collection capsule. Once enough samples are collected, the capsule closes, and the return journey commences.
The samples are expected to be around 250g, well in excess of the planned 60g. NASA would review the samples and share them with other space research agencies from across the world. The quantity collected will be sufficient for many more researchers to afford a chance to evaluate these exciting space rocks.
What Lies Ahead?
OSIRIS-REx does not finish its mission here. In 2022, it was commissioned to extend its mission to head to another Near-Earth Asteroid, 99942 Apophis, with a new call sign – OSIRIS-ApEx (which stands for Apophis Explorer). It will rendezvous with Apophis in 2029, and conduct extensive imaging surveys.
With everything that is happening with Space, it is easy to miss out on this. But OSIRIS-REx may be part of a series of missions that help us understand asteroids much better, opening the floodgates to new interest in expanding the frontiers of human understanding.