What You Need To Know About Asteroids

Asteroids are actually minor planets which can neither be classified either as a planet or as a comet. These are generally in the direct orbit around the Sun, also known as the inner solar system. The larger forms of asteroids are also known as planetoids.

These are different from the minor planets in the outer solar system in their volatile-based surfaces much like comets. These are generally known as asteroid belt.
It is believed that these are mainly the remnants of circumstellar disk of gas around newly-formed star and the debris disk together. Their remains together are known as planetesimals. Mainly the known asteroids are present in the Jupiter Trojans or in the asteroid belt formed between the Mars’ orbit and that of Jupiter. Others are found in the solar system near Earth, termed as near-Earth asteroid. The size of asteroids can vary from 1000 km to 10 meter.

Classification Of Asteroids

asteroids moving towards earth

The 3 largest asteroids are almost spherical in shape like miniature planets, but with differentiated interiors. A majority of them, however, are small with irregular shapes. Depending on the chemical component, which is the carbon content, metal composition and silicate quantity, the asteroids are classified into 3 main groups:

1) C-type – These are the most common type of asteroids, consisting 75% of known asteroid population, also dominating the outer part of asteroid belt. All carbonaceous asteroids fall under this category. C-type asteroids are extremely dark in their nature with their reflection co-efficient ranging from 0.03 to 0.10.

2) S-type – The moderately-bright asteroids (albedo/reflection co-efficient- 0.10-0.22) with component mainly including iron and magnesium silicates. These are mainly found in the inner asteroid belt.

3) M-type– The asteroids with nickel and iron in its purest form are categorised under M-type. Sometimes these are also found with the presence of stones. Their brightness ranges from 0.1 to 0.2. All the asteroids are visible by binoculars except for one, 4 Vesta. This is the only asteroid that can be seen even without binoculars because of the relatively reflective surface. It is only rare that a passing asteroid becomes visible to naked eyes.

Formation Of Asteroids

Asteroids are leftovers from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today.

It is thought that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.

In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.

Characteristics

Asteroids can reach as large as Ceres, which is 940 kilometers (about 583 miles) across. On the other end of the scale, the smallest asteroid ever studied is the 6-foot-wide (2 meters) space rock 2015 TC25, which was observed when it made a close flyby of Earth in October 2015. The chances of it hitting Earth in the foreseeable future are small, Vishnu Reddy of the University of Arizona’s Lunar and Planetary Laboratory said in a statement.

“You can think of [an asteroid] as a meteorite floating in space that hasn’t hit the atmosphere and made it to the ground — yet,” Reddy added.

Nearly all asteroids are irregularly shaped, although a few of the largest are nearly spherical, such as Ceres. They are often pitted or cratered — for instance, Vesta has a giant crater some 285 miles (460 km) in diameter. The surfaces of most asteroids are thought to be covered in dust.

As asteroids revolve around the sun in elliptical orbits, they rotate, sometimes tumbling quite erratically. More than 150 asteroids are also known to have a small companion moon, with some having two moons. Binary or double asteroids also exist, in which two asteroids of roughly equal size orbit each other, and triple asteroid systems are known as well. Many asteroids seemingly have been captured by a planet’s gravity and become moons — likely candidates include Mars’ moons, Phobos and Deimos, and most of the outer moons of Jupiter, Saturn, Uranus and Neptune.

The average temperature of the surface of a typical asteroid is minus 100 degrees Fahrenheit (minus 73 degrees Celsius). Asteroids have stayed mostly unchanged for billions of years — as such, research into them could reveal a great deal about the early solar system.

Asteroids come in a variety of shapes and sizes. Some are solid bodies, while others are smaller piles of rubble bound together by gravity. One, which orbits the sun between Neptune and Uranus, comes with its own set of rings. Another has not one but six tails.

Asteroids become darker and redder with age due to space weathering.However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.

Discovery Of Asteroids

The first asteroid to be discovered, Ceres, was originally considered to be a new planet. This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term “asteroid”, coined in Greek as ἀστεροειδής, or asteroeidēs, meaning ‘star-like, star-shaped’, and derived from the Ancient Greek ἀστήρ astēr ‘star, planet’. In the early second half of the nineteenth century, the terms “asteroid” and “planet” (not always qualified as “minor”) were still used interchangeably.

Overview of discovery timeline;

• 10 by 1849
  • 1 Ceres, 1801
  • 2 Pallas – 1802
  • 3 Juno – 1804
  • 4 Vesta – 1807
  • 5 Astraea – 1845
  • in 1846, planet Neptune was discovered^[24]
  • 6 Hebe – July 1847
  • 7 Iris – August 1847
  • 8 Flora – October 1847
  • 9 Metis – 25 April 1848
  • 10 Hygiea – 12 April 1849 tenth asteroid discovered
• 100 asteroids by 1868
• 1,000 by 1921
• 10,000 by 1989
• 100,000 by 2005
• 1,000,000 by 2020

Impacts on Earth-Asteroid

Ever since Earth formed about 4.5 billion years ago, asteroids and comets have routinely slammed into the planet. The most dangerous asteroids are extremely rare, according to NASA.

An asteroid capable of global disaster would have to be more than a quarter-mile wide. Researchers have estimated that such an impact would raise enough dust into the atmosphere to effectively create a “nuclear winter,” severely disrupting agriculture around the world. Asteroids that large strike Earth only once every 1,000 centuries on average, NASA officials say.

Smaller asteroids that are believed to strike Earth every 1,000 to 10,000 years could destroy a city or cause devastating tsunamis. According to NASA, space rocks smaller than 82 feet (25 m) will most likely burn up as they enter Earth’s atmosphere, which means that even if 2015 TC25 hit Earth, it probably wouldn’t make it to the ground.

On Feb. 15, 2013, an asteroid slammed into the atmosphere over the Russian city of Chelyabinsk, creating a shock wave that injured 1,200 people. The space rock is thought to have measured about 65 feet (20 m) wide when it entered Earth’s atmosphere.

When an asteroid, or a part of it, crashes into Earth, it’s called a meteorite. Here are typical compositions:

Iron meteorites

• Iron: 91 percent
• Nickel: 8.5 percent
• Cobalt: 0.6 percent

Stony meteorites

• Oxygen: 6 percent
• Iron: 26 percent
• Silicon: 18 percent
• Magnesium: 14 percent
• Aluminum: 1.5 percent
• Nickel: 1.4 percent
• Calcium: 1.3 percent

Dozens of asteroids have been classified as “potentially hazardous” by the scientists who track them. Some of these, whose orbits come close enough to Earth, could potentially be perturbed in the distant future and sent on a collision course with our planet. Scientists point out that if an asteroid is found to be on a collision course with Earth 30 or 40 years down the road, there is time to react. Though the technology would have to be developed, possibilities include exploding the object or diverting it

For every known asteroid, however, there are many that have not been spotted, and shorter reaction times could prove more threatening.

When asteroids do close flybys of Earth, one of the most effective ways to observe them is by using radar, such as the system at NASA’s Goldstone Deep Space Communications Complex in California. In September 2017, the near-Earth asteroid 3122 Florence cruised by Earth at 4.4 million miles (7 million km), or 18 times the distance to the moon. The flyby confirmed its size (2.8 miles or 4.5 km) and rotation period (2.4 hours). Radar also revealed new information such as its shape, the presence of at least one big crater, and two moons.

In a NASA broadcast from earlier in 2017, Marina Brozovic, a physicist at NASA’s Jet Propulsion Laboratory, said radar can reveal details such as its size, its shape, and whether the asteroid is actually two objects (a binary system, where a smaller object orbits a larger object.) “Radar is a little bit like a Swiss army knife,” she said. “It reveals so much about asteroids all at once.”

In the unlikely event that the asteroid is deemed a threat, NASA has a Planetary Defense Coordination Office that has scenarios for defusing the situation. In the same broadcast, PDCO planetary defense officer Lindley Johnson said the agency has two technologies at the least that could be used: a kinetic impactor (meaning, a spacecraft that slams into the asteroid to move its orbit) or a gravity tractor (meaning, a spacecraft that remains near an asteroid for a long period of time, using its own gravity to gradually alter the asteroid’s path.) PDCO would also consult with the White House and the Federal Emergency Management Agency (FEMA) and likely other space agencies, to determine what to do. However, there is no known asteroid (or comet) threat to Earth and NASA carefully tracks all known objects through a network of partner telescopes.

Exploration

The first spacecraft to take close-up images of asteroids was NASA’s Galileo in 1991, which also discovered the first moon to orbit an asteroid in 1994.

In 2001, after NASA’s NEAR spacecraft intensely studied the near-earth asteroid Eros for more than a year from orbit, mission controllers decided to try and land the spacecraft. Although it wasn’t designed for landing, NEAR successfully touched down, setting the record as the first to successfully land on an asteroid.

In 2006, Japan’s Hayabusa became the first spacecraft to land on and take off from an asteroid. It returned to Earth in June 2010, and the samples it recovered are currently under study.

NASA’s Dawn mission, launched in 2007, began exploring Vesta in 2011. After a year, it left the asteroid for a trip to Ceres, arriving in 2015. Dawn was the first spacecraft to visit Vesta and Ceres. As of 2017, the spacecraft still orbits the extraordinary asteroid.

In September 2016, NASA launched the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx), which will explore the asteroid Bennu before grabbing a sample to return to Earth.

“Sample return is really at the forefront of scientific exploration,” OSIRIS-REx principal investigator Dante Lauretta said at a press conference.

In January 2017, NASA selected two projects, Lucy and Psyche, via its Discovery Program. Planned to launch in October 2021, Lucy will visit an object in the asteroid belt before going on to study six Trojan asteroids. Psyche will travel to 16 Psyche, an enormous metallic asteroid that may be the core of an ancient Mars-size planet, stripped of its crust through violent collisions.

In 2012, a company called Planetary Resources, Inc. announced plans to eventually send a mission to a space rock to extract water and mine the asteroid for precious metals. Since then, NASA has begun to work on plans for its own asteroid-capture mission.

According to CNEOS, “It has been estimated that the mineral wealth resident in the belt of asteroids between the orbits of Mars and Jupiter would be equivalent to about 100 billion dollars for every person on Earth today.”

Credit:https://www.space.com/51-asteroids-formation-discovery-and-exploration.html

https://en.wikipedia.org/wiki/Asteroid#