This article is from WeChat official account:Science University (ID: kexuedayuan), author: Ziwei (CAS Purple Mountain Observatory), scientific review: Li Bin, Zhao Haibin, title figure from: vision China

According to the forecast of the International Minor Planet Center (MPC), at 08:42 am on November 20th, Beijing time, a piece of about 12 floors The tall near-Earth asteroid 2020 VA1 will fly over the earth about 4.4 million kilometers away, but it will not pose a threat to the earth.

However, this is not “new” news. Half a month ago, the near-Earth Object Telescope of the Purple Mountain Observatory of the Chinese Academy of Sciences discovered this asteroid.

2020 VA1 (target in the box) taken by the Near-Earth Object Telescope. Source: Purple Mountain Observatory

How do scientists monitor these uninvited guests who may pose a threat? Today, we will talk to you about “Asteroid Forecast”.

What is a near-Earth asteroid?

Before entering the topic, clarify two nouns.

Near-Earth Asteroids(NEA): Refers to the closest distance between the orbit and the sun(perihelion) less than 1.3 astronomical units (about 200 million kilometers) asteroids< /p>

Potentially threatening asteroids(PHA): Refers to the minimum orbital intersection distance with the earth less than 0.05 astronomical units(About 7.5 million kilometers) Near-Earth asteroids with a diameter greater than 140 meters.

Near-Earth asteroids that have been discovered: more than 20,000!

The cumulative number of near-Earth asteroids discovered │ Source: NASA

As of November 15, 2020, the total number of near-Earth asteroids discovered by humans is 24,285, of which 9,404 are greater than 140 meters in diameter, and 890 are greater than 1 km in diameter, which are potentially threatening There are a total of 2136 asteroids and 113 near-Earth comets.

The international search and discovery program for near-Earth asteroids has two main stages:

(1) In 1998, it took ten years to discover most of the near-Earth asteroids with a diameter of more than 1 km;

(2) In 2008, most potentially dangerous asteroids with a diameter greater than 140 meters have been detected,Tracking, classification and acquisition of physical characteristics.

At present, the first phase of the mission has been completed, and more than 95% of near-Earth asteroids with a diameter greater than 1 km have been discovered. A large number of discoveries were concentrated in the fifteen years from 1998 to 2012, and the main contributions came from LINEAR, NEAT and Catalina surveys.

Annual discoveries of near-Earth asteroids greater than 1 km in diameter │ Source: NASA

Astronomers are concentrating on the second phase of the mission-the discovery rate of near-Earth asteroids larger than 140 meters in diameter exceeds 90% (currently less than 30%) . Since 2000, the number of potentially threatening asteroids with a diameter greater than 140 meters has been maintained at about 300-500 each year, and there has been a continuous slow growth trend. The contributions mainly come from LINEAR, Catalina and Pan-STARRS survey relays. It is foreseeable that in the next 10 to 20 years, the discovery of near-Earth asteroids with a diameter between 140 meters and 1 km will still be the main task.

Annual discoveries of near-Earth asteroids greater than 140 meters in diameter │ Source: NASA

Who discovered the near-Earth asteroid?

A worker must first sharpen his tools if he wants to do his job well. Optical telescopes with powerful sky survey capabilities are the key to the discovery and monitoring of near-Earth asteroids. The sky survey speed and limit magnitude are the key indicators for near-Earth asteroid search, and the field of view and aperture are the telescope parameters closely related to these two indicators.

Among them, the large field of view is the decisive factor for large-scale discovery. There have been telescopes that have made outstanding achievements in the field of near-Earth asteroid search, and they are all known to be better than the sky survey speed. The Rubin Observatory under construction (LSST) will become the most powerful relay.

Table 1: Comparison of key indicators of major international survey telescopes (some equipment has been upgraded, WFST and LSST are under construction)

As can be seen from the above table, the United States is the absolute main force in international near-Earth asteroid observations. The discovery of near-Earth asteroids in the United States accounts for more than 98% of the total discovered. The planned relay development of sky survey equipment is The root of their victory.

What kind of asteroid has been discovered this time?

2020 VA1 was discovered by the Purple Mountain Observatory’s Near-Earth Object Telescope at 20:57:03 on November 6, 2020, Beijing time. The brightness was 19.9 and the apparent movement speed was 1.04 degrees/day.(relative to the stars in the background), approaching the earth. On November 8, the International Asteroid Center officially released this discovery.

2020 After the discovery of VA1, more than ten European asteroid monitorsThe telescope carried out follow-up observations, and the Purple Mountain Observatory also initiated the tracking observations of the Xing Mingwen Observatory in western my country, and quickly determined the orbital parameters of 2020 VA1: The semi-major axis a is 1.87 AU(AU is the astronomical unit, 1AU is about 150 million kilometers), the eccentricity e is 0.48, and the perihelion q is 0.97 AU. This is a typical Apollo-type near-Earth asteroid with a diameter of about 37 meters.

2020 VA1 is the fifth near-Earth asteroid discovered by the Purple Mountain Observatory’s Near-Earth Object Telescope this year. The telescope has discovered 25 near-Earth asteroids since its operation in 2006, accounting for the total number of near-Earth asteroids discovered in my country(including Taiwan)(31 pieces) about 80%.

2020 VA1 (green) and large planet orbit map (Mercury: white; Earth: blue) Source: Purple Mountain Observatory

How is the size of the asteroid estimated?

I often hear in the news that “an asteroid the size of a football field is approaching the earth” and “an asteroid the size of a truck will fly over the earth tomorrow night.” This time the asteroid is about 12 stories high. So, how do scientists estimate the size of these asteroids?

Actually, the number of near-Earth asteroids we know of the exact size and shape is very limited. Most asteroids are irregular in shape. What we usually call the size of an asteroid is to assume that it is equivalently spherical.Diameter d(km), this value can be calculated from the assumed geometric shape and uniform surface Estimate the asteroid’s geometric albedoPv and its absolute magnitude H:

Among them, the absolute magnitude of the asteroid H refers to the apparent magnitude when it is assumed to be placed at a distance of 1AU from the sun and the earth, and the phase angle is 0°; The geometric albedo of a planet Pv, that is, the ratio of the solar radiation reflected by a celestial body to the total solar radiation illuminating the celestial body, between 0 and 1, but this is usually an unknown quantity.

To facilitate the estimation of the size of near-Earth asteroids, a geometric albedo of 14% is usually assumed. Of course, if you know the spectral type of the asteroid, you can use the empirical relationship between the spectral type and the geometric albedo to determine the geometric albedo more accurately.

It can be seen that the Near-Earth asteroid size estimated by optical observations is very rough, which also introduces errors to related statistics. However, these statistics are useful for monitoring annual findings.

There are several types of near-Earth asteroids

Near-Earth asteroids are mainly divided into four categories according to their orbital characteristics, and their names come from ancient Greece, ancient Rome, ancient Egypt and Indian mythology.

Apollo type(Apollos), accounting for about 50%, named after the first discovered such asteroid 1862 “Apollo ”(Apollo)-the sun god in ancient Greek mythology.

Amor type(Amors), accounting for about 42%, named after the first such asteroid 1221 “Ah More”(Amor)——The god of love in ancient Roman mythology.

Ardenian type (Atens), accounting for about 8%, named after the first discovered such asteroid 2062 “Arden ”(Aten)——The sun god in ancient Egyptian mythology.

Atira type (Atiras), only more than 30 were found, named after the first such asteroid discovered, 163693″ Atira”(Atira)-the goddess of the earth and corn in the mythology of the American Indian Pawnee tribe.

Four main orbit types of near-Earth asteroids │ Source: Purple Mountain Observatory

Near-Earth asteroid monitoring needs long-term continuous development

In fact, the discovery of near-Earth asteroids is only one part of it, and the continuous monitoring of them is also important. The orbit of the near-Earth asteroid may change with the influence of the gravitational perturbation of massive celestial bodies and the Yakovsky non-gravitational effect. ThisIt is a long-term process. More than decades of continuous monitoring of near-Earth asteroids can obtain more observation arcs and determine the orbit more accurately. Therefore, long-term continuous monitoring is not only necessary, but also very necessary, and we hope to achieve full sky coverage in the shortest possible time. Through the global deployment of stations, building an effective ground-based monitoring and early warning network is a top priority.

The key to near-Earth asteroid monitoring is to spot targets that may pose a threat to the Earth in advance. The sooner you find the better. The focus of near-Earth asteroid defense is not to destroy them, but to try to change their orbits. In fact, it only takes a few millimeters/second to a few centimeters/second to change the speed, and after several years of cumulative action, a near-Earth asteroid that might have been hit by a near-Earth asteroid can pass by.

Therefore, as long as it is discovered early enough, with the continuous development of technology, humans have the ability to prevent collisions from happening. There are many related defensive technologies. Because they are not the focus of this article, they will not be described in detail. Interested readers can move here.(When an asteroid strikes, Besides letting the earth wander, what else can we do?).

a long way to go

The Purple Mountain Observatory is my country’s main force participating in the work of the International Asteroid Warning Network (IAWN). It is located at the near-Earth object at the Xuyi Observatory The telescope is currently the only backbone equipment for my country to contribute and share data, and it has been continuously developing near-Earth asteroid monitoring and early warning. So far, 4,625 new asteroids have been discovered, including 25 near-Earth asteroids and 5 potentially threatening asteroids. At the same time, long-term monitoring of more than 300 near-Earth asteroids is maintained each year. my country’s discovery of near-Earth asteroids accounts for only about one-thousandth of the world’s total, which is a significant gap with the United States.

Scientists are seeking opportunities through multiple channels to actively promote the construction of next-generation near-Earth object observation equipment and the construction of my country’s near-Earth object monitoring network. It is hoped that in the near future, ChinaIt can also become the main force for international near-Earth object monitoring, early warning, defense and research.

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This article is from WeChat official account:Science Compound (ID: kexuedayuan) author: Ziwei (CAS Purple Mountain Observatory), scientific review: Li Bin, Zhao Haibin