International Asteroid Day: Why It Is June 30 — and How to Observe Asteroids Yourself

Quick Answer: When Is International Asteroid Day?

International Asteroid Day is June 30 every year — the next edition falls on Wednesday, June 30, 2027. The date is fixed because it marks the anniversary of the Tunguska event of June 30, 1908, when an asteroid or comet fragment airburst over a remote region of Siberia and flattened roughly 2,150 square kilometers (about 830 square miles) of forest. In December 2016 the United Nations General Assembly formally declared June 30 International Asteroid Day to observe that anniversary and to raise public awareness of the asteroid impact hazard.

Unlike most "awareness days," this one is deeply relevant to telescope owners. The objects it celebrates are not abstract — the four largest asteroids in the main belt (Vesta, Ceres, Pallas, and Hygiea) are all within reach of modest amateur instruments, and Vesta is occasionally visible to the naked eye from a dark sky. Asteroid Day is a perfect excuse to add a moving dot to your observing list and watch it drift against the stars over a few nights.

What Is International Asteroid Day?

International Asteroid Day is an annual global event held on June 30 to raise public awareness about asteroids and what can be done to protect Earth from a potential impact. It grew out of a 2014 initiative — launched the year after the dramatic 2013 Chelyabinsk meteor airburst over Russia — and was formally adopted by the United Nations in December 2016. On the day itself, observatories, museums, science centers, and space agencies worldwide run public programs, and the asteroid-research community uses the occasion to explain both the threat and the science.

Three things are true at once about June 30, and they are worth holding in your head together:

• Asteroids are common and mostly harmless. Over a million orbit the Sun between Mars and Jupiter, and the vast majority never come near Earth. Most are small; the largest, Ceres, is about 940 kilometers across and is classed as a dwarf planet.
• Impacts are real and have happened. Tunguska in 1908 and Chelyabinsk in 2013 are the two most famous recent examples — both airbursts, both powerful enough to remind us the hazard is not zero.
• The hazard is being actively managed. Telescopic survey programs track near-Earth objects, and planetary-defense missions have already demonstrated that we can change an asteroid's orbit.

For observers, the day is also a genuine opportunity: the main belt's brightest members are well within the reach of the same telescopes we recommend for planets and the Moon. If you have never seen an asteroid, June 30 is the excuse you have been waiting for.

Why June 30: The Tunguska Event

On the morning of June 30, 1908, a large object exploded in the atmosphere over the remote Podkamennaya Tunguska river region of Siberia, Russia. The blast — an airburst rather than a ground impact — flattened an estimated 2,150 square kilometers (about 830 square miles) of forest, snapping trees in a radial pattern across the taiga. Because the region was almost uninhabited, few people were hurt, and the event remained poorly understood for years; scientists did not reach the site until the 1920s.

Tunguska is widely described as the largest asteroid impact in recorded history. No crater was found, which is consistent with a body — probably a few tens of meters across — that exploded several kilometers above the ground rather than striking the surface. It stands as the clearest modern reminder that a comparatively small object can still release enormous energy, and it is exactly why the United Nations chose June 30 for the annual observance.

The more recent reminder came on February 15, 2013, when a roughly 18-meter asteroid entered the atmosphere over Chelyabinsk, Russia, and disintegrated in a fireball. The blast injured more than a thousand people, mostly from shattered glass, and its shockwave was felt for hundreds of kilometers. At an estimated energy of a few hundred kilotons of TNT, it was the most energetic impact event recognized since Tunguska — and the fact that it happened over a city is precisely why Asteroid Day's founders wanted the world paying attention.

From a 2014 Declaration to a UN Day

The Real Hazard — in Numbers

Awareness days live or die on whether they give you a clear, honest picture. Here is the concise version, consistent with the definitions used by the United Nations and NASA:

The honest bottom line: no known asteroid poses a threat to Earth in the foreseeable future, the overwhelming majority of NEOs are small and harmless, and the reason we can say that confidently is that we keep looking. The day exists to make sure that looking continues — and that the public understands why it matters.

2029: Apophis and a UN Year

The reason Asteroid Day watchers are already excited about 2029 is a specific asteroid and a specific date: on April 13, 2029, the near-Earth asteroid 99942 Apophis will pass at a distance of about 32,000 kilometers from Earth's surface — closer than the ring of geostationary satellites. It poses no threat to the planet, but the flyby is extraordinary: an object of this size passing this close will be visible to the naked eye for large parts of the world, a once-in-a-millennium observing opportunity and a natural laboratory for asteroid science.

To take advantage of that moment, the UN General Assembly declared 2029 the International Year of Asteroid Awareness and Planetary Defence, aiming for a worldwide educational campaign around the close approach. For amateur observers it is a rare chance to watch a near-Earth asteroid with the unaided eye and to follow it through binoculars and telescopes as it races across the sky — a preview of the kind of object-surveying that planetary defense depends on.

How to Observe Asteroids Yourself

Asteroids look like stars, but they are not: a faint point of light that moves against the fixed star field from night to night (or hour to hour, for a close passer like Apophis) is an asteroid. The main-belt asteroids orbit roughly every three to six years between Mars and Jupiter, and the brightest of them are surprisingly easy:

The best observing window for any main-belt asteroid is at opposition, when it is opposite the Sun, highest at local midnight, and brightest. Because their orbits are long, each asteroid's opposition shifts from year to year — which is exactly why an up-to-date astronomy app (see our best astronomy apps) is the most useful "accessory" for asteroid hunting: it will tell you where each bright asteroid is on any given night.

Gear for Asteroid Observing

Asteroid observing has forgiving gear requirements — these objects are point sources, so the essentials are aperture to reach faint magnitudes, a steady mount to hold the field still, and dark skies to cut skyglow. The ladder:

How to Celebrate Asteroid Day

You do not need to be near an observatory to take part. Practical ways to mark June 30:

• Go asteroid hunting. Check where Vesta, Ceres, or another bright asteroid is that night, set up your scope or binoculars, and confirm its motion over a couple of nights.
• Join an event. Museums, planetariums, science centers, and astronomy clubs worldwide run Asteroid Day programs — many free and many streamed. Find a club via our club directory.
• Learn the hazard honestly. Read what NASA's planetary-defense program and the UN's IAWN and SMPAG actually do — the reality is more reassuring than the headlines.
• Start planning for 2029. April 13, 2029 — when Apophis becomes a naked-eye object — is worth putting on your calendar now, along with the International Year of Asteroid Awareness and Planetary Defence.

International Asteroid Day FAQ