Two asteroids are due to pass close to Earth this week: one large enough to destroy cities, the other large enough to end civilization.
Do not panic.
There's zero chance either will hit Earth, and depending on where in the world you are, you might be able to see one of them.
The larger of the two (415029), 2011 UL21, will travel more than 17 times farther than the Moon at 4:14 pm EST on Thursday. It's a whopping 7,600 feet long, but will be too far away to be easily spotted without a powerful telescope.
But in two days, a tiny space rock dubbed 2024 MK will get a lot closer to humanity. At 9:46 a.m. ET on Saturday, it will pass by Earth at 75 percent of the distance to the moon. If you have a decent backyard telescope, or even a good pair of binoculars, and a cloudless sky, you'll see the 400- to 850-foot-tall rock as a speck of light streaking across the starry sky.
“This object is moving very fast so it requires some technology to spot it,” said Juan Luis Cano of the European Space Agency's Planetary Defense Office.
Skywatchers in the United States, especially those in the Southwest, may be in the running to see the asteroid whizz past Earth. Those atop Hawaii's Mauna Kea volcano will be in a great position to watch the asteroid whizz past before sunrise. But people in South America may have the best viewing experience, said Andrew Rivkin, a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory.
Small asteroids and cometary debris occasionally crash through Earth's atmosphere, creating a harmless light show. Many pieces of rock and ice just graze the Earth, often ending up between the Earth and the Moon.
It's rare for an asteroid the size of 2024 MK to pass through the pinhole of a celestial object. “It's rare for something this big to pass so close, but it does happen on a decadal basis, and this is only the third time (that we know of) this century,” Rivkin said in an email.
If you can't spot 2024 MK, you don't have to feel left out for long: On April 13, 2029, the 1,100-foot-long asteroid Apophis will fly less than 20,000 miles from Earth's surface — closer than the orbit of a geostationary satellite, making it visible to the naked eye.
Such a close encounter will be useful to planetary defense researchers: This week's asteroid will be detected by radar arrays on Earth, making it possible to pinpoint its size and subsequent trajectory.
“These measurements significantly reduce the uncertainty about the asteroid's motion and allow us to calculate its trajectory well into the future,” said Lance Benner, principal investigator of the Asteroid Radar Research Program at NASA's Jet Propulsion Laboratory.
The double flyby also serves as a lucky preview of Asteroid Day on June 30, a United Nations-sponsored event to raise awareness about asteroid impacts.
On that day in 1908, a space rock roughly 160 feet in diameter exploded in the sky over remote Siberia, instantly destroying 800 square miles of forest – an area the size of the Washington, D.C. metropolitan area. The event is called the Tunguska event, named after the river that ran through the devastated area.
Although more near-Earth asteroids are discovered each year, most of the ones large enough to destroy cities remain a mystery. Fortunately, two new telescopes under construction, the multipurpose Bella C. Rubin Observatory in Chile and NASA's Near-Earth Object Explorer, may be able to find even more.
The 2024 MK asteroid is at least twice as long as the Tunguska impactor. It's certainly good news that this asteroid was discovered before it hits Earth, and it won't. However, it wasn't until June 16th that astronomers discovered this space rock.
“The case of 2024 MK is yet another reminder that there are many large objects yet to be discovered,” Dr. Cano said. Space agencies have the plans and technology to protect Earth from killer asteroids, but only if they find them before the asteroid finds us.