Apophis in 2029 — A Once-in-a-Millennium Observing Opportunity
Author: 一毫秒的永恒
On April 13, 2029, a speck of light in the sky will grow brighter and move faster. At one point it will cross more than the apparent width of the full Moon in a minute and shine about as brightly as the stars of the Big Dipper. It will be neither a satellite nor an airplane, but the roughly 340-meter near-Earth asteroid (99942) Apophis. The asteroid will skim past Earth about 31,000 kilometers above the surface, closer than some artificial satellites; geosynchronous satellites orbit roughly 36,000 kilometers above the equator. That may sound frightening, but more than a decade of tracking and orbital calculations has shown astronomers that Apophis will not hit Earth. Instead, the flyby offers a rare opportunity to learn about an asteroid in Earth’s neighborhood.
Apophis was discovered on June 19, 2004, by Roy Tucker, David J. Tholen, and Fabrizio Bernardi during a NASA-sponsored University of Hawaii asteroid survey at Kitt Peak National Observatory in Arizona. The team followed 2004 MN4, as Apophis was then known, over two nights. NASA initially estimated a collision probability as high as 1.6%, raising the asteroid to level 4 on the Torino scale. By December 27, the estimate had climbed to 2.7%, based on 176 observations. As more tracking and calculations refined its orbit, however, Apophis fell from level 1 to level 0 on August 5, 2006. Scientists can now be confident that it will strike neither Earth nor the Moon.
Scientists still have several years to prepare. Before the asteroid arrives, they can draw up a long observing wish list, design the best instruments for studying it from Earth, and decide how best to observe it from space.
Can the rest of us witness its arrival alongside the scientists? Certainly. Apophis is large and will come close enough to be seen with the naked eye. Roughly two billion people could watch it skim past Earth without a telescope. Around Beijing, the best viewing time should be approximately 1:30 to 2:30 a.m. local time on April 14, 2029.
Although researchers are confident that Apophis will not hit Earth in 2029, they cannot yet rule out a collision in the more distant future. Many other near-Earth asteroids also orbit the Sun in our vicinity. Planetary-defense specialists continually track them and prepare technologies to defend against any object that might pose a threat. Apophis appears similar to about 80% of the potentially hazardous asteroids discovered so far, so data from this encounter may improve our understanding of many others.
At the 2019 Planetary Defense Conference, researchers discussed how to observe the close “encounter” ten years later: how Earth’s gravity would change the asteroid’s orbit, whether Apophis could reveal the interiors of asteroids, and whether a spacecraft should be sent to it. Marina Brozovic, a radar scientist at NASA’s Jet Propulsion Laboratory (JPL), said: “The Apophis close approach in 2029 will be an incredible opportunity for science. We’ll observe the asteroid with both optical and radar telescopes. With radar observations, we might be able to see surface details only a few meters in size.”
Observation was not the conference’s only important subject. Researchers also asked how strongly Earth’s gravity will affect Apophis during the 2029 approach. Past objects passing close to Earth have had their orbits changed, and other asteroids should experience a similar effect. Another issue is equally important to planetary defense: as an asteroid absorbs sunlight and radiates heat, it receives a minute thrust. This phenomenon, the Yarkovsky effect, arises from the temperature difference between the asteroid’s day and night sides.
The Yarkovsky effect changes an asteroid’s orbit so slightly that distinguishing the thrust from an instrumental error or malfunction is difficult. Scientists have constrained Apophis’s trajectory to a corridor only 12 kilometers wide, but they still cannot entirely rule out other influences over the coming decades, partly because of uncertainty in the Yarkovsky effect.
Alongside major tasks for the next decade, the researchers discussed forward-looking mission concepts that could lay the groundwork for a probe to visit Apophis before, during, or after the close approach.
Over the previous year, a series of successes in space had provided a solid foundation. NASA’s InSight mission deployed the first robotic seismometer on another planet, accompanied by the first CubeSats sent to Mars, Mars Cube One (MarCO). NASA’s OSIRIS-REx and Japan’s Hayabusa2 had also carried out missions to rendezvous with small Solar System bodies.
Drawing on these achievements and experience, scientists considered how to design and launch spacecraft to observe Apophis. Some discussed the possibilities offered by CubeSat missions, including spacecraft modeled on MarCO.
One proposal would place a seismometer on the space rock, using a spacecraft designed to pierce Apophis like a fork. The instrument could detect minute vibrations in the asteroid and help reveal its internal structure, as well as the extent of Earth’s gravitational influence. Another idea would create an artificial crater on Apophis, as Hayabusa2 did on asteroid Ryugu (162173), to expose material hidden below the surface.
Some of these ideas may be too risky to justify. Scientists need to be certain that human intervention will not disturb Apophis’s presently safe orbit. “We have to be very careful and ensure that Apophis does not change its original orbit,” Arizona State University planetary scientist James Bell said in a presentation. “Even so, this is a once-in-a-millennium opportunity for NASA and other space agencies. I believe it will become the largest public event of 2029.”
This is the delicate balance asteroid scientists and planetary-defense specialists must strike: make full use of the scientific opportunity offered by Apophis’s close pass without causing panic—or accidentally creating a genuine danger. The most important observations will come in 2029, when asteroid scientists around the world can study Apophis’s size, shape, composition, and perhaps even its interior at close range.
“What is exciting is that an object this large comes this close to Earth only once every several thousand years,” MIT planetary scientist Richard Binzel said at the conference. “The key question is how we seize this extraordinary opportunity. We’ll find out in 2029, and we need to be fully prepared.”
References
爱发电(点击图片跳转)