Richard A Lovett
Scientists are gearing up for a near miss by the largest asteroid to pass close to the Earth in recent history.
The object, known as 99942 Apophis, won’t make its appearance until 13 April 2029. But when it does, it will pass within about 33,000 kilometres of the Earth’s surface – slightly closer than the orbits of communications satellites.
That’s close enough that as it sails across the skies of Australia and Africa it will be nearly as bright as the stars of the Southern Cross.
“An object this large comes this close about once per 1000 years,” says Richard Binzel, an asteroid researcher from Massachusetts Institute of Technology, US.
He was one of several scientists addressing matters related to the forthcoming near-miss at the recent International Academy of Astronautics’ 2019 Planetary Defence Conference in Maryland, US.
And while, shortly after its discovery a few years ago, there were concerns that the 350-metre rock – a rock 100 times more massive than the largest asteroid known to have hit the Earth in recorded human history – might impact the planet, that isn’t going to happen.
“Apophis will miss the Earth,” Binzel says. “It will not hit us. We’re going to have a close shave [but] again, it’s going to miss.”
What it will do is offer an unprecedented opportunity for research, including the opportunity for space missions that could do anything from a quick flyby to a rendezvous.
It’s even possible to do a landing, in which instruments could be deployed on the asteroid’s surface to monitor what happens as it encounters the Earth’s gravity field.
The Holy Grail, Binzel says, would be to put a seismometer on the surface to measure any shock waves created by stresses from the Earth’s much larger gravity.
Measuring how these waves propagate, he says, might provide clues to Apophis’s interior strength and structure.
He compares it to NASA’s InSight Mars lander, now seeking to probe the Martian interior via marsquakes.
“We can have InSight Apophis,” he says.
In addition to looking for stresses produced by Earth’s gravity, it is also possible to look for shock waves created by the impact of small meteorites, says Jeffrey Plescia, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory (APL), also in Maryland.
It could even be possible to plant small explosive charges on the surface, similar to those used to separate rocket stages during launch, to generate additional seismic waves, he says.
It’s also feasible to use an impactor fired at the asteroid from above, similar to that used by the Japanese in their current explorations of asteroid 162173 Ryugu, says Jakob Deller, an asteroid researcher at the Max Planck Institute for Solar System Research, in Göttingen, Germany.
Useful, too, is to measure how stresses from the flyby change Apophis’s rate of rotation, and whether they trigger landslides on sloping parts of its surface.
The first of these measurements would provide additional information about the asteroid’s interior, says Daniel Scheeres of the University of Colorado. The latter would help understand the strength of its surface materials.
Already, multiple mission concepts are in design, largely focusing on the use of inexpensive CubeSats, which are lightweight probes that can get off Earth via “ride shares” on other, larger, missions.
For example, says Fazle Siddique, a spacecraft mission design and navigation engineer at APL, a lot can be done with a vessel massing only 180 kilograms, using solar electric thrusters that, with relatively limited amounts of propellant, can allow it to chase down Apophis far out in its orbit, and rendezvous with it well before it swoops past Earth.
“You can [do] an Apophis rendezvous with about 40 kilograms of propellant,” he explains.
Visiting Apophis as it zips by, however, isn’t purely of scientific interest.
The rock is classified as potentially hazardous – and depending on just where its trajectory takes it in 2029, there is a small chance it will get just the right (or wrong) gravitational kick to put it on collision course with Earth the next time it comes close, in 2068.
Right now, the risk appears to be extremely small. But it is not zero, and the more we learn about its orbit as it approaches its 2029 flyby approach, the more we know about whether we have anything to worry about 39 years later.
More importantly, Apophis appears to be similar to most other potentially hazardous asteroids.
That means that even if we never have to worry about attempting to divert this one from hitting the Earth, anything learned about it may help better understand how to divert one of its kin, if the need arises.
“What we learn about Apophis is going to be broadly applicable to the whole potentially hazardous asteroid population,” Binzel says.
Not to mention that as it approaches, it is going to draw a lot of public attention.
“It will be visible to two billion people,” Binzel says. “Two billion people will see this asteroid streaking across the nighttime sky. The world will be watching, [and] it’s up to us to be ready.”
