We Plan to Fly to an Asteroid---But Which One is Best?

Webmaster: It has been a long time since I translated a popular science article. I am trying it again, still using the “Google + Kingsoft + Search + Understanding” mode. The level is still mediocre, and the writing style is still poor, haha. Any feedback is welcome.

On April 15, US President Barack Obama visited the Kennedy Space Center in Florida and delivered a speech, proposing a new US space plan: the future destination for US space exploration is Mars and asteroids, terminating the Constellation program proposed by the Bush administration. He strongly hit back at critics of his policy while calling on private enterprises to pave an innovative path to Mars, rather than relying solely on national power to demonstrate American superiority.

As we all know, a manned mission to an asteroid is much more difficult than a manned mission to the Moon. In addition to demanding technical conditions, there are not many asteroids suitable for landing. Is Obama’s new plan really feasible? Let’s wait and see!

We Plan to Fly to an Asteroid—But Which One is Best?

Sending astronauts to land on an asteroid is a completely feasible plan, but NASA must find suitable landing sites and solve the issue of safety.

Last month, US President Obama announced that the next stop for NASA is an asteroid. NASA plans to send astronauts to the surface of an asteroid by 2025 to accumulate experience in safely sending humans far from Earth, paving the way for the long journey to Mars. Understanding the interior of an asteroid up close is also of great importance—perhaps one day we will need to perform interstellar migration or deflection. However, this plan faces severe challenges. If it can be successfully implemented, it will mean that humans can overcome the dark and cold environment of space.

Before landing on an asteroid, a spacecraft must first be launched to orbit the asteroid for exploration, rather than simply flying past it. This is the way to obtain the information required for the project, such as speed and position, which helps determine the amount of fuel suitable for the rockets implementing the plan. And those asteroids that are closest to Earth during the scheduled launch window will be the only choices for launch targets (i.e., the opposition position).

Although there are many small celestial bodies such as meteorites in space, during the scheduled time (around 2025 and the years before and after), asteroids that are close to Earth in position are still very few. Martin Elvis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, pointed this out at the 41st Annual Meeting of the American Astronomical Society’s Division on Dynamical Astronomy in Boston.

In 2009, a study led by Paul Abell of NASA’s Johnson Space Center in Houston, Texas, found that between 2025 and 2030, among the more than 1,200 asteroids listed in “Near-Earth Objects” (Meteoritics & Planetary Science, 1825), only 7 were suitable for human visits. Even including new asteroids discovered up to 2006, this number only increased to 42. Furthermore, after taking further factors into account, many of these asteroids would also be rejected.

Currently, the rotation rate of most asteroids remains unknown, but any high-speed rotating body will be immediately rejected because it would be difficult for astronauts to stand on them. Combined with the possibility of launch delays, it is clear that we need to find more candidate bodies. Elvis said, “I don’t think people have realized the quantity required. NASA needs to survey a large number of asteroids to screen out the best one!”

Abell expressed optimism that some large telescopes, such as the Pan-STARRS observatory recently completed in Hawaii, will expand the list of candidate asteroids. “There will be many, many exploration targets,” he said.

However, Elvis pointed out that ground-based telescopes have significant limitations because asteroids running in orbit are often drowned out by the strong light of the sun, just like the Earth. He advocates launching a space telescope into a solar orbit near Venus, which could better observe near-Earth asteroids—a proposal that has been discussed for a long time but has never been funded.

Daniel Scheeres of the University of Colorado at Boulder, who performed simulations of orbits, stated that even if suitable exploration targets are found, there are still many problems to solve. Small and irregularly shaped asteroids have lumpy gravity fields (as shown in the picture), so the probe would enter a chaotic orbit, making navigation much more troublesome than orbiting the Moon or the Earth.

“The surfaces of some asteroids may also be unstable, so astronauts could accidentally cause a landslide,” Scheeres added, noting that it would be wise to send robots up before a manned landing. “Before sending astronauts, we lack the necessary data,” he said.

English Original:

We’re flying to an asteroid – but which one?

DECIDING to send astronauts to an asteroid is all very well, but now NASA will have to find the few space rocks that are suitable to visit, and work out how to rendezvous safely.

Last month, US president Barack Obama announced the next destination for NASA astronauts would be an asteroid, as early as 2025. The goal would be to gain experience of safely sending humans far from Earth, as a stepping stone towards longer journeys to Mars. Studying the interior of an asteroid up close could also prove important if we ever need to deflect one. Yet achieving the goal will mean overcoming daunting challenges.

Before landing on an asteroid, a spacecraft must enter its orbit, rather than simply whizzing by. That means matching the object’s speed and direction of motion, which in most cases would require burning too much rocket fuel to be practical. The only way round this would be if the asteroid’s motion happened to be very similar to Earth’s at the time of its closest approach.

Even if a space rock passes that test, few have close approaches to Earth in the right time frame, in 2025 or the following few years, points out Martin Elvis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was to speak on the subject this week at a meeting of the American Astronomical Society’s Division on Dynamical Astronomy in Boston.

A 2009 study led by Paul Abell of NASA’s Johnson Space Center in Houston, Texas, found only seven asteroids that could be visited between 2025 and 2030, from a list of more than 1200 near-Earth objects (Meteoritics and Planetary Science, vol 44, p 1825). New discoveries since that study - which only included asteroids known in 2006 - has increased that number to 42, but many of these could be rejected when further criteria are applied.

The rotation rate of most asteroids is unknown, but any fast-spinning objects will be off limits because they would be difficult for astronauts to hang onto. Combine that with potential mission delays, and it becomes clear that many more candidates are required, Elvis says. “I think people have not appreciated how many you need,” he says. “NASA will need to survey huge numbers of asteroids to sift out the limited number of really good ones.”

Abell is optimistic that telescopes like the Pan-STARRS observatory that recently opened in Hawaii will expand the list of candidates. “There could be many, many targets to go to,” he says.

However, ground-based telescopes are hampered because asteroids in orbits similar to Earth’s are often hidden by the glare of the sun, Elvis says. He advocates launching a space telescope to orbit the sun near Venus, from which it could look outward to see asteroids near Earth’s orbit - an idea that has long been discussed by astronomers but never funded.

Even if enough suitable targets can be found, there are more problems to overcome. Small, irregularly shaped asteroids have lumpy gravity fields, so an orbiting spacecraft would follow a chaotic trajectory, making navigation much trickier than around Earth or the moon, says Daniel Scheeres of the University of Colorado in Boulder, who has simulated such orbits (see picture).

The surfaces of some asteroids may also be unstable, so astronauts could accidentally set off a landslide, Scheeres says, adding that it would be wise to send robots before humans. “We don’t have the sort of data that you might want before you send an astronaut,” he says.

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