Search for the City Killers July 3, 2012Posted by skywalking1 in Space.
July 3, 2012
The B612 Foundation announced last week that it will raise funds to launch and operate a space-based telescope to search for dangerous near-Earth asteroids (NEAs). The hazard from these bodies is real. The Tunguska impact in Siberia, in 1908, was caused by Earth’s collision with a small asteroid, about 40 meters across, which exploded with the force of 3 to 5 Megatons of TNT, enough to level a major city. There are about a million near-Earth asteroids big enough to penetrate Earth’s atmosphere and cause city-wide or regional destruction; Tunguska-sized impacts occur every few centuries. Each year, about thirty explosions as powerful as the Hiroshima A-bomb occur in the high atmosphere from small asteroid impacts.
Today, we know of less than 10,000 of these objects. The number of detections is growing rapidly, thanks to NASA’s search program. The agency spends $20M per year on asteroid detection and research. One good result is that NASA-funded Earth-based telescopes have discovered about 90% of all NEAs > 1 km (about 5/8 mile) in diameter. These objects could cause global damage if they struck Earth. Fortunately, none is on a collision course in the next century (see the most worrisome impact threats here). In fact, today we know of know of no asteroids with a high probability of impact.
There’s the rub: We have detected and have orbits for less than 1% of the asteroids capable of causing extensive damage on Earth. We need a much more thorough search. At the current pace of NASA funding, we will not find these numerous small asteroids for decades. To fill this detection gap, the private B612 private foundation (full disclosure: several friends are involved in running B612) wants to launch Sentinel, a sun-circling space telescope to find these dangerous asteroids.
Two astronaut colleagues are principals in the B612 effort: Rusty Schweickart (Apollo 9) and Ed Lu (shuttle, Station). Sentinel will be privately funded, and once launched, will deliver NEA detection results to NASA, and through the Minor Planet Center, to other space agencies and scientists.
Sentinel has a main mirror about half a meter wide, and stands about eight meters tall. The relatively simple spacecraft is based on the Kepler planet-hunting telescope and the detectors are based on those of the WISE infrared telescope. The mission will cost about $500M over a decade, about the same as a typical Mars orbiter or Discovery-class interplanetary mission.
Our ground-based asteroid observatories search only at night, and can’t look too close to the Sun, missing those NEAs which spend much of their time in Earth’s daytime sky. To overcome those limitations, Sentinel will orbit the Sun in an orbit similar to that of Venus, about 0.7 the distance of Sun to the Earth. Racing on the inside track interior to Earth’s orbit, Sentinel will discover NEAs more efficiently than ground-based telescopes: it will sweep through the asteroid swarm more rapidly than Earth, and looking outward past Earth, it will see small asteroids inside Earth’s orbit, those usually invisible to terrestrial telescopes.
Sentinel will detect heat, or radiation in the infrared portion of the spectrum, where asteroids shine brightest. It will ignore background stars and look for rapidly moving objects—NEAs. The telescope will relay the orbits of asteroids it detects to the Minor Planet Center catalog. NASA will then run an analysis of these thousands of new orbits to look for possible future impacts.
If funds are available, Sentinel can launch in about five years. Once in its Venus-like orbit, the mission will take about 5.5 years to detect and map most of the small asteroids bigger than 50 m across.
NASA itself has studied NEA search missions like Sentinel, but in its current budget straits the agency says it cannot afford it now. The White House has chosen not to budget for such a telescope and its “asteroid insurance” policy, waiting for Congress to appropriate the funds. For its part, Congress in 2005 directed NASA to find 90% of the NEAs that are larger than 140 m, representing most of the remaining impact risk to Earth. But it has failed to deliver explicit funding for the telescope mission.
If we find a NEA on a collision path, we have the technology to divert it –– and prevent a cosmic disaster. We only have to change the velocity by ~ 1mm/sec to make an asteroid miss its appointment with Earth, years later. Three methods are promising:
- the gravity tractor (slight force on an asteroid exerted by gravity of hovering spacecraft)
- kinetic impact (smacking an NEA with a hypervelocity slug to change its velocity)
- nuclear explosive (vaporizes thin top layer of NEA surface, pushing asteroid in opposite direction of blast. Rarely needed, but can divert large NEAs, comets, or those discovered very late)
[Read more about how we’ll decide on preventing an impact through the work of the Association of Space Explorers.]
The enabler for all three solutions is early warning. As JPL comet and asteroid scientist Don Yeomans says, the top three priorities for diverting an NEA are:
- Find them early!
- Find them early!
- Find them early!
That’s where Sentinel comes in. The Sentinel project or one like it is the fastest way for us to reveal the cosmic shooting gallery within which Earth orbits. B612 has decided not to wait for NASA, and to start the discovery process now.
Astronaut colleague Rusty Schweickart says that right now, we are driving around the solar system without insurance. The B612 team plans to fly Sentinel, and work with NASA along the way to get this vital NEA tracking information to society. An important side benefit is that Sentinel will also tell NASA which asteroids are in the best, most accessible orbits for future astronaut expeditions to the nearby NEAs.
Tom Jones is a planetary (asteroid) scientist, veteran NASA astronaut, speaker, and the author of “Planetology: Unlocking the Secrets of the Solar System.” (Chapter 2 is all about asteroid impacts.)