[Danuri Launch] Chose the Most Difficult Path to the Moon

Choosing a Ballistic Lunar Transfer Orbit Taking 4.5 Months
Saving Over 20% Fuel Enables Extended Mission Duration

[Asia Economy Reporter Kim Bong-su] South Korea's first lunar probe, 'Danuri,' was launched at 8:08 a.m. Korean time on the 5th from Launch Pad 40 at the U.S. Space Force Base in Cape Canaveral, Florida. Although the straight-line distance from Earth to the Moon is about 380,000 km, Danuri has chosen a longer route, requiring about four and a half months of travel. It has taken the 'most difficult path' to the Moon. Why did Danuri choose this 'arduous journey'? 'Danuri Launch'

◇ Why take 4.5 months for a 3-day distance?

There are three ways to travel to the Moon. The first is the direct transfer method used during the U.S. Apollo missions, which heads straight from Earth to the Moon. After escaping Earth's gravity, the trajectory is adjusted to head directly to the Moon, but this requires an enormous amount of fuel. It can reach the Moon within five days, making it suitable for manned missions vulnerable to space environments. There is also the phasing orbit method, which involves orbiting Earth several times in a very elongated elliptical orbit before gently entering the Moon's orbit. This takes about one month. Although it consumes less fuel than the direct transfer, a considerable amount is still needed to enter lunar orbit. South Korea uses neither of these two methods but instead employs the Ballistic Lunar Transfer (BLT) method.

About 40 minutes after launch, Danuri separates at an altitude of 1,650 km and heads toward the Lagrange point where Earth's and the Sun's gravitational forces balance. At this time, the distance from Earth extends up to 1.56 million km. Then, it changes direction and uses Earth's gravity to swing back and enter the Moon's Earth orbit. This requires traveling a long distance over about four and a half months. It involves the challenging task of remote control while communicating with the spacecraft over a long-distance orbit. Among the three lunar orbit insertion methods, this takes the longest time but consumes the least fuel. Japan succeeded with this method in 1990, and the U.S. in 2011. South Korea chose the BLT method considering Danuri's limited weight of 678 kg, the target mission duration of one year, optimized fuel tank size, payload, and expected consumption period. In other words, it reduces fuel consumption by more than 20%, extending the probe's mission duration. However, the downside is the increased difficulty in communication and control as it travels more than 1.5 million km away from Earth and returns.

◇ Unable to use Nuri rocket, rented SpaceX

Danuri was launched aboard SpaceX's Falcon 9 rocket. This rocket was being reused for the sixth time and carried only Danuri for a solo launch. It is 70 meters long, with an external diameter of 3.7 meters, and has thrust capable of placing 22.8 tons into low Earth orbit using its first and second stage liquid-fueled engines. The launch pad is located at 28.29 degrees north latitude and 80.34 degrees west longitude. Being closest to Earth's equator in the U.S., it minimizes fuel consumption by utilizing Earth's rotational speed. Danuri, weighing 678 kg, was flown from Incheon Airport in South Korea on the 6th of last month and arrived at Cape Canaveral two days later. It underwent system checks, communication tests, and leak tests. The launch was originally scheduled for the morning of the 3rd but was delayed by two days due to issues with the Falcon 9 rocket.

Unfortunately, South Korea's Nuri rocket, which was successfully launched on June 21, cannot be used. Danuri separates at about 40 minutes after launch at an altitude of 1,650 km to enter the lunar transfer orbit. The problem is that Nuri cannot carry Danuri to that altitude. Nuri can only place 1.5 tons of payload into an orbit of 600 to 800 km. Moreover, although Nuri has just succeeded in development, its performance, safety, and reliability have not yet been fully verified. During its first launch in October last year, the third-stage engine shut down prematurely, failing to place the satellite mock-up into orbit and thus failing the mission. It only demonstrated full performance during the second launch on June 21. The government plans to conduct four more launches to improve reliability. At one point, the government considered launching Danuri using a Korean launch vehicle, but due to scheduling conflicts, it decided early on to use a foreign launch vehicle. Meanwhile, Danuri's total weight was somewhat increased from 550 kg to 678 kg. The government plans to develop an upgraded version of Nuri with significantly enhanced performance through the next-generation launch vehicle development project and use it for lunar exploration scheduled for 2030.