Despite some recent delays, NASA’s Artemis program is still largely on track to establish a permanent human presence on the moon. Artemis astronauts will rely on a lot of logistical information, both when traversing the lunar surface, and for regular traffic to and from Earth. To prepare for this, NASA is testing a variety of tools to ensure safe, accurate and reliable lunar stays for visitors – and one of the space agency’s most successful recent experiments uses tactics long used on Earth to achieve success first in the month.
According to NASA, the Lunar Reconnaissance Orbiter (LRO) – currently in lunar orbit – recently aimed a laser altimeter about 100 km (about 62 miles) below towards the Indian Space Research Organization (ISRO) Vikram lander, near the Manzinus crater in the South Pole region. After releasing a series of five pulses on December 12, 2023, LRO then recorded signals reflected from a small retroreflector aboard Vikram, confirming the success of this method on the moon for the first time.
Measuring the return time of the laser from the retroreflector can help accurately estimate the distance and location of the object. Laser altimeters are often used to track satellites above Earth, although in last month’s tests, pulses were typically fired from equipment on the surface to orbiting satellites, not the other way around. Retroreflectors were also used during the Apollo missions to measure the moon’s distance from Earth – a distance that was revealed to be increasing by 1.5 inches every year.
However, the retroreflector used in the Apollo missions was much larger than the one aboard the Vikram. In this comparison, the Laser Retroreflector Array on the Vikram lander is only a 2 inch diameter round aluminum frame. No power source is required to do its job – all one has to do is wait for the laser to bounce off one of the eight quartz prisms at the corners of its cone back to the beam source.
And speaking of waiting, there really is quite a bit of time involved. LRO’s Lunar Orbiter Laser Altimeter (LOLA) produces pulses that cover only a 32-foot-wide portion of the lunar surface. The gap between each pulse ensures there is only a small chance of hitting the retroreflector as the LRO passes the Vikram lander.
“The altimeter is great for detecting craters, rocks and boulders to create global elevation maps on the Moon. But they are not ideal for pointing with one-hundredth of a degree accuracy to the retroreflector, which is necessary to consistently achieve accurate results,” NASA explained last week. Therefore, it took eight attempts to finally successfully contact Vikram.
The LRO altimeter is currently the only laser instrument orbiting the moon, so many other instruments are needed to ensure consistent and accurate measurement readings from the retroreflector. After that, future laser systems could be used to help Artemis astronauts land in complete darkness on the moon, as well as mark the locations of previously landed spacecraft. A similar set of retroreflectors is currently being used to help ship cargo dock automatically with the International Space Station. Think of them like little lunar air traffic controllers who help direct navigation and safety for astronauts.
Until then, more retroreflectors are on the way – JAXA’s SLIM lander was among one when it landed last week, and another is slated to be in the company’s private launch in mid-February.
2024-01-22 18:10:24
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