SpaceX’s USDV, which will be responsible for sending the ISS to the bottom of the Pacific Ocean

The International Space Station (ISS) is the largest structure ever launched into space by mankind. But with a mass close to 450 tons, it also poses a danger if for some reason it were to fall back to Earth uncontrollably. Currently, the ISS is still active, and its orbit is regularly raised by Russian Progress cargo ships or the engines of the Russian Zvezda module (more recently by American Cygnus cargo ships, although not regularly). These ignitions are necessary to compensate for the small but constant atmospheric braking that occurs at an altitude of 400 kilometers. However, the ISS has an expiration date set by the project’s partner countries: 2030. This means that some kind of plan is needed to safely lower this huge structure from orbit.

The idea is for the station to re-enter the remote South Pacific region, specifically the SPOUA area (Uninhabited territory of the South Pacific Ocean), not far from the famous Pacific Pole of Inaccessibility, better known as Point Nemo. Two years ago, it was planned to use three Russian Progress spacecraft and the Zvezda module’s engines. The reason for using so many ships is that the Progress main engine has a thrust of only 300 kgf and allows ignition for up to 900 seconds (the Zvezda engines have a thrust of 315 kgf and there is no specific limit on the duration). (thanks to layer-by-layer cooling of the injectors – but the fuel reserves are limited). In a model scenario developed several years ago, the ISS will be deorbited in December 2030. Depending on solar activity, which regulates atmospheric braking, the station’s orbit will be lowered from 2026 or 2028. In June 2030, the ISS’s altitude will decrease to below 330 kilometers around, and the crew will evacuate the station. Between this month and November 2030, three Progress spacecraft will be launched to lower the station’s orbit. After reaching an altitude of 270 kilometers at perigee, Zvezda’s engines were to be fired to ensure a controlled entry into the Pacific Ocean.

There are a number of flaws in this plan, including the uncertainty of solar activity and the problem that a delay or cancellation of one of the Progress launches would entail. But the main one, logically, is that it depends entirely and exclusively on the participation of Russia, a country that, since its invasion of Ukraine in 2022, has repeatedly reiterated its determination to abandon the ISS as soon as possible or to separate part of the Russian segment from the ISS. to create its own station. Roscosmos’s current plans include abandoning the ISS in 2028 and building a new all-Russian station called ROS. So while that date will likely be delayed – just as the ISS will likely remain active beyond 2030 – when NASA wants to deorbit the ISS, it certainly can’t count on Russia. Or, at least, it can’t risk Russia deciding not to cooperate with the station’s dismantling.

For that reason, NASA decided in March 2023 to build a specific spacecraft that would allow the ISS to be safely deorbited sometime in 2030 or 2031, or later if the partners decide to extend the station’s life. NASA weighed other options for destroying the ISS, such as gradually dismembering it, selling it to a private contractor, or putting it into high orbit, but concluded that deorbiting it was best (Axiom will begin assembling its station docked to the ISS before separating it, but that’s a different story). The vehicle that will destroy the largest, most expensive, and most complex space structure in history is called the USDV (US Descent Apparatus), and on June 26, the space agency awarded SpaceX a contract worth $843 million for its development. Unlike the cargo Dragons and Crew Dragons that SpaceX launches to the ISS, USDV will be owned by NASA, not Elon Musk’s company, and will be operated by the space agency.

Neither NASA nor SpaceX have provided design details for the USDV, but it is expected to be a relatively large vehicle capable of carrying enough fuel to safely re-enter the atmosphere (on the order of 20 tons). According to agency requirements, the USDV must be docked with the station for up to a year and must be capable of generating at least 3.24 kN of thrust to achieve the Delta-V (41-47 m) required for deorbiting. /c) within one hour. Of course, the thrust cannot be greater than 6.2 kN so as not to compromise the integrity of the ISS structure (the final ignition must have a Delta-V of 30 m/s for up to 40 or 60 minutes to ensure a perigee of 50 kilometers and atmospheric destruction at South Pacific). NASA has said that the USDV will be developed from the Dragon, although it is unlikely that the USDV will be similar to those spacecraft because it does not require a reentry capsule—the heat shield mass means less fuel to slow the station down—and because the Dragon’s four main engines ” Draco” are directed in the wrong direction. So we know that the USDV will be some kind of modified Dragon, probably without the capsule, with the “trunk” modified so that the kit can carry enough fuel (over 8 tons) to deorbit the ISS. It is also possible that the USDV shares design elements with the Dragon XL cargo ship that will be used on the Gateway lunar station. And before anyone asks: no, the USDV will not be a starship option because such a large ship is not needed and because the Raptor’s power could destroy the structure of the station (in addition to the difficulties associated with docking such a large ship). ship from the ISS).

My personal favorite option for the ISS is to leave it in orbit above 600 kilometers, where it will remain for many decades or centuries as a monument to human creativity and the ability of our species to cooperate, but I realize that this is not feasible due to the cost (it would require many cargo ships) and the risks: the ISS could be fragmented by meteorite or space debris strikes, or by depressurization of some parts, which would create a very serious space debris problem. And the higher the altitude, the more likely it is to suffer a destructive impact. An energetic impact could completely fragment the ISS and leave low orbit virtually unusable for centuries (laugh at the movie Gravity). In any case, the later we see USDV in action, the longer the ISS will continue to host astronauts from all countries… except China.

References:

• https://www.nasa.gov/news-release/nasa-selects-international-space-station-us-deorbit-vehicle/
• https://www.nasa.gov/wp-content/uploads/2024/06/iss-deorbit-anaanalysis-summary.pdf